Astrobiology Workshop Final Report NASA Ames Research Center December 1996 Edited by: D. DeVincenzi Prepared by: G. Briggs, M. Cohen, J. Cuzzi, D. DesMarais, L. Harper, D. Morrison, A. Pohorille Table of Contents Introduction 1 Current State of Knowledge in Key Areas of Astrobiology 2 New Cross-Disciplinary Research Programs 7 Additional Activities 13 Appendices Workshop Program A1 Abstracts A4 Attendees A61 Introduction Astrobiology is defined in the 1996 NASA Strategic Plan as "The study of the living universe." At NASA's Ames Research Center, this endeavor encompasses the use of space to understand life's origin, evolution, and destiny in the universe. Life's origin refers to understanding the origin of life in the context of the origin and diversity of planetary systems. Life's evolution refers to understanding how living systems have adapted to Earth's changing environment, to the all-pervasive force of gravity, and how they may adapt to environments beyond Earth. Life's destiny refers to making long-term human presence in space a reality, and laying the foundation for understanding and managing changes in Earth's environment. The first Astrobiology Workshop was held at Ames on September 9-11, 1996, bringing together a diverse group of researchers to discuss the following general questions: ¥ Where and how are other habitable worlds formed? ¥ How does life originate? ¥ How have the Earth and its biosphere influenced each other over time? ¥ Can terrestrial life be sustained beyond our planet? ¥ How can we expand the human presence to Mars? The objectives of the Workshop included: discussing the scope of astrobiology, strengthening existing efforts for the study of life in the universe, identifying new cross-disciplinary programs with the greatest potential for scientific return, and suggesting steps needed to bring this program to reality. Ames has been assigned the lead role for astrobiology by NASA in recognition of its strong history of leadership in multidisciplinary research in the space, Earth, and life sciences and its pioneering work in studies of the living universe. This initial science workshop was established to lay the foundation for what is to become a national effort in astrobiology, with anticipated participation by the university community, other NASA centers, and other agencies. This workshop (the first meeting of its kind ever held) involved life, Earth, and space scientists in a truly interdisciplinary sharing of ideas related to life in the universe, and by all accounts was a resounding success. It was broadly interdisciplinary in attendance, with the following breakdown of the invited participants: 23 astronomers and physicists, 37 Earth and planetary scientists, and 38 life scientists. Attendance was 250 on the first day. The smaller workshop held on the next two days was nominally restricted to about 100 invitees, but in fact it attracted an overflow crowd. Peak attendance was actually reached during the final afternoon. Numerous phone calls were received from the public wanting access to additional information. The news media called several times after the workshop to request updates on and access to the latest thinking, discussion, and speculation. This report is a summary of the highlights of the workshop. The first section deals with the current state of knowledge in the fields that comprise astrobiology as presented by the invited speakers. This was widely considered to be one of the most significant aspects of the workshop, as participants were appraised of the latest thinking in fields outside their own. The next section identifies new cross-disciplinary research topics which resulted from new information exchanged among all of the relevant fields. These topics were developed during small group discussions organized around the 5 key questions noted above and occurred during two "working lunches." They were summarized and discussed during the final afternoon plenary session. The last section contains suggestions for follow-on activities which were proposed by workshop participants during the final afternoon plenary session. The report concludes with appendices containing the workshop program, abstracts, and participant list. There was no attempt made at the workshop to reach consensus on research priorities, recommendations, or funding requirements. Rather this workshop was intended to stimulate cross-discipline thinking and new ideas for productive research. Current State of Knowledge in Key Areas of Astrobiology Formation and Diversity of Planetary Systems Fundamental to understanding the distribution of life in the cosmos is understanding the formation and diversity of planetary systems, which are the retinues of planets and satellites of different mass and composition orbiting stars of different luminosities. The conditions under which these systems form and evolve will determine the diversity of habitable environments in space and in time. Understanding planetary phenomena will rely on three key approaches: direct, multi-wavelength observations of planetary systems across the entire range of formative and mature stages; theoretical studies of the behavior of multiple, complex, and interacting processes under diverse conditions; and laboratory and astronomical measurements of primitive materials preserved since the formative stages of our own system. A consensus theory of planetary formation is generally in hand: gradual accumulation of solids within a primarily gaseous, flattened circumstellar accretion disk, which itself is a byproduct of the formation of its parent star from a dense, rotating interstellar cloud of gas and dust. However, this theory has been studied in only a very narrow range of initial conditions, possibly important physics has been neglected, and it has little or no predictive capability. For example, recent discoveries of giant planets in circular orbits very close to solar-type stars were unexpected and are still not completely understood. There are, as of this writing, eight new giant planets known to orbit solar-like stars; at least one of these orbits within the "habitable zone" of its parent star. These new data provide not only a challenge to the current theoretical paradigms, but clear direction as to parts of parameter space in which both theoretical models and observations of extrasolar systems need more exercise. Furthermore, given the wide range of conceivable environments, we might ask "what makes a planet habitable?" (an associated question is "habitable for what kind of organism?"). Advances in technology are enabling not only new observations of these mature (if unanticipated) extrasolar planetary systems, but also of "protoplanetary nebulae" within which the planetary formation process is still ongoing. These observations are capable of telling us the extent, mass, gas and solid content, and thermal structure of the material from which planets form. In order to comprehend the new, surprising diversity of planetary systems, we must continue to study the early stages of planetary formation under a range of conditions, as well as to establish the full range of ultimate outcomes of the process. In addition to observations of remote extrasolar planetary systems from ground and space, we are fortunate to have in hand, or accessible by spacecraft, actual material which survives from the days of the early accumulation of our own planets. So-called "primitive material" preserves clues as to the materials from which, and the processes by which, planets formed. To be found in these primitive materials are presolar grains which carry clues as to the variety and number of stellar precursors of our own system, complex organic material which might preserve the signature of interstellar chemistry, once-molten silicate "chondrules" with composition, size, and mineralogy diagnostic of the pre-accretionary environment, and, in one recent case, suggestive evidence for past life on another planet. Origin of Life The occurrence of organic compounds in interstellar clouds, planets of the outer solar system, comets and meteorites suggests a chain of astrophysical processes which link the chemistry of interstellar clouds with the prebiotic evolution of organic matter in the solar system and on the early Earth. Although there is no record of the evolutionary pathway from this simple organic matter to present-day life on Earth, the main steps along this pathway can be deduced from basic physical and chemical principles, environmental conditions on the early Earth, and the cellular biology and phylogeny of contemporary organisms. There is compelling evidence that cellular life existed on Earth 3.56 billion years ago. Recently, a persuasive argument was made that terrestrial life was already present toward the end of the period of heavy bombardment of the early Earth by asteroids and comets from 4.0 to 3.9 billion years ago. This implies that ancestors of contemporary life emerged rather quickly, on a geological time scale, and perhaps also survived the effects of large impacts. Such catastrophic events would have strongly favored survival of thermophilic organisms which thrive at high temperatures. This scenario is consistent with the phylogenetic record, which indicates that the last common ancestor was thermophilic. This record also supports the view that life might have arisen first near marine hydrothermal vents. The possibility remains, however, that the first common ancestor lived at moderate temperatures and only later adapted to thermophilic conditions, in which case ocean surfaces and near-shore shallow environments might have spawned life. All present-day forms of life are cellular, with lipid bilayer membranes forming the primary barrier that separates the interior of a cell from the external environment. It has been proposed that similar, encapsulating structures (vesicles) made of simple membrane-forming material could have self-assembled in the protobiological environment. The presence of such membrane-forming material in carbonaceous meteorites is consistent with this idea. Furthermore, recent experiments showed that vesicular lipid bilayer structures can grow by spontaneous addition of membrane-forming material from the surrounding medium, and can encapsulate both ions and macromolecules. Besides separating intracellular components from the diluting effect of the environment, cell membranes also provide a barrier for separating charges, a fundamental process in bioenergetics. From phylogenetic data we infer that the earliest cells probably used chemical rather than photochemical energy sources. It has also been proposed that membranes helped stabilize the secondary structure of peptides (protein precursors) having appropriate sequences of polar and nonpolar amino acids. Some of these peptides may have been capable of performing basic protocellular functions, such as catalysis, signaling, and energy transduction, without requiring the existence of separate molecules capable of storing and transmitting genetic information (i.e., nucleic acids). Alternatively, it has been postulated that there was a time in protobiological evolution when RNA played a dual role as both genetic material and a catalytic molecule ("the RNA world"). However, this appealing concept encounters significant difficulties. RNA is chemically fragile and difficult to synthesize abiotically. The known range of its catalytic activities is rather narrow, and the origin of an RNA synthetic apparatus is unclear. Therefore, it may be more likely that RNA and proteins co-evolved in protocells, rather than evolving independently. The co-evolutionary process leading to division of cellular functions between these molecules, however, is not at all clear. Understanding the emergence of life requires studies that extend beyond the origin of biopolymers and cellular structures. All these components necessarily assembled into auto-catalytic, self-reproducing systems capable of evolution and selection. Based on theoretical arguments, it has been suggested that sets of mutually catalytic molecules can reproduce and evolve without templating, resulting in a primitive metabolism without a genome. However, only a limited number of experimental studies have been performed in this area. The recent discovery of organic, possibly even biogenic, material in a martian meteorite (ALH84001) opens the exciting possibility of extending the search for the origin of life to places beyond the Earth. Although current findings on ALH84001 are inconclusive regarding possible life on Mars, future exploration might lead to fundamentally new insights into prebiotic chemistry and protobiological evolution, the record of which is lost on the Earth. Interactions Between Earth and Its Biosphere The history of life on Earth was directed, at least in part, by changes in the surface environment. Today we are experiencing rapid environmental changes of our own making, and our biosphere must adapt and, perhaps eventually, evolve to a different state. Environmental change surely has occurred in the past, but can studies of our past help to predict our future? Also, to the extent that rocky planets have followed similar evolutionary paths, at least during the early chapters of their history, can studies of our own biosphere assist us in our search for extraterrestrial life, past or present? The processes which modified the environment vary widely both in their magnitude and time scales. For example, the increase in solar luminosity, the declining rates of comet and meteorite impacts, the exchange of volatile materials between Earth's mantle and crustal reservoirs, and the stabilization of continents have all exerted dominant controls on the surface environment. However, because these processes themselves evolved very slowly, they required 108 to 109 year time scales to cause global changes. The effects of plate tectonics, erosion, sedimentation, and glaciation acted more quickly, causing changes over 104 to 108 year time scales. Faster still have been the effects of ocean and climate dynamics and ocean-atmosphere-biosphere interactions, which can vary on 1 to 104 year time scales. Already, human activity has dramatically altered patterns of erosion, sedimentation, climate patterns, species biodiversity, primary productivity and ocean-atmosphere-biosphere exchange. These changes are happening over a few decades. In the earlier "natural" world, such changes would have required typically thousands to millions of years to occur. How will plants, animals and the microbial world respond to such rapid change? Microorganisms are supremely adapted for coping with change. Should global conditions deteriorate, the small size of microbes allows them to "hide" in niches. Small cell size imparts a high surface/volume ratio, which allows rapid rates of chemical exchange with the cell's surroundings. Thus microbes can rapidly exploit favorable conditions. The diverse biochemistry of microbes permits them not only to survive, but even to prosper under environmental extremes. Already by 3.5 billion years ago, widespread microbial communities accommodated large meteorite impacts, UV irradiation, desiccation, wide excursions in temperature and salinity, and a long menu of chemical substrates as sources of energy and organic matter. For example, our early biosphere adapted to major changes in volcanism, coastal environments, atmospheric composition, and the oxidation state of the oceans and atmosphere. On the other hand, microorganisms can themselves contribute to environmental change by, for example, affecting rates of erosion and sedimentation or by influencing the atmosphere's inventory of reactive gases. Microbes responsible for infectious diseases evolve to circumvent medical treatments, thereby continually challenging human populations. In contrast with the bacteria, plants and animals are much larger, more complex and highly specialized. They typically depend upon a more limited suite of nutrients and a relatively narrow range of conditions for their survival. Accordingly, environmental change, human-induced or otherwise, can more easily trigger catastrophe within ecosystems which sustain these complex eukaryotic organisms. Modern challenges to the biosphere include rising atmospheric levels of CO2, SO2, CH4, CO, and N2O due to fossil fuel burning and agriculture (causing greenhouse climate effects as well as direct biospheric effects), declining ozone levels (leading to increased ultraviolet radiation), invasions of foreign species, and land use changes whose effects include the following: soil salinization, overgrazing, increased soil erosion, altered energy balance, loss of biodiversity, species extinctions, declines in food and fisheries, and chemical pollution. While large meteorite impacts, such as the one which marks the Cretaceous/Tertiary boundary, were perhaps more severe than modern human-induced changes, impacts still serve as useful models for the effects of catastrophic change on the biosphere. For example, the severe "winter" which had been predicted to follow a large impact alerted us to the "nuclear winter" which might follow thermonuclear war. Also, impacts remind us that catastrophism probably does play at least a limited, but still important, role in the long-term evolution of our biosphere. The role of impacts in evolution was perhaps most pronounced during the earliest stages of Earth's history, when impact rates were much higher. Sustaining Life in Space Because life evolved and developed on the Earth, it is uniquely adapted to function on this planet. To sustain life beyond the Earth's biosphere for prolonged periods of time will require a better understanding of the processes underlying biological adaptation and the interactions among organisms and their environments. The relationships among the behavioral, structural, and genetic bases of survival remain to be elucidated. Adaptability in biological systems is a given, but the limits of adaptability and the issue of irreversibility of adaptive changes are major concerns. A concerted effort in enhancing our knowledge of biological adaptation, and developmental and evolutionary biology, will be needed if we are to sustain terrestrial life beyond the Earth's biosphere. Electromagnetic radiation and gravity are two fundamental environmental variables that dramatically affect biological systems. On Earth, gravity is effectively constant in magnitude and direction, and the natural radiation environment has modest variability. These physical variables are difficult to control in space, and consequently can severely limit our ability to sustain life beyond the surface of the Earth. How the radiation environment beyond the Earth affects biological systems is only partially understood. In space, galactic cosmic rays and particles from solar events can be lethal to terrestrial life forms. We have a very limited ability to predict solar events, and our understanding of shielding techniques to manage radiation risks is poor. Further, our ability to characterize the radio-biological effectiveness of various ionized and non-ionized particles, is limited. Space travelers beyond low Earth orbit must, therefore, monitor the Sun for solar storms as a matter of life or death. Clearly, the effects of various forms of radiation on RNA and DNA are issues of major concern. Currently we are ignorant of the relationships among chromosomal damage, chromosomal aberrations, and carcinogenesis. The direct effects of high energy particles on the nervous system are also poorly understood, as are biological mechanisms for the repair of radiation damage. Gravity profoundly affects many biological systems, both directly and indirectly. The cardiovascular, musculoskeletal, and neurovestibular systems all undergo dramatic changes in space, where organisms are deprived of terrestrial gravity. For example, fluids shift from the lower limbs and lower torso to the upper torso and the head; blood volume is reduced; anti-gravity muscles in the lower limbs and torso tend to atrophy; bones that formerly supported the organism against gravity become less dense and more fragile; vestibular-ocular reflexes are altered, and the nervous system re-calibrates itself to function in the absence of gravity. Although these changes are generally benign for functioning in space, they can seriously compromise an organism's ability to function in a new gravitational environment and upon return to the Earth. Humans currently use multiple countermeasures to minimize the effects of non-terrestrial environments on physiological systems for periods of more than one year. These countermeasures, which include training procedures, protective garments, physical exercise, conditioning devices, and various pharmacological agents, may be of only limited value to sustain life beyond the Earth's biosphere for prolonged periods of time that ultimately will include multiple generations. Artificial gravity, provided by continuous or intermittent centrifugation, lower-body negative pressure exercise chambers, or other techniques, may be necessary. Our experience with artificial gravity for humans in space is limited to a single, brief, Gemini flight experiment, and our current knowledge base is inadequate to assess the need for artificial gravity to sustain life beyond the Earth's biosphere. Critical psychological variables in small group interactions during prolonged isolation in a perpetually hostile environment away from the home society are not well understood. The interactions of gravity, radiation, and isolation in non-terrestrial environments have never been studied systematically. Thus, many fundamental questions in the life sciences will need to be answered before we can assure that terrestrial life forms can be sustained beyond the Earth's biosphere for prolonged periods. With current technology, we are able to maintain terrestrial life beyond the Earth for periods in excess of one year. To sustain terrestrial life beyond the Earth for longer periods, it is necessary to create a micro-environment that is similar to that on Earth, at least initially. This environment must provide an atmosphere with a ppropriate partial pressures of O2 and allow for gas exchanges to support metabolism; it must provide adequate liquid water, appropriate microorganisms, adequate gravity, food, thermal protection, and radiation protection; it must allow for the partial recycling of nutrients and waste-products; finally, it must be stable and reliably sustainable for an indefinite period of time. Human Exploration of Mars As described in the section above, we still lack much of the fundamental knowledge necessary to send humans on extended space journeys beyond the protection of the Earth's biosphere (including its magnetic field). Only modest progress is being made towards actually carrying out the life science experiments and technology tests needed to ensure that a crew arriving at Mars will be at a sufficient fitness level (albeit that fitness level needs definition) to assure their well being and the success of their mission. Thus, fully effective countermeasures to deal with long duration exposure to microgravity have not yet been demonstrated, and the appropriate shielding requirements to deal with extended exposure to heavy galactic cosmic rays have not been fully defined. However, these issues appear tractable if appropriate experiments are conducted on the International Space Station and if appropriate particle accelerator experiments are carried out. A program to extend human presence to Mars will inevitably have both exploration and what we may term habitability goals. If evidence that life once evolved on Mars is discovered, human explorers will provide much of the scientific capability needed (beyond robotic capabilities projected for the next several decades) to investigate how the pre-biotic seeds of microbial life evolved and subsequently prospered or perished. Theory, laboratory experimentation, subterranean terrestrial sampling and meteoritic evidence suggest that microbial life could have evolved on early Mars. Our present lack of direct knowledge about subterranean martian environments should make us cautious, therefore, about concluding (as seems common) that any such early life would inevitably have become extinct on a planet where present surface conditions are indeed extremely hostile. To answer questions about possible extant life we need to explore the subsurface below the cryosphere, which extends to kilometer depths, and into the warmer martian hydrosphere. Although a thorough exploration of the martian subsurface by robots alone is feasible in principle, the combined effects of great communication distances and intrinsically limited machine intelligence might well require postponement of such exploration for many generations. Therefore, some astrobiologists are considering whether human exploration of Mars may be legitimately identified as a real scientific priority as the only efficient and timely way in which we will be able to study, at first hand, a second sample of life (all terrestrial life being linked to a common ancestor). The consequences of the discovery of life, past or present, on Mars in the coming decades will have profound implications beyond just the intense interest of molecular biologists. (Likewise, although it will be much harder to disprove the case, the determination that Mars never evolved life would also have profound implications.) Scientists and non-scientists alike will immediately appreciate the improbability that humans are "alone" in our galaxy. The discovery of life on Mars will surely add priority to the search for life elsewhere in our solar system (e.g. in the subterranean oceans of Europa), to the search for Earth-like planets orbiting other stars in our galaxy, and to the search for extraterrestrial intelligence. More generally, the stimulation of such a discovery of martian life is also likely to lead us to a recognition that, having the technological means at hand, we can be on the verge of becoming a multi-planet civilization, with Mars as our second abode. New Cross-Disciplinary Research Programs Formation and Diversity of Planetary Systems It was recommended that part of NASA's vision should be to understand the planetary formation process in enough depth to be able to predict, or at least constrain, the diversity of habitable planetary systems. Within this vision, primary science goals might include: What are the fundamental processes and conditions that lead to planetary formation? What kinds of planets form, around what kinds of stars, and at what distances from their parent stars? What defines and determines "habitability"? There are several well-posed problems that are ready for accelerated study by astronomical observations. Recent indirect radial velocity observations of extrasolar planets, and millimeter-wave, infrared, and HST observations of circumstellar gas and particle disks, are outstanding examples of the tip of this iceberg. Certainly, observations of mature planetary systems around a large variety of stellar types of various ages will be needed to determine what kinds of planets form around what kinds of stars. Direct or indirect detection of planets in "mature" systems is the focus of NASA's planning efforts to date. However, there remain critical gaps in our understanding of the earlier "protoplanetary" stage; these include the actual absolute (not assumed relative) abundances of gas and solids, the nebula radial extent under different initial conditions, its radial and vertical temperature structure, the particle-to-planet accumulation time scale, the role and distribution of angular momentum of in-falling material, the properties of stellar winds, and the role of magnetic fields. We are completely ignorant of whether any of these properties vary with stellar type and/or with star formation environment (i.e., solitary or densely clustered), and there are hints in current data that protoplanetary systems in the two different star forming regions in Taurus and Ophiuchus have rather different properties. The observations need to be conducted over a wide range of wavelengths between the short microwave (millimeter) and near-infrared spectral regions to penetrate the thick nebular dust envelopes and sample the mid-plane where planet formation is occurring. Infrared (Keck and follow-on) and millimeter-wave interferometers are needed to resolve protoplanetary disk structure at 1 AU resolution. NASA's planned "Origins" program proposes a series of space-based infrared telescopes and interferometers; other approaches were mentioned which are less connected technologically but are also worthy of consideration (photometric detection, balloon missions, HST upgrades, far-IR [100 micron] interferometer, microlensing, etc.). Several key theoretical questions are ripe for interdisciplinary attack. These include the properties of a densely clustered protostellar environment, the role of ionization, grain charging, and electromagnetic forces, the role of global wave modes and/or energetic infall itself in nebula evolution, the presence, extent, duration, and energetics of nebula turbulence, the possibly wide-ranging migration of protoplanets (and solid material in general) within the nebula and even relative to the nebula gas. It was noted that the Sun-Earth Connections theme of the Space Science Enterprise might be tapped to a larger extent for its expertise in electrodynamics and stability or instability of weakly ionized media, properties of stellar and solar winds, and of dusty plasmas. It was generally felt that augmentation of the numbers of primitive meteorites returned, catalogued, and analyzed from the Antarctic would yield commensurate rewards. Much of the nation's analytic resources are Apollo-era and worthy of considerable upgrading. Of particular interest might be the sort of ultra-high resolution analytic equipment capable of studying the internal structure of putative nano-fossils (such as found in ALH84001) or of obtaining accurate age dates and/or isotope data on small mineral samples. Exploration of this sort of "inner space" is probably just as demanding of technology and expertise, and as rewarding in terms of understanding of the planetary formation process, as comparable efforts devoted to exploring "outer space." Concerning the issue of habitability, there are many unknowns (even in the case of our own planet). For instance, what stellar and/or planetary conditions are most important for the origin of life, or for its evolution and increasing complexity? Are stellar photons of extreme energies and charged particle fluxes positive or negative factors? What is the role of internal planetary activity (tectonics) in truncating or prolonging the habitable era? Water is generally agreed to be the sine qua non of life, but how is water distributed across growing planets or reintroduced on mature planets that lost it or never had it at all? Does chaotic planetesimal dynamics spray icy objects from the outer solar system onto mostly formed inner planets? What is the composition of the "primitive" objects that even today impact the terrestrial planets? What is the mass and extent of the Kuiper belt of primitive planetesimals? Are terrestrial-sized satellites of close-in giant planets orbiting M dwarf stars habitable? Are they dynamically stable? Studies of the "habitable zone" using planetary scale climate evolution models might profit from more association with the sophisticated modeling supported in the Earth science community (such as to treat cloud feedback effects). Origin of Life The origin of protobiological self-organization and complexity. The main thrust in this area should be to establish the principles of organization and complexity that led a collection of organic molecules to assemble into the earliest ancestors of contemporary cells. With these principles as a basis, attempts should be made to create laboratory versions of cellular, self-reproducing and evolving systems starting from material that might have existed under prebiotic conditions. Even though enough knowledge appears to exist to make fundamental progress in this direction, this research area remains severely under-represented in the current exobiology program. The expertise of cellular and molecular/structural biologists and bio-organic and physical chemists will be required, guided by collaboration between experimentalists and theorists. The goals of this interdisciplinary effort are to establish protobiological versions of basic cellular functions such as energy capture, chemical catalysis, and transport of solutes across membrane boundaries. These processes must be accomplished by simple molecules that self-assemble to form auto-catalytic, self-reproducing systems that evolve in response to environmental pressures. The laboratory experiments must be guided by theoretical work aimed at discovering general principles of organization and complexity from simulations that include realistic descriptions of intermolecular interactions, energetics and chemical kinetics. Conditions on the Earth between 4.6 and 3.8 billion years ago. Based on current evidence that life on Earth originated earlier than 3.8 Gyrs ago, it becomes especially important to establish plausible conditions on the Earth during the prebiotic period. This would be a truly interdisciplinary effort involving astronomers, geologists, chemists, and planetary and atmospheric scientists. Among the main unanswered questions are: (a) How did the temperature and the chemical composition of the atmosphere, crust and the oceans evolve over this time? (b) What were the global environmental effects of impacts of varying degrees of severity on the origin and survival of life? (c) What was the inventory of organic material on the prebiotic Earth, and what were the relative contributions of terrestrial and extraterrestrial sources? (d) Could life have been transported between Earth and Mars? From the biological side, it would also be important to determine what can be learned about the early evolution of life and its environment by phylogenetic characterization of the last common ancestor. Origin of life elsewhere in the solar system. An unambiguous answer to the question about extant or extinct life on Mars will likely come only from direct exploration. Nevertheless, there is a considerable body of research that should be done prior to or in parallel with missions to Mars in order to interpret results of analyses of martian samples. Perhaps the most urgent task is to broaden and extend the study of the ALH84001 meteorite and Earth analogs with the goal of better understanding the morphological, chemical and isotopic characteristics attributable to micro-organisms. Collecting and analyzing more Mars meteorites is also a key task. Developing reliable criteria for distinguishing between biogenic and abiotic structures in the Mars meteorite is critical. Besides the rock record, information about prebiotic chemistry and possible life on Mars may be gained by comparing conditions on the early Mars and early Earth. Since liquid water is required to support life, other, possibly transient, sub-surface micro-environments in the outer solar system and large asteroids could have been conducive to the origins of life, a prime example being Europa. These environments should be considered in future missions and their prebiotic and biological potential should be assessed by model studies. Organic chemistry in astrophysical and planetary environments. The occurrence of organic matter in interstellar clouds, star-forming regions, comets and carbonaceous meteorites point to a chain of processes linking interstellar material to solar system formation and perhaps even to prebiotic evolution on Earth and Mars. The complexity of molecular structure that can be achieved in astrophysical environments remains, however, to be fully explored. In particular, little is known about what survives molecular cloud collapse to become incorporated in planetary materials. Are there amino acids, nucleic acid bases or sugars in interstellar dust and comets? Now that optically active amino acids have been found in Murchison meteorite, what evidence can be found for astrophysical mechanisms capable of such stereoselectivity? These questions can be addressed by astronomical observations and further laboratory and theoretical studies of primitive materials. Interactions between Earth and its Biosphere The concept that the evolution of the biosphere and its environment are inextricably related should be investigated in detail. Far beyond simply demonstrating that such a relationship exists, such an investigation offers many conceptual and practical benefits. Understanding the extent to which biological evolution has been a product of environmental change will reveal the mechanisms of the evolutionary process. The consequences of human-induced environmental change will therefore be easier to forecast. The search for life beyond the earth will be improved by a firmer understanding of how planetary environments influence the survival of biospheres. Studies of ancient ecosystems could explore the relationship between the microenvironment and the diversity of microbiota and how these changed over time. Comparative studies of modern and ancient ecosystems could identify those aspects of the microenvironment which are crucial to microbial diversity and evolution and how they changed over geologic time. Microbial communities in hydrothermal systems (including hot spring deposits) and in groundwater are especially important analogs both for understanding the very early fossil record on Earth and for guiding the search for evidence of past life on Mars. Also, the changes in morphology and chemistry which accompany fossilization should be examined. Regarding Earth's "macroenvironment," we should identify those mechanisms which directed the long-term increase in atmospheric O2 and the decline in atmospheric CO2 levels. To accommodate these changes, microbes modified their pathways of CO2 uptake, invented protocols for detoxifying oxidants, devised new O2-requiring biosynthetic pathways, and so forth. What were the nature and timing of these innovations? What were the composition and abundance of trace biogenic gases in the ancient atmosphere, particularly before significant levels of O2 were attained? What were/are the significant feedback effects involving biota, trace gases and climate? How did oxygen-utilizing eukaryotes evolve in response to these changes? Given anticipated land-use changes today, what role will trace gases play in future climate change? How does an entire ecosystem, including its microbes, respond to abrupt environmental perturbations? The natural microbial world is a rich source of information about the mechanisms which could permanently change those ecosystems which sustain plants and animals. Can these microbial effects be detected before permanent change occurs? Ecosystem-level studies could monitor the effects of change. For example, such studies could explore the following: (a) the ecology of microbial communities which are still relatively unaltered by human activity, (b) the sensitivity of ecosystems to changes in specific parameters, singly or in combination (e.g., CO2 levels, UV irradiation, soil acidity, reductions in biodiversity, etc.), and (c) the relationships between the biota, climate, geography and hydrology. Studies also could be pursued for plants and animals, specifically in the following areas: a) the influence of extraterrestrial phenomena such as impacts upon evolution, b) the physical and biological drivers of mass extinctions, and (c) ecosystem, hydrologic and climate changes which impact the natural ecosystem and also public health. Extrasolar planets will eventually be examined to search for other biospheres. Life should ultimately be detectable through spectroscopic analyses of a planet's atmospheric composition. Under what conditions does the presence of abundant atmospheric O2 definitely indicate life? Aside from abundant O2 levels, what other atmospheric compositions are definitive indicators of a biosphere? Is the early history of our own atmosphere actually representative of other evolving, habitable planets? Obviously an effective research and exploration program requires that new cross-disciplinary technologies be developed to exploit novel approaches for getting answers. These involve, for example, the development of new microsensors for probing the dynamics of microbial ecosystems, field sensors to monitor gas exchange between ecosystems and the atmosphere, new approaches in remote sensing and so forth. An effective technology program is one which is closely integrated with the research program and responds effectively to new needs as they arise. Integrated quantitative models should be constructed which help to develop a deeper understanding between physiology, ecology, and the environment. Such models could account for changes over various time scales, and ultimately they should be able to predict community responses to perturbations. We also should model other planets which might still be habitable but which are different from Earth. How would different planetary sizes, solar insolation or volatile inventories affect the evolution of the planet and its biosphere? Such insights would greatly enrich our understanding of Mars as well as those extrasolar rocky planets which we are destined to discover. Sustaining Life in Space Perspectives from the early evolution and development of life on Earth can provide perspectives for developmental biology in space. The same mechanisms that allowed terrestrial life forms to adapt to the earthly environment will probably be at work in allowing terrestrial life forms to adapt to non-terrestrial environments. Similarly, experimental studies investigating gravitational and radio-biological influences on genetic material and developmental processes can provide perspectives for an understanding of the evolution of life on Earth. The new field of astrobiology provides a framework in which this integration can take place. To sustain terrestrial life beyond the Earth's biosphere for prolonged periods of time will require new fundamental knowledge and an integration of that knowledge in many disciplines. Further, we need a more profound understanding of closed or semi-closed ecological systems. Interdisciplinary studies involving radiation physics, gravitational biology, genetics, neurobiology, and developmental biology are required to provide the critical understanding. The International Space Station is an essential evolutionary test bed for research on the effects of the space environment in biological development and evolution, as well as the only place where the effects of gravity on living systems can be investigated systematically. For example, we do not fully understand the role of gravity in development. Are there thresholds and critical periods for the effects of gravity; how are phenotypes and genotypes affected by gravity at the cellular, system, and organism level? Improved models for definition and prediction of solar events, the development of advanced radiation shielding techniques, and enhanced understanding of genetic biological radiation repair mechanisms are of particular importance. Other interdisciplinary studies involving gravitational physics and life sciences are also needed. Just as importantly, we need a new interdisciplinary perspective to integrate the information that will assure the long term survival of terrestrial species beyond the Earth's biosphere. A regenerative life support system (i.e. one which can be fully restored/replenished), will ultimately be needed to sustain terrestrial life beyond the Earth. Thus, a biodome will be required. Unfortunately, we cannot fully specify all the necessary characteristics of the required micro-environment at this time because we do not understand all the control mechanisms that function to maintain closed or nearly-closed ecological systems. A research biodome is an important tool that will be needed to help us examine these relationships. The transition from constant re-supplying to the use of in situ resources will eventually be necessary to sustain terrestrial life beyond the Earth's biosphere. Cross-disciplinary studies continue to provide insights into the Earth's complex ecosystem, and these can ultimately be applied to developing artificial life support systems. This remains a promising area for future cross-disciplinary efforts, for the better we can characterize those events that alter the Earth's biosphere, the more adequately we will be able to specify what is needed to provide stable and sustainable life support systems in space. Human Exploration of Mars Progress in understanding the origin and evolution of life includes further efforts to explore the subterranean portion of the Earth's biosphere. The development of improved technologies to do so should include equipment that could later be used on Mars; e.g., light-weight, semi-automated drill rigs. Moreover, techniques should be evolved from current procedures to ensure that as samples are acquired from new subsurface environments, those samples are protected from contamination -- all the way from their source to their detailed examination within a well-equipped Mars base laboratory. Such aseptic protection of the samples must be carried out in a way to ensure that the samples are effectively quarantined until adequately determined to be free of pathogenic properties. If and when samples of extant martian life are indeed discovered, we must be prepared to proceed with their fundamental characterization and to have clear procedures established to determine if and when such samples should be returned to Earth. The importance of this issue will surely influence astronaut selection and training requirements which will likely include participation in the exploration of the extremes of our own planet's biosphere, e.g. desert (including Antarctica) and subterranean environments. If Mars does have an extant subterranean biosphere, then our exploration of that biosphere raises the serious environmental and ethical questions that we face on our own world where species are endangered and lost sometimes even before we have discovered and characterized them. The need to avoid contaminating and changing a presently unknown biologic environment was raised at the workshop and acknowledged to be not only a potential scientific catastrophe but also a real policy issue, as yet without an assigned advocate. Further, even if it should turn out that Mars is now a sterile planet, environmental issues will confront us -- issues relating to the control of the pollution and waste associated with an expanding human base. In the much longer term our technology may provide us with the ability to seriously consider "terraforming" regions of Mars (or even the entire planet). Today terraforming another planet amounts to little more than a thought experiment, but human history demonstrates that such conjectures can indeed become reality, usually with severe unintended consequences. In the absence of any other organization likely to grapple with the ethical dilemmas involved in the future expansion of humans beyond the Earth, the astrobiology component of NASA's space research program appears to be the natural home for analysis of exploration ethics. Additional Activities 1. New Programs a. A number of the promising new research directions fit within existing NASA programs. In these cases, it is recommended that solicitations be included within existing NRAs to recruit research offerings in these areas. Because of the multidisciplinary nature of these proposals, it is also recommended that existing peer review panels be supplemented with reviewers possessing relevant skills. High quality research would be funded by the sponsoring program. b. Several important new research directions have been proposed which cross traditional NASA discipline and programmatic boundaries. For these, a funding commitment to support new research is recommended. This program should be designed to support an average grant interval of approximately three years (the length of time usually needed for a graduate thesis). It is recommended that astrobiology research proposals be solicited using a NASA Research Announcement and peer reviewed by a multidisciplinary panel organized by the Chief Scientist's office at NASA Headquarters (HQ) or "hosted" by each of the participating HQ Offices in turn: Space Science, Life and Microgravity Science, and Mission to Planet Earth. c. Based on the high level of interest expressed by the Workshop participants, a funding commitment from Ames Research Center is recommended to sponsor one focused conference (nano-fossils was proposed as a topic to follow-on to the ALH84001 results) per year to follow up on the high priority research topics that require further refinement and one general conference every 3 years to present the latest results of research important to astrobiology. The results will be made available to HQ and its advisory committees for consideration and programmatic action as appropriate. 2. Cross-agency and Cross-disciplinary Collaboration. a. In many areas of research summarized earlier, it was felt that an increased level of NASA-NSF collaboration would yield great rewards. Several powerful observational tools currently on the drawing boards are ground-based facilities (Keck interferometer, mm Array, etc.) and lie in a grey area between NASA (which traditionally emphasizes flight missions) and NSF (which emphasizes ground-based facilities). In the area of primitive materials, NSF and NASA already share the load in collecting and analyzing meteorites. Much theoretical modeling is highly computer intensive, yet both NASA and NSF are rethinking and/or downsizing their dedicated computational facilities. An increased level of interagency cooperation in all of these areas, and perhaps others, might be both appropriate and desirable. b. While fostering cross-agency cooperation, and with the likely support of Congress and the public, the workshop participants recommended that NASA also rededicate an appropriate fraction of its R&A to "interdisciplinary" research. Teaming across discipline boundaries is not always efficient at the beginning, but can be a critical step towards creative insights as each team member assimilates the knowledge of other disciplines while remaining an expert in their own right. Providing tangible incentives for working scientists to move in this direction is perhaps the most obvious tack. Policy-level adjustments to the R&A program priorities or practices might be appropriate; short "learning" sabbaticals might be encouraged to a greater extent under the grants programs; NAS-NRC associateships in interdisciplinary "new direction" research might be endowed. 3. Outreach and Communication Activities. There was widespread public as well as scientific interest in the Astrobiology Workshop and its results. Beyond its deep fundamental scientific value, it was felt that a driving force for astrobiology is unquestionably the enormous public interest it excites. Hopefully, this interest will not flag at any point along the long path which now lies ahead. Opportunities should be built in from the start to engage the public and to provide them with new results and information appropriate to their level of public investment. A more problem- or theme-focused approach perhaps provides more appeal to the public and to Congress than a traditional method- or discipline-oriented approach. Workshop participants urged more multidisciplinary discussions and interactions. Some Workshop participants are interested in teaching courses in astrobiology. Workshop participants also urged that a special effort be made to ensure that scientists interested in astrobiology have easy access to the latest findings, including access to the results of new research efforts described above. Four mechanisms are proposed to respond to this interest. a. In addition to future astrobiology workshops and conferences, astrobiology may be a topic or session at other scientific conferences such as the American Society of Gravitational and Space Biology, COSPAR, the International Society for the Study of the Origin of Life, the Gordon Conference, and others. b. A Web page will be established that will not only present formal work but will also allow discussion and interaction via chat rooms and dialog groups. Part of this Web page may be an "electronic textbook" accessible to the public to support education efforts in this area. Interdisciplinary communication can be furthered by using web-based connections to allow motivated discipline experts better insight into other communities. c. A lay person's summary of the results of the current Astrobiology Workshop will be developed for presentation in a publicly accessible medium such as The Planetary Report, Discover Magazine, Scientific American, etc. d. Special workshops will be convened for educators and students. Appendices Astrobiology Workshop September 9-11, 1996 Program Astrobiology is defined in the 1996 NASA Strategic Plan as "The study of the living universe. This field provides a scientific foundation for a multidisciplinary study of (1) the origin and distribution of life in the universe, (2) an understanding of the role of gravity in living systems, and (3) the study of the Earth's atmosphere and ecosystems." Ames Research Center has been assigned the lead role for astrobiology within the agency. In a response to the challenge from the NASA Administrator to develop new cross-disciplinary programs and strengthen existing efforts for the study of life in the universe, Ames will host a scientific workshop, organized around several major questions in astrobiology: ¥ How does life originate? ¥ Where and how are other habitable worlds formed? ¥ How have the Earth and its biosphere influenced each other over time? ¥ Can terrestrial life be sustained beyond our planet? ¥ How can we expand the human presence to Mars? Session 1 (Monday morning, Conference Center Ballroom) 0800 Registration 0900 Harry McDonald Welcome and introductions 0930 Carl Pilcher Astrobiology and space science 0955 Arnauld Nicogossian Astrobiology and life science 1020 William Townsend Astrobiology and earth science 1045 Frank Martin Astrobiology and exploration 1110 David Morrison Key questions in astrobiology 1130 Lunch break Session 2 (Monday afternoon, Conference Center Ballroom) Co-chairs: MRC Greenwood & David Morrison 1300 Stuart Kauffman Chemical and physical pathways to complexity 1340 Geoff Marcy Detection of planets orbiting Sun-like stars 1420 James Kasting Habitability of planets 1500 Chris McKay The search for life on Mars 1540 William Sprigg Near-term evolution of Earth's climate 1620 Emily Holton Gravity and biology 1730 Reception Session 3 (Tuesday morning, Space Science Auditorium) Co-chairs: Warren Gore & Kevin Zahnle 0800 Scott Sandford Complex molecules in the interstellar medium 0830 John Cronin Organic chemistry in the early solar system 0900 David Des Marais Evolution of the early Earth and its biosphere 0930 Brian Toon Extinctions due to impacts, past and future 1015 David Peterson Response of Earth's ecosystem to global change 1045 Ken Nealson Response of microbial ecosystems to global change 1115 Anne Erlich The current great extinction 1200 Working lunch Session 4 (Tuesday afternoon, Space Science Auditorium) Co-chairs: Nancy Daunton & Charles Fuller 1330 Lewis Feldman Evolution of light- and gravity-sensing genes in plants 1400 Debra Wolgemuth Vertebrate development in space: clues and complications 1430 Muriel Ross Gravity sensor plasticity in the space environment 1500 Ben Levine Human cardiovascular adaptation to altered environments 1530 Amy Kronenberg Biological responses to exposure to the space radiation environment Co-chairs: Alan Hargens & Sam Pool 1615 Mike Duke Science and habitability goals for Mars exploration 1645 Larry Young Artificial gravity for human missions 1715 Scott Parazynski Destination Mars: An astronaut's perspective Session 5 (Wednesday morning, Space Science Auditorium) Co-chairs: Jeffrey Bada, Robert Pepin, Frank Shu, & Don DeVincenzi 0800 Jack Welch Observations of planetary system formation 0830 Pat Cassen Theory of planetary system formation 0900 Don Brownlee Primitive materials and planetary formation 0945 Norman Sleep Planetary perspective on life on early Mars and the early Earth 1015 David Deamer Origin of protocells 1045 Norman Pace Biological perspective on the Earth and the chemistry that spawned life 1115 David McKay Evidence for past life on Mars 1130 Jack Farmer Exploring Mars for evidence of past or present life 1200 Working lunch Session 6 (Wednesday afternoon, Space Science Auditorium) 1330 Frank Drake Intelligent life in the universe 1415 Final panel and discussion session; formulation of recommendations 1730 Adjourn Astrobiology Workshop ABSTRACTS Understanding the Role of Biology in the Global Environment: NASA's Mission to Planet Earth William F. Townsend Headquarters, National Aeronautics and Space Administration Understanding our Changing Planet NASA has long used the unique perspective of space as a means of expanding our understanding of how the Earth's environment functions. In particular, the linkages between land, air, water, and life-the elements of the Earth system-are a focus for NASA's Mission to Planet Earth. This approach, called Earth system science, blends together fields like meteorology, biology, oceanography, and atmospheric science. Mission to Planet Earth uses observations from satellites, aircraft, balloons, and ground researchers as the basis for analysis of the elements of the Earth system, the interactions between those elements, and possible changes over the coming years and decades. This information is helping scientists improve our understanding of how natural processes affect us and how we might be affecting them. Such studies will yield improved weather forecasts, tools for managing agriculture and forests, information for fishermen and local planners, and, eventually, an enhanced ability to predict how the climate will change in the future. NASA has designed Mission to Planet Earth to focus on five primary themes: ¥ Land Cover and Land Use Change ¥ Seasonal to Interannual Climate Prediction ¥ Natural Hazards ¥ Long-Term Climate Variability ¥ Atmospheric Ozone Mission to Planet Earth is far more than a NASA endeavor; indeed, it is NASA's contribution to the multi-agency U. S. Global Change Research Program. Because the challenge of understanding the global environment is so broad, Mission to Planet Earth is a program with extensive contributions and collaborations with many other organizations. In addition to providing a broader and more reliable quantity of data, this cooperation fosters broader confidence in the results of such research and the decisions that may be considered as a consequence of the findings. The program benefits from active partnerships with other nations (including Japan, ESA, Russia, Canada, Brazil, and many others), Federal agencies (such as NOAA, EPA, USGS, and NSF) and the broad community of researchers. NASA is also seeking to dramatically expand its partnerships with the commercial community, recognizing the inherent overlap between commercial and research needs as well as emerging commercial capabilities for provision of data and application of results. Biological Research as Part of Mission to Planet Earth Broad-scale biological studies have long been an important part of NASA's Earth science research. Using space as a platform from which to observe changes in ground cover and ocean productivity enables us to examine areas of the Earth that would be impractical (or very difficult) to study globally from the ground. Moreover, the broader perspective available from satellites and planes is critical to establishing how biological changes occur over time and aid in explaining why those changes are happening. The last few years offer some notable examples of how biological research from Mission to Planet Earth has significantly advanced our understanding of the nature of biological activities and changes. Examples include: · Biology and Weather: Satellite, aircraft, and ground observations by the U. S. and Canada have revealed that the air over the boreal forests in northern Canada is actually much drier than previously expected, due to much slower release of water by the trees. These findings are already being used to substantially improve weather models (and thus forecasts) and will also help scientists better understand this critical ecosystem. The experiment also enabled accurate measurement of the rate of growth of trees over a large area for the first time. · Biology and Land Use: Working closely with their Brazilian counterparts, U. S. researchers used satellite and ground observations to accurately estimate the rate of deforestation in the Amazon. Their studies also revealed surprising findings regarding the effect of such deforestation on biodiversity and viability of individual ecosystems within the rain forest. Using these new techniques and findings, scientists now have a powerful tool in better understanding forests across the globe. · Biology and the Carbon Cycle: As an example of the linkage between biological and physical processes, NASA is looking at changes in forest acreage and density as a key source or sink in the global distribution of carbon, which also involves the oceans and human activity. · Biology and Agriculture: A combination of satellite, aircraft, and ground data has been used to help California wine growers mitigate the effects of an infestation of phylloxera in the vineyards, thus helping to protect the multi-billion dollar wine industry in the state. NASA research has also examined the health and productivity of Chesapeake Bay marshes, growth rates of phytoplankton in the world's oceans, and the patterns of crop use and growth in America's breadbasket. · Biology and the Oceans: Working with data from satellites and surface observations, scientists will be able to precisely track changes in phytoplankton productivity in the oceans; such measurements will enable researchers to better understand the behavior of fish, to assess changes in ocean chemistry, and to track water-borne pollution and river discharge. Of course, research in Mission to Planet Earth is closely tied to that of other NASA science efforts. In turn, these activities build heavily on biological studies by other Federal agencies and especially by the nation's universities and research centers. NASA's primary role in this area is to explore the connections between biological processes and other facets of the Earth system, primarily through the collection and analysis of remote sensing data. A New Era of Earth Observations Having built a solid record of scientific research, Mission to Planet Earth has entered an unprecedented period of observational capability and international coordination. Over the coming years, NASA and its international partners will launch dozens of spacecraft to observe the Earth. In 1998, NASA will begin launches of the Earth Observing System, a multi-spacecraft system which will offer the first integrated measurements of the Earth's process. EOS will generate a 15-year database focusing on the complex issues associated with climate change. Many of these new capabilities and research will observe the biological components of the dynamic Earth system. Notable among these will be instruments on the EOS AM-1 spacecraft (including global vegetation dynamics, land cover changes, and ocean productivity), Landsat-7 (continuing the long-term global land cover change record), and SeaStar (examining ocean productivity for both research and commercial customers), as well as a varied set of international field and research efforts. In the NASA tradition, Mission to Planet Earth is a constantly evolving program, investing in new technology today in an effort to both increase program capabilities and reduce program costs. It is a program that seeks to understand the global and regional, so as to help answer questions that are regional and local. The natural link between biological research and Mission to Planet Earth is strong, and will only grow stronger in the coming years. Key Questions in Astrobiology David Morrison NASA-Ames Research Center Astrobiology is a new name for a range of interdisciplinary studies related to life in the universe. The term is defined in the 1996 NASA Strategic Plan as "The Study of the living universe. This field provides a scientific foundation for a multidisciplinary study of (1) the origin and distribution of life in the universe, (2) an understanding of the role of gravity in living systems, and (3) the study of the Earth's atmosphere and ecosystems." Concepts associated with life in the universe have a long and checkered history in science. Giordano Bruno was burned at the stake in part for his speculation concerning other inhabited worlds, a fate unlikely to be associated today with astrobiology. By the late nineteenth century the pendulum of public thinking had swung to the opposite extreme, with a widespread belief (supported by Percival Lowell and other astronomers) in the presence of intelligent life on Mars. If we can judge by the science fiction of the time, our great-grandparents also believed that humans could survive travel in space and explore other worlds like the Moon with relative impunity. By the mid-twentieth century, however, these optimistic ideas had been largely abandoned. Scientists may have been disappointed by the negative results of the Viking life-detection experiments on Mars, but they were not really surprised. And when NASA first began to consider human space flight, it was felt necessary to send chimpanzees into space first to verify survivability in the space environment, even for flights of only 15-minute duration. Today the pendulum is swinging back. We have analyzed complex organic chemistry in interstellar clouds of gas and dust and have discovered other planets circling distant stars. On Earth, life has been found at environmental extremes from the Antarctic ice to boiling hot springs, and from thermal vents in the deep ocean to aquifers buried kilometers below the land surface. We know that liquid water, the one essential ingredient for life as we know it, once flowed on the surface of Mars and probably exists today below the icy crust of Europa. Life on Earth has been traced back 3.8 billion years to the period of heavy cometary bombardment, an era that simultaneously brought life-giving water and organic compounds to the terrestrial planets while battering them with lethal quantities of impact energy. We are discovering both the fragility and the robustness of life, as we investigate the history of mass extinctions on our planet (including the extinction taking place today), the subtle alterations in climate triggered by both volcanic eruptions and human industry, and the destruction of our protective shield of ozone by non-toxic, chemically inert, and cheap industrial chemicals. As more and more humans venture into space, we celebrate their ability to live and work and achieve wonderful feats of engineering in this hostile environment, at the same time facing baffling physiological and chemical changes experienced by astronauts on long-duration missions. Space and life interact on many levels and time scales, in ways that we are only beginning to explore. Astrobiology is playing an increasing role in a wide variety of NASA programs. The primary objectives of the Earth Observing System are related to the effects of human activity on the climate, atmosphere, and ecosphere of our planet. We are building the international space station in part to learn how humans can live and work for long periods in space. The centrifuge facility being constructed for the space station will provide a unique facility for basic science related to the effects of microgravity on living things. We have located 3.6-billion-year-old evidence in a martian meteorite that suggests a warm wet climate on Mars and hints of biological activity. We are planning a series of a dozen spacecraft to Mars, including sophisticated surface rovers and sample return, much of this work motivated by the search for ancient life on that planet. We are accelerating the search for other planets, focused eventually on the discovery of a "pale blue dot," an inhabited planet in some distant solar system. Generous private donations are supporting the most comprehensive search ever undertaken for radio signals from intelligent life in the Galaxy. Life in the cosmos strikes a resonant chord with the public and the science community alike. We are here today for the world's first scientific conference on astrobiology, to help define this field and to develop new cross-disciplinary programs for the study of life in the universe. The meeting has been organized around five key questions that illustrate the breadth of this subject: ¥ How does life originate? ¥ Where and how are other habitable worlds formed? ¥ How have the Earth and its biosphere influenced each other over time? ¥ Can terrestrial life be sustained beyond our planet? ¥ How can we expand human presence to Mars? This is just one of many ways of looking at astrobiology. Many studies can be included under this umbrella. Most of these are not new. What may be new is the emphasis we are trying to bring to the interdisciplinary aspects of these studies. We seek to bring life scientists and physical scientist together, to help break down the barriers that exist between academic disciplines. If we can talk to each other, we are bound to gain insight and generate new ways of looking at old problems. That is the primary purpose of this astrobiology workshop. Detection of Planets Orbiting Sun-Like Stars Geoffrey W. Marcy and R. Paul Butler San Francisco State University and University of California, Berkeley During the past 11 months, astronomers have finally discovered planets orbiting Sun-like stars. A total of eight planets has been detected by the Doppler technique, and there are possible planets detected by astrometry around one other star. Some of the new planets exhibit properties similar to those in our Solar System. But many of them have properties that were unexpected. Several planets are more massive than Jupiter, and some orbit their host star in orbits smaller than Mercury's orbit. Equally unexpected is that three of these planets have non-circular orbits. Current theory of the formation of planetary systems is challenged to account for these new planetary properties, but several models are emerging, involving gravitational scattering of planetesimals and viscous or tidal decay of orbits. The occurrence rate of true analogs of our Solar System will soon be determined with the detection of long-period gas giants analogous to Jupiter. During the upcoming five years, several new techniques for detection of extrasolar planets will be tested and implemented. Most immediately, the Keck 10-meter telescope will advance the proven Doppler and astrometric (MAP) techniques to find smaller giant planets in surveys of about 400 stars. The Keck surveys will be sensitive to Saturn-mass and Jupiter-mass planets within 5 AU, as well as the close-in "51-Peg-like" planets. Ground-based interferometry will begin this year (at Palomar) to provide astrometric precision of 50 milliarcsec, enabling detection of Neptune-mass planets within 5 AU. A Keck version of this interferometry, using both Kecks, may achieve precision of 20 milliarcsec, enabling detection of sub-Neptune planets. Direct detection of planets from the ground is being explored by several groups using ground-based super-AO (R. Angel, C. Max). Balloon-borne telescopes may supersede these efforts cheaply and quickly to make direct detections. Habitability of Planets James F. Kasting Penn State University The habitable zone (HZ) around a star is defined as the region in which an Earth-like planet could support liquid water. The continuously habitable zone (CHZ) represents the overlap of the HZs at two different instants in time. HZs move outward with time because main sequence stars get brighter as they age. The inner edge of the HZ is set by loss of water by way of photodissociation followed by escape of hydrogen to space. A conservative (i.e., pessimistic) estimate for the solar flux at which this phenomenon occurs is 1.1 S0, where S0 is the present solar flux at Earth's orbit, 1370 W/m2 (1). The outer edge of the HZ is set by CO2 condensation, which shuts off the stabilizing feedback provided by the carbonate-silicate cycle. Within the HZ, atmospheric CO2 concentrations should increase with orbital distance as a consequence of this cycle. A conservative estimate for the solar flux at the outer edge of the HZ is 0.53 S0 (1). In terms of distance, the HZ for our own Solar System extends from at least 0.95 AU to 1.37 AU, and the 4.6-Gyr CHZ extends from at least 0.95 AU to 1.15 AU. Corresponding fluxes and distances for other types of stars are tabulated in ref. (1). The number of planets expected to lie within the HZ or CHZ around a given star depends on how planets are spaced and on how rapidly the star evolves. If planets are spaced geometrically, as they are in our own Solar System (i.e., according to Bode's Law), then roughly equal numbers of instantaneously habitable planets are expected around all types of stars. Early-type stars (type O, B, and A) will have fewer planets that remain continuously habitable because they evolve in luminosity much more rapidly than does the Sun. Late-type stars (late K and M) may have few continuously habitable planets because their HZs lie within the tidal locking radius of the star. Any potentially habitable planets are likely to become locked in synchronous rotation, allowing their atmospheres to condense out on their dark sides. Stars not too different from the Sun (early F to mid K) have about a 50% chance of harboring a long-lived habitable planet if Bode's Law spacing is obeyed (1). A glaring deficiency in the climate model on which these predictions are based is that it fails to account for the apparently warm climate of early Mars (2,3). Either greenhouse gases other than CO2 and H2O were important, or some other aspect of the Martian climate system has been poorly represented. New ideas about how to solve this problem will be discussed if circumstances permit. In any case, the existence of a warm climate on early Mars implies that HZ around a star is probably wider than has been calculated to date. The exciting development in this field is that it now appears feasible to look for Earth-sized planets around other stars using a space-based, infrared interferometer and to examine their atmospheres spectroscopically (4). The practical way to search for life on extrasolar planets is to look for the 9.6-µm band of O3 (5). O3 is a sensitive indicator of atmospheric O2, and O2 is, under most circumstances, a strong indicator of photosynthetic life. Exceptions to this rule include planets on either side of the HZ, as such planets can conceivably accumulate large amounts of O2 abiotically (7). A planet with O3 in its atmosphere and liquid water on its surface, however, is very likely to be inhabited. The fact that we are on the verge of being able to identify such planets suggests that building such an instrument should be a top NASA priority. References: 1. J. F. Kasting, D. P. Whitmire, and R. T. Reynolds, Icarus 101:108-128 (1993). 2. J. F. Kasting, Icarus 94:1-13 (1991). 3. S. W. Squyres and J. F. Kasting, Science 265:744-749 (1994). 4. J. R. P. Angel and N. J. Woolf, Scientific American, April, 60-66 (1996). 5. A. Leger, M. Pirre, and F. J. Marceau, Astron. Astrophys. 277:309-313 (1993). 6. J. F. Kasting, H. D. Holland, and J. P. Pinto, J. Geophys. Res. 90:10,497-10,510 (1985). 7. J. F. Kasting, in Proceedings of NASA Ames Bluedot Workshop, D. Des Marais, ed., June, 1996. The Search for Life on Mars Christopher P. McKay NASA-Ames Research Center Although the Viking results may indicate that Mars has no life today, the possibility exists that Mars may hold the best record of the events that led to the origin of life. There is direct geomorphological evidence that in the past Mars had large amounts of liquid water on its surface. The Mars meteorites also suggest that early Mars was wet with conditions suitable for organic material. Atmospheric models would suggest that this early period of hydrological activity was due to the presence of a thick atmosphere and the resulting warmer temperatures. From a biological perspective, the existence of liquid water by itself motivates the question of the origin of life on Mars. From studies of the Earth's earliest biosphere we know that by 3.5 Gyr. ago, life had originated on Earth and reached a fair degree of biological sophistication. Surface activity and erosion on Earth make it difficult to trace the history of life before the 3.5 Gyr. time frame. If Mars did maintain a clement environment for longer than it took for life to originate on Earth, then the question of the origin of life on Mars follows naturally. Near-Term Evolution of Earth's Climate William A. Sprigg National Research Council Earth's climate is in constant evolution. Our view of a climatic optimum has existed for but a brief period in the history of climate so far as we have been able to reconstruct it. In the near term, climate will likely continue in this climatic optimum. But "the devil may be in the details." Current research to understand monthly to decadal time-scales of climate variability attempts to unravel the intricate processes of variability and change. Anticipating the monthly, seasonal, and interannual variability of climate over the coming decades would be valuable, indeed. The challenge to do so reaches into many disciplines of science and engineering. Climatology entered a new era in the early 1970s. El Nino, an aperiodic warming of water in the central and eastern equatorial Pacific Ocean, became a target of intense scientific interest. Long rooted in Peruvian folklore and economy, El Ni–o was seen connected to a complex, interactive ocean-atmosphere system. El Ni–o became a rallying point for multidisciplinary study. The hope for seeing predictability in the system became a reality. And the several cooperating disciplines matured into a broadly international, interdisciplinary science. The study of this system continues. Now have climatologists, in partnership with doctors, epidemiologists, plant and animal ecologists, found another rallying point for interdisciplinary research? Infectious diseases are the world's leading cause of premature death. While it is widely known that living conditions and sanitation habits are most important in determining human health, finding the extent to which weather and climate are a factor is important also. Drought can lead to famine, weakened immune systems, and disease. And disease outbreaks have been linked to variations in weather and climate. Eruption of meningococcal meningitis follows the timing of dry and wet seasons in the African Sahel. Where carriers, or vectors, of disease are involved, weather and the ecosystem are principal factors, as in the 1993 outbreak of hantavirus in the Southwestern United States. Some vectors, such as the mosquito Aedes aegypti, the carrier of dengue fever, are geographically limited by temperature or other environmental condition. Observations, analyses, and forecasts of weather, climate, and plant and animal ecology can assist in the control of disease vectors, in activating stepped-up disease surveillance strategies, and in developing health-care services. A new interdisciplinary rallying point is suggested. It could begin by understanding the legacies and lessons learned from studies of El Nino. Gravity and Biology Emily R. Morey-Holton NASA-Ames Research Center Gravity has been the most constant environmental factor throughout the evolution of biological species on Earth. Organisms are rarely exposed to other gravity levels, either increased or decreased, for prolonged periods. Thus, evolution in a constant 1G field has historically prevented us from appreciating the potential biological consequences of a multi-G universe. To answer the question "Can terrestrial life be sustained and thrive beyond our planet?" we need to understand the importance of gravity on living systems, and we need to develop a multi-G, rather than a 1G, mentality. The science of gravitational biology took a giant step with the advent of the space program, which provided the first opportunity to examine living organisms in gravity environments lower than could be sustained on Earth. Previously, virtually nothing was known about the effects of extremely low gravity on living organisms, and most of the initial expectations were proven wrong. All species that have flown in space survive in microgravity, although no higher organism has ever completed a life cycle in space. It has been found, however, that many systems change, transiently or permanently, as a result of prolonged exposure to microgravity. Although our knowledge of the biological consequences of spaceflight has increased significantly, we have only snapshots of biological changes in multiple species. Humans adapt to spaceflight with fluid redistribution, altered cardiovascular function, disrupted visual/sensory/motor orientation, transient anemia, loss of muscle mass, and site-specific bone loss. Juvenile rats adapt to altered gravity with changes in many physiological systems resembling the adaptation process of humans. Suckling rat pups flown in space for two weeks during early development and then returned to Earth appear to have a permanent locomotor deficiency. Pregnant rats flown in space for 11 days and returned to Earth gave birth to pups with exaggerated head movements in response to directional changes for the first few postnatal days. Quail eggs fertilized on Earth and incubated on Mir did hatch, but the hatchlings did not develop the skills necessary for prolonged survival. Frog eggs fertilized in space showed evidence of defects that would be predicted to be lethal or at least seriously harmful in larvae, yet the tadpoles appeared normal externally when they hatched. However, the tadpole lungs were smaller until they returned to Earth and began to gulp surface air. Plants are known to respond to gravity; roots grow in the direction of the gravity vector and away from sunlight, while shoots grow against gravity and toward sunlight. The Russians have grown plants for a single generation on Mir with few notable changes, yet some shuttle experiments suggest that chromosomal abnormalities may occur. Biological changes are observed even when cells are isolated from the whole organism and grown in culture. Most scientists predicted this would not occur since cells were considered too small to be affected by gravity changes. Recent shuttle results suggest that flight may alter the characteristics of cultured cells. For example, spaceflight may reduce the activity of bone cells, decrease the ability of muscle cells to fuse upon return to Earth, and decrease the mass of muscle fibers by decreasing the rate of protein synthesis. These data suggest that spaceflight directly affects cells and tissues even when isolated from systemic factors. Modern molecular techniques and spaceflight hardware are providing tools for future space research. Recent studies with fish eggs are providing a new understanding of vertebrate embryonic development in space through real-time visualization, which ultimately may allow monitoring gene expression in vivo while the embryos are developing. Advances in cell membrane biology and identification of molecules that attach both to the internal cell skeleton and to the external matrix have focused interest on the potential importance of this system as the possible transducer of mechanical signals to biological effects at the cell nucleus. Development of science and hardware for Space Station and beyond should allow us to better understand the role of gravity in biology. In summary, data from flight experiments suggest that some organisms may only be transiently affected by decreased gravity while others may require gravitational loading for normal development and even survival. Gravitational biology research in space has revealed the inadequacies of our understanding about life on Earth, caused major revisions in widely held scientific assumptions, and opened new avenues of scientific inquiry. Ames and its university and industry collaborators pioneered the field of gravitational biology, frequently inventing the techniques and technologies that made biological research in space possible. We have created an inventory of specialized flight equipment that allows frequent, low-cost access to space for research. Ames has a capability unmatched in the world for the study of gravitational effects on living systems, providing the international science community with access to instruments and facilities that allow a broad range of living systems-from single cells through plants and animals to humans-to be studied across the full range of the gravity spectrum, from the microgravity of orbital space to hypergravity on centrifuges. The International Space Station (ISS) will provide continuous access to space with accommodations for the most sophisticated array of biological laboratory equipment ever flown. ISS will enable the first multi-generational studies of living organisms in extraterrestrial environments. The centrifuge and gravitational biology facilities being developed for ISS will provide long-duration laboratory facilities for scientists. The centrifuge can be used not only as an onboard 1G control but also to simulate gravity levels on the Moon and Mars, to determine the gravity level required for stimulating physiological systems, and to provide data on intermittent gravity as a potential countermeasure. These Space Station facilities will not only allow researchers to examine unknown areas of terrestrial physiology to benefit life on Earth but will also allow scientists to evaluate the potential of terrestrial life to thrive in environments beyond Earth as a prelude to exploration of the solar system. Gravitational biology research conducted beyond Space Station and during explorations of Mars missions would allow us to address profound questions about our future: Is terrestrial life limited to Earth? Or can we-if we choose or if we must-live, reproduce, and thrive on other worlds? Complex Molecules in the Interstellar Medium Scott Sandford NASA-Ames Research Center I will briefly review our current state of knowledge concerning the composition of complex molecules in the interstellar medium (ISM). Given the limited time available, it will not be possible to give a full discussion of the composition of everything in the ISM, and I will largely restrict myself to the discussion of materials in interstellar dense molecular clouds, since they are where stars and planetary systems form. I will also concentrate largely on solids because they contain a major fraction of the heavier elements in these clouds and because these materials are the most likely to survive incorporation into new planetary systems and participate in the subsequent formation and evolution of life. However, dense clouds are not well-defined, long-lived entities but are dynamic objects that are formed from materials in the diffuse ISM and destroyed on time scales of 106-108 years. As a result, materials in space are probably constantly being mixed between the dense ISM and the more diffuse intercloud ISM, and some discussion of the materials found in the diffuse ISM is also merited. It has been known for some time that the interstellar medium (ISM) contains gas and dust. This was initially demonstrated in 1930 by Trumpler, who noted that disparate measurements of interstellar distances could only be explained by the presence of a general "absorption" produced by the interstellar medium itself. However, it was another 30 years before the first molecules were detected by optical and radio spectroscopy. Of these techniques, spectroscopy at millimeter and radio wavelengths has proven to be one of the most powerful techniques for the detection of molecules in the gas phase. The current list of gas phase molecules uniquely identified in space using radio techniques contains close to 100 molecules ranging in size from 2 to 13 atoms. These molecules are believed to be formed by a combination of ion-molecule reactions and gas-grain surface reactions in dense molecular clouds. An additional family of carbon-carrying molecules, polycyclic aromatic hydrocarbons, has also been identified in the gas phase by their characteristic infrared emission features. While radio astronomy has amply demonstrated that the interstellar medium contains molecules and must therefore support ongoing chemistry, it is only capable of identifying materials in the gas phase. The composition of the solids in the interstellar medium remained essentially a mystery until the advent of infrared astronomy, particularly through the use of infrared spectroscopy. We now know that a large portion of the heavier elements in both the diffuse and dense ISM are in molecular form in solid grains and (in the dense clouds) in ices, rather than in the gas phase. The presence of ices in dense clouds is not surprising since the grains in these clouds typically have temperatures of less than 40 K. At these temperatures, most gas phase atoms and molecules will freeze out onto the grains. The formation of ice mantles on interstellar grains leads to several additional chemical processes. First, the condensation process itself generates new species through gas-grain reactions. Such reactions can lead to several different classes of molecules. Grains in environments where H/H2 > 1 will produce hydrides like NH3, H2O, CH4, etc. Environments where H/H2 < 1 will produce mantles that more closely reflect the abundance of molecules in the gas phase and will contain species like O2, N2, CO, and CO2. Recent infrared telescopic and laboratory studies of the CO ice band indicate that these disparate types of environments probably do exist in dense clouds. Some of the more common and better identified constituents of interstellar ices are H2O, CH3OH, CO, CO2, H2, CH4, XCN, HCO, H2CO, and OCS. In addition, there exists good indirect evidence for the abundant presence of a number of other molecules, including N2, O2, and NH3. A second way in which the presence of ices leads to additional chemistry is through their interaction with ionizing radiation in the form of high-energy charged particles and ultraviolet photons. Exposure of mixed-molecular ices to UV and cosmic ray irradiation and subsequent warming results in the destruction of some of the original ice species and the creation of new and more complicated compounds. Laboratory studies indicate that such processes can be expected to produce, among other things, ethers, alcohols, nitriles, and isonitriles, amides, ketones, compounds related to polyoxymethylene (POM), and hexamethylenetetramine (HMT). The latter is of interest since it is known to produce amino acids when placed in acidic solutions. There is already some evidence for some of these compounds in the interstellar medium. For example, the infrared absorption spectra of protostars often show an absorption band near 2165 cm-1 characteristic of the CºN stretch in a nitrile or isonitrile. This band is produced in virtually all laboratory ice analogs irradiated in the laboratory. In addition, it has been demonstrated that more than 5% of the carbon in the diffuse ISM resides in grains rich in aliphatic organics. While the exact nature of this material has yet to be determined, it has a CH2/CH3 ratio of ~2.5, implying a substantial degree of complexity. The profile of the C-H stretching feature of this material is quite similar to some of the more abundant carbonaceous components of meteorites and the refractory organic residues produced when interstellar ice analogs are photolyzed in the laboratory. While these molecules are not complex by the standards of molecules in living systems, they represent a major step up the ladder of complexity from the simple 2-, 3-, and 4-atom molecules many people normally associate with space. The presence of this molecular complexity may have played a significant role in terms of the seeding and maintenance of early life on the Earth. Carbonaceous Chondrites: A Window on Organic Chemistry in the Early Solar System J. R. Cronin Arizona State University Origin of life hypotheses postulate prior formation of some minimum set of molecular components by strictly abiotic processes. In most cases, the spontaneous synthesis of organic compounds of considerable complexity is proposed. Knowledge acquired over the last 40 years of stellar, interplanetary, and planetary processes now allows us to place organic chemical evolution in a cosmochemical context in which each of these locales plays a unique and significant role in the development of organic structural complexity. The organic matter of carbonaceous chondrites provides important insights to these processes by bridging the gap between stellar and interstellar chemistry, and the final steps that were played out on the surface of the early Earth. The study of these primitive meteorites allows us to see how stellar and interstellar processes are made manifest in/on planetary bodies and also provides a data base which might prove useful in extrapolating toward the as yet uncharted territory of a pre-RNA world. The organic carbon indigenous to carbonaceous chondrites is found both in an extended macromolecular phase and in a complex suite of soluble, low molecular weight compounds. Since the fall of the Murchison meteorite (1969), analytical work from several laboratories has contributed to a detailed, although still incomplete, inventory of these compounds [1]. Many common biomolecules have been found-e.g., eight of the 20 protein amino acids; however, meteoritic organic compounds are, in general, distinctive and show (i) complete or nearly complete structural diversity, (ii) exponentially declining concentrations within homologous series, (iii) predominance of branched chain isomers, and (iv) unusually high contents of 2H (D), 13C, and 15N relative to terrestrial organic matter. The finding of substantial stable isotope enrichments in meteoritic organic compounds suggests that they are, or are closely related to, interstellar organic matter [2]. Taking into account the fact that the organic-rich I- and M-type carbonaceous chondrites are uniquely those that have experienced aqueous processing suggests the following formation hypothesis: (i) accretion of the parent body from volatile-rich, icy planetesimals containing interstellar organic matter, (ii) warming of the parent body and generation of an aqueous phase in which the interstellar organics underwent various reactions, and (iii) mild thermal processing with eventual loss of residual volatiles, leaving the suite of nonvolatile compounds that now characterize these meteorites. Meteorites have delivered organic matter to the Earth throughout its existence, and it is possible that this organic matter represents an essential step in a continuous process of organic chemical evolution that began in the presolar molecular cloud and continued prebiotically and biologically on the early Earth. If so, it is also possible that meteoritic organic matter may hold clues to the origin of chiral selectivity, a question fundamental to understanding the origin of life. The suggestion that UV synchrotron circularly polarized light emitted by neutron stars might introduce an enantiomeric bias into the organic matter of interstellar clouds [3] has led us to seek evidence for remnant enantiomeric excesses in the organic matter of carbonaceous chondrites. Prior analyses of the optical activity of meteorite extracts and the enantiomeric ratios of meteorite chiral compounds have given negative or unconvincing results. We have renewed the search for enantiomeric excesses, focusing on molecular chiral centers that (i) might have been formed in the presolar molecular cloud or within the parent body, (ii) would have avoided racemization during aqueous alteration of the meteorite parent body, and (iii) are located in molecules that are unlikely to have been contaminated by corresponding terrestrial compounds. Analyses of selected chiral amino acids from the Murchison meteorite suggest L-enantiomer excesses of the order of 5-10%. In general, the finding of enantiomeric excesses in extraterrestrial molecules supports the hypothesis that exogenous delivery made a significant contribution to organic chemical evolution leading to the origin of life. The finding of these enantiomeric excesses specifically in a-substituted amino acids may have implications for the chemistry of a pre-RNA world insofar as it suggests the possibility that these unusual, but meteoritically abundant, amino acids were early biomonomers. [1] Cronin J. R. and Chang S. (1993) in The Chemistry of Life's Origins (J.M. Greenberg et al., eds.) Kluwer, pp. 209-258. [2] Epstein S. et al. (1987) Nature, 326, 477-479. [3] Bonner W. A. and Rubenstein E. (1987) BioSystems, 20, 99-111. Evolution of the Early Earth and Its Biosphere David J. Des Marais NASA-Ames Research Center The history of life on Earth is a rich tapestry of adaptation and innovation which was shaped, at least in part, by the changing surface environment of our planet. To the extent that all rocky planets have followed similar evolutionary paths, studies of our own biosphere can guide us in our search for extraterrestrial life. Understanding the nature, timing, and causes of long-term changes in the global environment is a key objective. Our surface environment reflects a delicate balance between processes operating within the Earth and those processes which govern the exchange of matter and electromagnetic radiation between the planet and outer space. Solar radiation and the heat flow from Earth's interior are the two most important agents which impinge upon and influence the surface environment. Over the past 4 billion years, solar luminosity has increased by almost 30 percent and heat flow has declined about threefold. These changes affected the atmosphere dramatically. Given the geologic evidence that liquid water existed 3.8 billion years ago and that the sun was less luminous at that time, the ancient atmosphere's greenhouse properties, hence its composition, must have differed substantially from today. A combination of higher levels of the greenhouse gases carbon dioxide (CO2) and methane (CH4) seems most likely. In addition, a higher interior heat flow caused reduced volcanic species to be emitted at much higher rates, rates sufficient to overwhelm any source of molecular oxygen (O2). Thus, 4 billion years ago, the atmosphere had perhaps tens of percent of CO2, a few tenths of a percent or less of O2, and also nitrogen, water vapor, and hydrogen. Plate tectonics, driven by the heat flow engine, transformed the continents from smaller, thinner, less stable sheets into stabilized, thickened, and areally more extensive platforms. Early life both adapted to and exploited the atmospheric changes and the transformations in crustal configuration. To compensate for a declining source of reduced species for energy and the synthesis of organic compounds, life invented photosynthesis. To adapt to the changing atmosphere, life invented respiration and various strategies for mitigating the toxic effects of O2 and the declining availability of CO2. Ultimately, complex cells developed which were the single-celled eukaryotic forerunners of plants and animals. These cells actually required a dependable supply of O2. Important changes over shorter, 100-million-year time scales also influenced our biosphere's evolution. Tectonic processes, which are crustal movements driven by heat flow, altered the continental configurations and global climate. It is already well-known that continental movements have influenced the isolation and development of plant and animal species. Even billions of years ago, continental breakups and collisions influenced the processing of the biologically important elements carbon (C), sulfur (S), phosphorous (P), and probably nitrogen (N). Because C and S exist in the crust in both oxidized and reduced forms, the tectonic processing of their crustal reservoirs has affected the composition and oxidation state of the atmosphere. As changes in the global environment become better documented, we should explore in more detail their implications for the evolution of the early biosphere. Some interesting correlations are already evident. The bacteria which tolerate the highest temperatures live in volcanic hydrothermal environments and are most closely related to the ancient common ancestor of life. Evidence of O2-requiring eukaryotic cells first appears about 2.1 billion years ago, at the time when atmospheric O2 levels increased substantially. The CO2-fixing enzyme in algae and green plants is more highly selective for CO2 in the face of high O2 levels, than is the "more primitive" bacterial CO2-fixing enzyme. Biosynthetic pathways which require O2 have apparently evolved relatively more recently than other, non-O2-requiring pathways. Thus, major trends in the evolution of the early, unicellular biosphere appears to have been broadly compatible with presently-known changes in the global environment. Perhaps lessons learned from Earth's early biosphere can be applied to the search for Martian life. Clearly Mars' environment has also witnessed dramatic changes. For example, as the volcanism waned and surface temperatures may have declined on Earth, thermophilic bacteria retreated deeper into Earth's crust and to a few scattered near-surface hot sites associated with volcanism. Did the same retreat occur on Mars? Did life survive the retreat? The recent studies of Martian meteorite ALH84001 established that mineralogical and chemical (including organic) information can be preserved even at the submicron scale. What can fossil information recorded at this spatial scale in terrestrial samples tell us about ALH84001 or about early life on Earth? Human activities presently contribute to global environmental change. Records of past change on Earth should help us to predict the consequences of our activities, both for Earth's climate and for our biosphere. Response of Earth's Ecosystem to Global Change David L. Peterson NASA-Ames Research Center The Earth is in the midst of rapid and unprecedented change, much of it caused by the enormous reproductive and resource acquisition success of the human population. For the first time in Earth's history, the actions of one species-humans-are altering the atmospheric, climatic, biospheric, and edaphic processes on a scale that rivals natural processes. How will ecosystems, involving those manipulated and managed by humans largely for human use, respond to these changes? Clearly ecosystems have been adjusting to change throughout Earth's history and evolving in ways to adapt and to maintain self-organizing behavior. And in this process, the metabolic activity of the biosphere has altered the environmental conditions it experiences. I am going to confine this presentation to a few thoughts on the present state of terrestrial ecosystems and the urgency that changes in it is bringing to all of us. Virtually all of human population growth occurs within the terrestrial ecosystems, as the ever-expanding human civilization brings the goods and services of the ecosystems under their control to sustain and improve the human condition. This fact is exerting tremendous pressures on Earth's life support capacity. Many of the environmental problems that challenge human society are fundamentally ecological in nature. In response to this situation, the Ecological Society of America proposed the Sustainable Biosphere Initiative (Lubchenco et al. 1991), an ambitious call-to-arms to focus ecological research in three areas: global change, biological diversity, and sustainable ecological systems. Recent analyses by Kates et al. (1990) concluded that humans now dominate roughly half of the ice-free terrestrial surface, and by Vitousek et al. (1986) estimate that nearly 40% of potential terrestrial net primary productivity is being appropriated for human use. One of the consequences of this alteration of the landscape is to force the remaining millions of species (plant and animal) into a much smaller and often fragmented fraction of land and resources. Much of this human-dominated land is devoted to food production. Virtually all arable land is now in agricultural production or being lost from production due to mismanagement. Over the past 50-60 years, progress in agricultural science and practices (the Green Revolution) has helped to keep production in pace with population growth. Until recently! A disturbing trend in some recent analyses has indicated a flattening of per capita grain production (only eight grains fulfill the majority of food production) over the past seven years (Brown et al. 1990). And there are disturbing patterns of inequities in food delivery due to droughts, distribution, corruption, and severe weather. Even as late as 1986, many agricultural analysts were predicting that the Earth could sustain population growth by another 2 billion to 2030, thanks to the kinds of technological innovations which typified the Green Revolution. One of the victims of this landscape fragmentation and simplification into just a few crop and forest types is the irreversible loss or decline of biodiversity. Loss of biodiversity has a multitude of consequences including certain loss of medicinally useful plants, loss of ecosystem interactions through the modification of the genetic composition of populations and species, and reduced biotic control on competing populations with sometimes adverse effects such as disease outbreaks. Despite the importance of land use change, no global inventory of land cover, its change, much less the uses to which land is being put, exists at the scale at which the billions of individual actions alter the land. This is one of the five priority science strategy themes of Mission to Planet Earth (NASA HQ, 1996). One of the principal ways by which the biosphere affects the climate systems is through feedback of biogenic emissions to atmospheric chemistry. The exchange of carbon dioxide between the biosphere and the atmosphere is dominated by oxygenic photosynthesis, and this annual cycle is clear in continuous measurements of CO2 partial pressure (Keeling et al. 1989) and in models of net photosynthesis and respiration (Potter and Klooster 1996). Isotopic analyses indicate that the cause for the steady increase in CO2 is attributable almost entirely to the combustion of fossil fuels. CO2 concentrations have fluctuated throughout history, across glacial and interglacial periods. However, the recent climb to over 300 ppm is unprecedented in both its magnitude and its rate of change (Barnola et al. 1987). In general, the biosphere is in a state of dynamic equilibrium between assimilation and respiration on an annual time scale. This sudden increase in CO2, sometimes called a fertilization effect, is projected to have ecosystem effects which range from competitive advantages for C3 plants over C4 leading to changes in ecosystem structure and function, increased water and nutrient use efficiencies leading to lowered C:N ratios in foliage and effects on herbivore populations, and increased sequestration of carbon in biomass. There is some carbon budget and transport evidence (Tans et al. 1990) that a missing sink of carbon exists in the northern terrestrial ecosystems that as yet remains undiscovered. And recent analyses by Keeling's group indicate the timing of seasonality in northern ecosystems is lengthening, implying a potential sudden shift in the range of these ecosystems (Keeling et al. 1996). As a global greenhouse gas, the abundance and radiation cross-section of CO2 makes it the largest contributor to greenhouse warming. However, several other gases, notably nitrous oxide and methane, though lower in abundance have much higher radiation cross-sections, and their concentrations in the atmosphere are largely controlled by ecosystem processes. Methane is generated in ecosystems from the metabolism of anaerobic organisms and emitted through several physical and biological processes. Inundated lands in the tropics, in the polar regions, and in agriculture (rice fields) account for the major sources, while biomass combustion contributes a lesser amount. Changes in nitrogen biogeochemistry, particularly the man-made production of nitrogen fertilizer which is nearly equal to natural processes, is contributing to increases in N2O. Nitrous oxide is very stable and long-lived in the troposphere, where it acts as a greenhouse gas, and when it makes it to the stratosphere, it removes ozone. Since removal processes are slow, any increases now will stay in the atmosphere for many years to come. Human actions alter the nitrogen cycle through fixation of nitrogen in fertilizer and its over-application to crops in excess of demand (resulting in leakage of N2O to the atmosphere), through fossil fuel burning, and through land conversion, exposing soils by removing plants to take up nitrogen and encouraging nitrification/denitrification to liberate N2O. The effect of these two gases, plus CO2, is toward global warming. One of the initial international policy efforts is to stabilize the global concentrations of these trace gases by the year 2000 (IPCC 1995). The combination of shifts in climate (rainfall redistribution; precipitation timing, duration, state, and chemistry; air temperature increases; weather extremes; changes in cloudiness) coupled with alterations in nitrogen availability can lead to many forms of ecosystem response. In the cold regions, temperature increases lead to more rapid nutrient turnover and decomposition, increased plant growth, and richer foliage (C:N decreases), accompanied by a loss of insulating moss on the forest floor, potentially leading to release of the large quantities of storage carbon below ground (Hom 1986). Ecosystem studies in the northeast U.S., receiving acidic precipitation from upwind power plants in the Midwest, have found ecosystems respond to this excess nitrogen by increases in foliar nitrogen content, increased productivity, and eventually lowered lignin content, making the plants potentially more susceptible to insect attack (Aber et al. 1989). In California, for example, positioned at the convergence of two different climate systems (monsoonal and arctic), changing patterns of precipitation and higher temperatures would result in the migration of communities to higher elevations and northern latitudes, replacement of Douglas fir-a high value commercial species-by Ponderosa pine, and greater susceptibility to insect and fire effects (California Energy Commission, 1989). As these ecosystems move up, they also force out relict alpine communities. Species migration is very slow, hampered by biological enertia, as seen in pack rat middens in the Grand Canyon, indicating that the present community took almost 2000 years to establish after the end of the last ice age. The changes I have mentioned thus far represent only a few of the serious challenges we must confront. A short list would also include: a) increasing ultraviolet radiation as ozone thins in the stratosphere; b) species migrations and invasions as alien plants and animals catch a ride on modern modes of transportation; c) salinization of croplands due to irrigation; d) disturbance of watersheds leading to freshwater quantity and quality changes; e) overgrazing of grasslands producing albedo changes with a positive feedback effect on regional climate and desertification; f) redistribution of freshwater for irrigation having downstream and coastal zone problems; g) soils erosion and degradation reducing crop yield; h) increases in fire frequency leading to savannahization; i) losses of key ecosystem components, especially birds and pollinators due to habitat destruction; and j) the intense application of chemical means to control pests and plant pathogens and to remove vegetation. The deepening concern in all these changes has led to a socio-political shift from dealing with scarcity towards concepts of sustainable development. In this concept, the goal is to leave the Earth's ecosystems in a state (goods and services) that preserves for future generations what we have today. In the early seventies, the Club of Rome commissioned a modeling study that revealed the limits to growth and drew widespread controversy, criticism, and concern. The concern was amplified in the second book, Beyond the Limits, (Meadows et al. 1990) with model projections giving only a fairly short time horizon for humanity's response, about 30-50 years before a virtual collapse in resources and in the human population. Their recommendations centered on aiming for zero growth and sustainability. And Mathews (1983) added to this concern by discussing how resource competition will likely be the new basis for national security and the motive behind future wars. Many analyses trace the present pattern of overuse of resources to the increasing disparities between rich and poor. The proportion of people living under crushing poverty is increasing in all countries and seems to be a paradox in a world with so much productive capacity (Brown 1990). The poor, most of whom are agricultural workers, are being forced into more and more marginal lands. One growing area of alarm is infectious disease. Some components of the ecosystems capable of keeping up with the rate of change are the microbial communities. As humans have tried to usurp the power of the genetic machinery to combat disease through the production of antibiotics and vaccines, they have inadvertently contributed to the adaptation of the microbial community to more virulent strains of disease. The disruption of the ecosystems and of the habitats of rodents and insects, coupled with the failures to control them by chemical agents, are also enabling vectors that transmit disease to thrive. The poorest members of the human society are at greatest risk as epidemics become more common worldwide (Garrett 1994). The problem is not confined to only the poorest countries or the poor in the First World, as the disease epidemic in the U.S. and the E. coli outbreaks in several large metropolitan regions testify recently. While the attempts for treatment and vaccination continue to have trouble, it is clear that understanding the ecological basis for the transmission of infectious disease can help in the prevention of human exposure to diseases. In conclusion, what can and should be done? There are innumerable suggestions and recommendations being made, many of them laced with socio-political agendas. First, efforts such as the Sustainable Biosphere Initiative to bring ecological knowledge to bear on these ecologically-based problems is vital. Second, education is probably a key factor, especially if accompanied by actions the individual can make everyday. Third, conversion from fossil fuel dependence to, perhaps, solar energy is worth exploring. Fourth, a worldwide improvement in the condition and status of women is badly needed. There is a clear indication that poverty is being feminized. Fifth, conversion to sustainable agro-ecosystems is crucial to preserve and rebuild soils and nutrient supplies and to maintain biological diversity. Overall, a sustained global commitment and cooperation is necessary to accomplish these goals. Literature Cited: Aber, J. D., et al., BioScience 39(6):378-386 (1989) Barnola, J. M., Nature 329:408-414 (1987) California Energy Commission, The Impacts of Global Warming on California, Interim Report (1989) Garrett, L., The Coming Plague, Farrar, Strauss and Giroux (1994) Hom, J., Ph.D. Dissertation, Chapter 1, Univ. of Alaska-Fairbanks (1986) Intergovernmental Panel on Climate Change (IPCC), Second Synthesis, UNFCCC, (1995) Kates, R. W., et al., in The Earth as Transformed by Human Action, Cambridge Univ. Press, pp. 1-17 (1990) Keeling, C. D., et al., in Aspects of Climate Variability in the Pacific and the Western Americas, AGU Geophysical Monographs (1989) Keeling, C. D., et al., Nature 375:666-670 (1996) Lubchenco, J., et al., Ecology 72(2):371-412 (1991) Mathews, J. T., Foreign Affairs (1983) Meadows, D., et al., Beyond the Limits, Universe Books (1990) NASA Headquarters, MTPE, Strategic Enterprise Plan 1996-2002 (1996) Potter, C. A., and S. Klooster, in press, Tellus (1996) Tans, P., et al., Science 247:1431-1438 (1990) Vitousek, P. M., Ecology 75(7):1861-1876 (1994) Vitousek, P. M., et al., BioScience 36:368-373 (1986) Brown, L., et al., State of the World 1990, Norton and Co. (1990) Response of Microbial Ecosystems to Global Change Ken Nealson Center for Great Lakes Studies In order to consider the question of global change as it relates to microbial ecosystems, one must be in the right spatial scale and recognize some fundamental properties of microbial ecosystems and microbes themselves. That is, we must ask what is the environment that a microbe actually sees; when this is done, we can conclude that large size and complexity are decided disadvantages when global change occurs. As size decreases, the ability to escape from bulk phase conditions increases, and except for true ecological calamities, the changes that dramatically affect the larger eukaryotes probably have little effect on the microbes on a global scale. When one also considers the metabolic advantages of a high surface to volume ratio enjoyed by the bacteria, it is easy to understand why the microbes have remained small-if it was an advantage to be large, they would be large (given 3.8 billion years)! The simplicity in structure of microbes has allowed a large radiation of metabolic types to fill the many niches in which energy can be gleaned on our planet. Probably from times as old as 3.8 billion years ago the planet was inhabited with abundant bacteria forming dynamic ecosystems known as stromatolites, and doing quite well in what by modern standards could only be considered to be a very harsh environment. An examination of today's bacteria emphasizes this point; bacteria are found in environments ranging from -5û C to over 120û C, from pH values of 0-10, salinities ranging from distilled water to hypersaline brines. In these structurally simple creatures there is not much to break!...no complex chromosomes, no meiosis or mitosis, no nucleus or nuclear membrane, and-with only a few exceptions like bacterial flagella and some photosynthetic membranes-no complex structures that can be destroyed by extreme conditions. These creatures are the "Timex watches" of the biological world and stand in stark contrast to the eukaryotic "Grandfather clocks" with their complex structures and organelles that reflect their evolution and behavior and that can be damaged or rendered non-functional by rather minor environmental changes. If one considers survival rather than growth, the point is even more strongly made. It is rather routine (and was done for years in my microbiology course at Scripps Oceanographic Institution) to take a seawater sample of 4-10û C in aerobic waters and culture from it thermophilic bacteria (those able to grow above 60û C) which will not grow at temperatures below 50û C, or sulfate-reducing bacteria that will not grow in the presence of oxygen. While these bacteria are not abundant, they are nevertheless present in a viable state, ready to prosper when the right niche is encountered. Adding to all of this is the rather remarkable metabolic versatility of microbes, the ability to glean energy for survival and growth from almost any redox couple that is present on Earth, as long as the reductant or oxidant is not itself toxic. The versatility is expressed both in terms of electron donors (fuels) and electron acceptors (oxidants). Eukaryotes, for the most part, have evolved to use rather simple carbohydrates (often glucose) as their fuel and oxygen as their oxidant. The absence of either of these would have drastic consequences on eukaryotic ecosystems. In contrast, prokaryotes have evolved to utilize many different electron donors and electron acceptors; microbial ecosystems would be altered, but not catastrophically, by the removal of easily metabolized carbohydrates and/or oxygen. So what types of global change can affect these apparently ubiquitous and adaptable prokaryotes? To answer this question we may like to look at the geological evolution of the planet and ask whether any of the changes we have observed would be dramatic or even calamitous. Here we can examine a few key variables that may have represented major factors in microbial ecosystem structure and abundance. I begin with three types of change that must have been truly dramatic if and when they occurred. Temperature: If global impacts heated the world oceans to very high temperatures (greater than 50-100û C) after the evolution of life, it could have had a major impact on those species that survived. Such events were apparently common on the early Earth, and it is no surprise that most of the oldest lineages of extant bacteria are thermophiles or hypothermophiles. One can argue that these are the survivors of major events-irrespective of whether they were the organisms that evolved first. Global change of a few degrees is of very little consequence to the microbes, who characteristically can survive and grow over a range of 10-20 degrees with little effect. The effects might well be on the removal of predators that would be much more susceptible to temperature changes. Oxygen: One of the major global changes that occurred in terms of bacteria was the introduction of oxygen. This compound is toxic by virtue of the generation of toxic byproducts of oxygen (singlet oxygen, superoxide, hydroxyl radicals, and peroxides) and must have posed a major problem for organisms encountering it. Even today, we have many bacteria whose growth is inhibited by oxygen and others that are killed by it. However, the introduction of oxygen must have led to the production of a wide array of electron acceptors that could be used for metabolism (Fe3+, nitrate, oxidized sulfur compounds, etc.), and the radiation of bacteria into these energetic "niches," and finally to the evolution of the ability to use oxygen as the electron acceptor of choice-as evidenced by the universal use of oxygen for respiration by eukaryotes. Eukaryotes: The origin and radiation of the eukaryotes must certainly have been among the most influential of the global "changes" endured by the prokaryotes. Imagine the prokaryotic world-metabolic competition, with simple organisms virtually incapable of predator-prey, and in reasonable metabolic balance. In this scenario, bacterial stromatolites probably dominated the face of the planet. When the eukaryotes began to emerge and blossom, three major effects would be expected: 1) competition for space and metabolites, 2) grazing, and 3) the creation of new niches. Certainly the inhabiting of the environment by the hungry eukaryotes would have challenged some of the prokaryotes, but because of size and metabolic differences, these effects could be expected to be minor. Of more impact, however, was the appearance of heterotrophic grazers. This probably resulted in the widespread disappearance of stromatolitic microbial mat environments due to the grazing of the eukaryotes. Today such stromatolitic environments are found primarily in hypersaline or hyperthermal environments, places with conditions too harsh for the eukaryotic grazers. However, in terms of today's microbial ecosystems, the tables have turned, and there are few eukaryotes who are not dependent on symbiosis, tolerant of commensalism, or impacted by parasitism of prokaryotes. As the eukaryotes radiate, they create a wide array of new niches. Evolution of Light- and Gravity-Sensing Genes in Plants Lewis Feldman University of California, Berkeley Organisms perceive and respond to a wide variety of physical stimuli, including gravity and light. Following perception, translation of physical stimuli into a biological response occurs via a number of interlinked steps collectively designated "signal transduction." When and from where did the gravity signal transduction pathway evolve? The hypothesis advanced here is that the signal transduction pathway for gravitaxis (gravitropism) evolved by building on and/or incorporating signal transduction steps which evolved earlier in connection with prokaryotes responding to light. In the earliest prokaryotes, light-absorbing pigments may have been used to absorb excess light energy (UV?) that was harmful to cells growing in the sea in surface communities. Absorption of the light could either have served as a means of directly screening the cell's light-sensitive machinery from UV, or the light absorbed may have stimulated a light-avoidance response (negative phototropism), with the organisms actively moving away from the light. A light avoidance response would necessitate the development of a transduction pathway to convert the physical stimulus of light into a response (avoidance). Of the extant prokaryotes, about half are capable of directed movement, including mechanisms involving flagella, which may be scattered over the entire cell or concentrated at one or both ends of the cell. A second type of motility mechanism characterizes a group of helical-shaped bacteria called the spirochetes in which several filaments spiral around the cell under the outer sheath of the wall structure. A third type of mechanism for motility found in prokaryotes involves the secretion of slimy substances allowing a gliding movement that may result from the presence of flagella motors. During early prokaryotic evolution it is believed that organisms existed as chemoautotrophs nurtured by the free organic compounds generated in the primordial seas via abiotic synthesis. With the depletion of these compounds it is hypothesized that earlier-evolved, UV-absorbing, energized pigments were coupled with electron transport systems to drive ATP synthesis. In modern archaebacteria (the extreme halophiles), a pigment that captures light energy (bacteriorhodopsin) is built into the plasma membrane, leading to the relatively direct establishment of a proton gradient. By about 3.5 billion years ago, photosynthesis was likely well-established on earth, and it is now believed that these ancient organisms had metabolic capabilities similar to modern cyanobacteria. What is the evidence to support this hypothesis that the gravity signal transduction pathway evolved from and/or built upon elements of the phototropic pathway? From unicellular organisms the data are few, but there are a few supportive facts. 1. In many motile unicellular plants, gravitaxis is impaired by UV light, suggesting that an efficient gravitactic system evolved only after an oxygen/ozone atmosphere was produced, via photosynthesis, to screen out the UV light. 2. Flagellar responses and phototropic stimuli are coupled in algae through calcium ions and calcium-mediated membrane phenomena. Calcium and calcium-modulated events are central, as well, to the gravitropic response in plants, suggesting common steps in the two pathways. Stronger evidence that the gravity signal transduction pathway developed and evolved from the light processing pathways comes from studies of higher plants, specifically corn, in which much effort has focused on calcium and its mechanism of action. Calcium is hypothesized to have a central role in gravity signal transduction. The processing of the gravity stimulus is believed to involve a redistribution of calcium and a subsequent interaction of calcium with target proteins, such as the calcium-binding protein calmodulin, and calcium-activated protein kinases. For studies of calcium action, roots serve as a model system. In most plants roots typically show a "complete" gravity (gravitropic) response and grow vertically downward. This directional response occurs in either light or darkness. However, in many plants if roots are maintained in darkness their direction of growth is instead nearly parallel to the soil surface. Subsequently, when these roots are illuminated (light can penetrate to considerable depths in soil), the direction of growth alters and the roots now grow downward. Work has shown that in roots exhibiting this light-dependent gravitropism that an unusual calcium-regulated protein kinase is involved in the gravity response. Application of drugs which specifically target this kinase can inhibit this light-dependent gravity response. However, in roots of species showing the complete gravity response in darkness (light-independent gravitropism), the previously used drugs are without effect and the treated roots exhibit vertical, downward growth. Using results from these experiments, we propose that the original gravitropic signal transduction pathway was not only light-requiring but also incorporated steps of the earlier evolved light transduction pathway. We consider that the light-independent gravity signal transduction pathway evolved later. Moreover, while this "new" pathway still makes use of calcium in transducing the gravity stimulus, additional calcium target proteins evolved, separate and independent from those used in light signal transduction. Vertebrate Development in Space: Clues and Complications Debra J. Wolgemuth Columbia University College of Physicians and Surgeons The development of an animal includes embryogenesis, growth, and reproduction. The successful completion of this program requires the capability to survive in and adapt to altered environments. On Earth, the gravitational field represents a constant force in the environment which may have influenced the size, shape, and behavior of animals throughout evolution. The extent to which altered gravitational environments can affect the normal developmental program in higher organisms is poorly understood. Space flight, especially for extended periods of time, represents an altered environment in which animals will be required to undergo these various aspects of the developmental program, not only in the virtual absence of gravity but also in the presence of other unique environmental features such as radiation and alterations in atmospheric pressure. Underlying the question of how this altered environment will affect development is the hypothesis that normal development depends on the embryo's ability to maintain a programmed temporal and spatial coordination of morphogenetic events, even under abnormal conditions, and that normal adult functions require the same plasiticity of responses. Clearly animals are able to survive for limited periods of time in the microgravity environment, and limited data from space flight experiments demonstrate that at least certain aspects of reproduction can occur as well. However, it may well be that some alterations in morphology or function do occur in a microgravity environment, and although subsequent development may appear overtly normal, subtle effects with potential long-term ramifications may result. All of our analyses to date, without exception, are hampered by the paucity of experimental subjects, the relatively short duration of exposure to the space flight environment, and the lack of biomarkers and endpoints sufficiently sensitive to detect these effects. Thus, one of the greatest needs in evaluation of the effects of the space flight environment on the biology of terrestrial organisms is the development of appropriate experimental paradigms and sufficiently sensitive experimental endpoints to test and predict such effects. In this discussion, we will summarize some of the observations of vertebrate development in microgravity and consider various stages of development in vertebrates that might be expected to be differentially sensitive to altered gravity conditions. While we emphasize mammalian development, we discuss the suitability of various model systems for examining such effects in other species. Furthermore, we consider recent developments in our understanding of the molecular genetic program regulating embryogenesis and reproduction that could serve as markers for assessing perturbations of development. A question of particular importance focuses on the effects of the space environment on the stability of the eukaryotic genome, with the potential short- and long-term effects that such perturbation might have. Understanding the influence of space flight environment on the developmental processes is important not only in terms of evaluating possibilities of human survival in space but also in understanding how animals could adapt to and be influenced by a constant microgravity environment. Selected References Alberts, J. R., Serova, L.V., Keefe, J. R., and Apenasenko, Z. 1986. Early postnatal development of rats exposed in utero to microgravity. In: Final reports of U. S. monkey and rat experiments flown on the Soviet satellite Cosmos 1514. Eds: Mains, R.C., Gomersall, E.W. NASA TM-88223. Dubrova, Y. E., Jeffreys, A. J., and Malashenko, A. M. 1993. Mouse minisatellite mutations induced by ionizing radiation. Nature Genetics 5:92-94. Mains, R. C., and Gomersall, E. W. 1986. Final reports of U. S. monkey and rat experiments flown on the Soviet satellite Cosmos 1514. NASA TM-88223. Pedersen, R. A. 1986. Potency, lineage, and allocation in preimplantation mouse embryos. In: J. R. Rossant and R. A. Pedersen (Eds.), Experimental Approaches to Mammalian Embryonic Developmentt. New York: Cambridge University Press, pp. 3-33. Wolgemuth, D. J., and Murashov, A. K. 1995. Models and Molecular Approaches to Assessing the Effects of the Microgravity Environment on Vertebrate Development. ASGSB Bulletin 8:63-71. Gravity Sensor Plasticity in the Space Environment Muriel D. Ross NASA-Ames Research Center The ability of the brain to learn from experience and to adapt to new environments is recognized to be profound. This ability, called "neural plasticity," depends directly on properties of neurons (nerve cells) that permit them to change in dimension, sprout new parts called spines, change the shape and/or size of existing parts, and to generate, alter, or delete synapses. (Synapses are communication sites between neurons.) These neuronal properties are most evident during development, when evolution guides the laying down of a general plan of the nervous system. However, once a nervous system is established, experience interacts with cellular and genetic mechanisms and the internal milieu to produce unique neuronal substrates that define each individual. The capacity for experience-related neuronal growth in the brain, as measured by the potential for synaptogenesis, is speculated to be in the trillions of synapses, but the range of increment possible for any one part of the nervous system is unknown. The question has been whether more primitive endorgans such as gravity sensors of the inner ear have a capacity for adaptive change, since this is a form of learning from experience. Gravity sensors are very simple parts of the nervous system. There are two of them on each side of the head: a utricular macula and a saccular macula. Each is about the size of a period in newsprint. Each consists of a test mass of crystallites (otoconia), a layer of two kinds of sensory cells (type I and type II), and another layer that amounts to a neural plexus (tangle) of nerve fibers, their complex branches and expanded sensory endings (calyces), and small-diameter nerve fibers of central origin. Maculae sense linear accelerations, translational and gravitational, while other endorgans of the inner ear sense angular accelerations. Information sent from the sensors to the central nervous system is used for the reflex control of eye movements, posture, and balance. It was originally proposed that gravity sensors would prove to be the Achilles heel of human spaceflight. It was suggested that humans would suffer continuous intolerable nausea, weakness, and disorientation in a weightless environment, making human exploration of space impossible. Spaceflight, however, demonstrated the ability of humans to survive and thrive in weightlessness after an initial adaptive period of illness or malaise called "Space Adaptation Syndrome" (SAS). The syndrome was endured for a period that varied between 1 and 14 days, depending on the individual. Once returned to Earth, another adaptive period resulted. These adaptations were attributed to central rather than peripheral mechanisms since the prevailing theory is that central conflict between mismatched visual, kinesthetic, and gravity sensor inputs is the cause of "Space Adaptation Syndrome." In contrast, the hypothesis tested in the work to be reported here was that the periphery also adapts to a novel gravitational environment (exhibits neural plasticity) by changing the number of synapses in the sensory cells. Ability to function effortlessly in weightlessness signals that adaptation has been completed. The initial test of this hypothesis was an experiment conducted in microgravity on rodents as the model for mammalian gravity sensors. This took place on the first Space Life Sciences Mission, SLS-1, which lasted nine days. A second experiment was conducted on SLS-2, a 14-day flight. The basic premise was that synapses in the receptor cells would change in number in an altered environment. Serial thin sections were cut from the medial part of the utricular macula of flight animals and controls, and studied at the level of the transmission electron microscope. Synaptic bodies (ribbon synapses) were photographed and counted in 50 serial sections per gravity sensor for the SLS-1 experiment, and in 100 serial sections for each macula for SLS-2. Data from three rats per experimental group were pooled for SLS-1, and from two rats per group for SLS-2. Ribbon synapses were further characterized by the appearance of the central dense body as spherular or rod-like. Synapses in pairs or in groups were noted. Every seventh serial section was photographed in its entirety so that the sensory cells could be numbered and the section used as a map. The sections crossed the entire medial aspect of the macula. Thus, more than 100 sensory cells were profiled in each section. For each experiment, ~ 6000 synapses in over 1000 hair cells were counted and analyzed statistically, using analysis of variance (Super ANOVAª software) with the level of significance set at p < 0.05. Graphs were prepared using DeltaGraph¨ Pro 3 and CANVASª. For SLS-1, results were obtained from tissues collected ~ 4.5 to 6 hours post-landing (recovery day, or R + 0) and on the ninth post-landing day (R + 9). The data showed that the means of synapses per hair cell had increased by ~ 41% in type I cells and by ~ 55% in type II cells (p < 0.0001 for both) on R + 0. There was a shift toward the spherular form of ribbon synapse in both types of sensory cells of flight animals (p < 0.0001), a near doubling of pairs in the flight rats (p < 0.0001), and an increase by a factor of 12 of groups of synapses (p < 0.0001). However, there was no significant difference in synaptic mean counts between flight and control animals on R + 9. High counts were retained in flight animals, while counts were elevated to comparable levels in control animals. These findings at R + 9 were attributed to stress induced by experimental treatments post-flight that included withdrawal of blood from the tail vein and numerous injections of radioactive materials required by other experimenters. The SLS-1 study demonstrated the labile nature of cell-to-cell communication in gravity sensors but left unanswered the question whether counts would have been higher if the tissues had been collected in space. That is, had the gravity sensors already begun to adapt to earth by 4.5 hrs post-landing? The SLS-2 experiment answered this question. Macular tissues were collected and fixed in-flight on day 13, when it was anticipated that the animals would be adapted to microgravity. The results showed that, in type II cells of in-flight experimental animals, means of counts had increased by 100% compared to ground controls (11.4 versus 5.4, p< 0.0001). In contrast, the mean number of type I cell synapses was not much different from that obtained in the SLS-1 experiment. There was a 44% increase in the mean in-flight (p < 0.034) and little change in mean synaptic number in type I cells post-flight. In the case of type II cells, by R + 0 the mean number of synapses in type II cells had declined to 9.3, a figure comparable to that obtained at R + 0 on SLS-1 (also 9.3). The mean number of synapses in type II cells was further reduced post-flight (to 8.7 at day 14 post-flight). Neither post-flight mean in flight animals differed significantly from their controls (controls, 7.5 at R + 0 and 7.2 at R + 14). Additional results showed that the trends toward spherular synapses, pairs, and groups were replicated in the maculae collected in-flight. These results are the first to demonstrate the plasticity of the sensory connections in the gravity sensors in a model mammalian system. It is evident that exposure to the altered gravity of space has a profound effect on communication sites between the sensory cells and the nerve fibers ending on them. The increase in spherular synapses as a result of spaceflight is interesting because it has been proposed that these synapses are the more primitive. That is, spherular synapses develop into the rod-like form. The larger numbers of pairs and groups could function to increase the efficacy of the synapses at those specific sites. There is additional evidence, not recounted in this abstract, that handling the rats and/or human interaction with them also affects synaptic counts in maculae. The reason for this surprising finding, whether hormonal or simply resulting from increased activity, is unknown. However, it is clear from these studies that controls for space experiments (and all others as well) must be carefully thought out. Finally, the differences in increments in synaptic means noted between type I and type II cells in both experiments can be accounted for on the basis of the neural circuitry. The existence of microcircuits in maculae was discovered in ground-based studies and verified by computer-based 3-D reconstruction methods using transmission electron microscopy. Type I cells synapse only with expanded nerve fiber endings called calyces, but type II cells are inserted into microcircuits that include feed-forward and feedback connections. An hypothesis resulting from this research is that this siting of type II cells accounts for the larger increments in synapses shown by these cells in microgravity. It is postulated that synaptogenesis is an attempt by the system to return the output to a more normal level in the absence of gravity. This hypothesis can be tested in later space flights. It is also clear that the unusual level of synaptic plasticity shown by gravity sensors in space makes flight experiments a practical way to study the molecular basis of neural plasticity. Insights obtained should be applicable to neural plasticity wherever it occurs in the nervous system. Human Cardiovascular Adaptation to Altered Environments Benjamin Levine, M.D. University of Texas Southwestern Medical Center The human cardiovascular system has an extraordinary adaptive range. Even the most unfit person can, within minutes, increase the amount of blood pumped by the heart and distributed to skeletal muscle by an order of magnitude. Endurance athletes, who place regular and sustained demands on the cardiovascular system, may have an additional two-fold capacity above more sedentary individuals. In contrast, bed rest, space flight, or other causes of disuse (i.e., illness or injury) result in a reduction in maximal capacity by 25 - 30%. Thus the cardiovascular system can be viewed in terms of its acute ability to respond to a sudden increase in metabolic demand, and a more chronic adaptation to match the range of the system to the regular requirements that are placed upon it. In order to understand how the human cardiovascular system adapts to stress, it is helpful to think about the system as made up of "the plumbing," comprised of the "pump" (the heart) and "pipes" (the blood vessels), and the "control system" (the autonomic nervous system, hormones regulating salt and water balance, and local endothelial derived mediators of microvascular flow). For acute demands, such as during exercise or rapid changes in posture (i.e., gravitational vectors), higher order centers in the brain initiate an increase in the heart rate, termed "central command." Sensors located in skeletal muscle respond to changes in both metabolic and mechanical state and send back signals to the brain reflective of the intensity of effort. The heart itself is a sensory organ and detects the adequacy of hydration and cardiac filling through "mechanoreceptors". Pressure sensors or "baroreceptors" in the walls of the large blood vessels detect the pressure within the vasculature. These signals are integrated in special centers in the brain which respond by regulating both the strength and frequency of the heart's contraction and the resistance of the blood vessels, primarily by neural mechanisms. Because humans are predominantly upright creatures, with the majority of the blood volume located below heart level, gravity has a profound effect on the mechanical distribution of fluid within the cardiovascular system. When hydrostatic gradients are removed, such as changing from the upright to the supine position or exposure to microgravity, blood is translocated from the lower part of the body towards the chest virtually doubling the amount of blood within the heart. The heart responds to this volume load by increasing the amount of blood it pumps (Starling's Law of the Heart), and by initiating both a redistribution and elimination of plasma. Although Gauer hypothesized in the 1950s that the regulated level for humans was the upright posture, data from space flight and bed rest studies suggest that in actuality, the system settles on a pressure and volume approximately 2/3 of the way between upright and supine. It is important to emphasize that this adaptation is rapid and appropriate, being complete within 48 hours of microgravity conditions. In contrast to bone and skeletal muscle, for which load and thereby structural integrity is dependent on gravity and physical activity, the heart is constantly "exercising" with each heart beat. There does not appear to be any further change in cardiovascular loading with longer exposures. Because cardiac work is reduced somewhat in space, the heart appears to atrophy slightly with a 15-20% reduction in cardiac muscle mass. The blood vessels also appear to get slightly smaller and stiffer. However this adaptation is a physiological, rather than a pathological response and does not appear to be associated with impaired function. In fact, the maximal capacity of the cardiovascular system, as reflected by maximal exercise performance, is well maintained during short term space flight. For longer term space missions, such as the 84 day Skylab mission, some astronauts actually improved their fitness, probably because the rigorous exercise requirements of the mission exceeded their pre-flight training practices. There is no evidence to date of any physiologically meaningful alteration in the cardiovascular control systems in space. Although the adaptation to microgravity is physiologically appropriate, immediately upon return to a 1G environment, it becomes dysfunctional. Nearly 2/3 of astronauts suffer from orthostatic intolerance, a condition whereby they are unable to stand continuously for 10 minutes, after return to earth. In addition, maximal exercise capacity is reduced by approximately 25%. These problems appear to result because the heart empties precipitously when head-to-foot gravitational forces are restored and blood pools below the heart. Because the heart is slightly smaller and stiffer than it was before adaptation to space, this emptying is relatively greater than it was before flight. Reflex mechanisms attempt to compensate by increasing heart rate and vascular resistance. However in some individuals who are prone to orthostatic intolerance, their blood vessels appear unable to constrict to the degree necessary to compensate for the empty heart. It is not yet clear whether this trait is a result of a unique adaptation to space flight, or an inherent characteristic of these individuals. Regardless of the mechanism, orthostatic tolerance is restored rapidly - virtually all astronauts are able to remain upright within 24 hours after return. Maximal exercise capacity is restored to pre-flight levels within a week, at least after short term space flight. The data for long term space flight are more scarce, though the Skylab experience would suggest that detraining is not dependent on the duration of the flight. In summary, despite the dire predictions of early physiologists, the cardiovascular system adapts well to space. Blood pressure and organ perfusion are well maintained and maximal exercise capacity is preserved. There is no evidence for a progressive deterioration of cardiovascular performance with longer term space flight and on theoretical grounds, the heart should tolerate even longer missions required to support a mission to Mars without trouble. Although transiently upon return to earth there may be a problem with orthostatic intolerance, this does not seem to be chronic and astronauts re-adapt to earth as quickly as they adapted to space. The cardiovascular system is not likely to be a significant impediment to long term space flight. Biological Responses to Exposure to the Space Radiation Environment Amy Kronenberg Lawrence Berkeley National Laboratory The environment in space is different from that on Earth in many ways. One of the environmental factors that is unique is the space radiation environment. This environment may play an ongoing role in the evolution of life in the universe in addition to being an important consideration for human exploration in space. The radiation environments encountered in space are different from the natural background radiation on Earth and are complex in nature. Both bacteria and eukaryotes have evolved a series of mechanisms to respond to similar damages on Earth. The space radiation environment is largely comprised of positively charged particles such as protons and helium ions and negatively charged electrons. The Earth's magnetic field serves as a shield against charged particle radiations. The primary sources of radiation in space are traditionally classified into trapped particle radiation, galactic cosmic radiation, and solar particle radiation. The trapped particle radiations are located in a series of radiation "belts" that surround the Earth and are comprised primarily of protons and electrons. The galactic cosmic radiation (GCR) is the major radiation in interplanetary space and is comprised primarily of charged particle radiation. Of the different charged particles that make up the GCR, 87% are protons, 12% are helium ions, and 1% are heavier ions that are of high energies. Iron is the most important of these heavier ions due to its abundance and the amount of energy each iron ion can deposit as it interacts with matter. The sun contributes in a dynamic way to the space radiation environment. Solar particle events (SPE) are large intermittent emissions of protons, helium, and sometimes heavier ions from the sun. The frequency of SPE varies within the solar cycle. In addition to providing protection against GCR, the Earth's magnetic field also shields terrestrial life against intermittent SPE. The different charged particle radiations in space have biological consequences that are dependent upon the spatial distribution of the energy they impart to the cell or tissue. As charged particles pass through matter, they slow down. The energy deposited is a function of the energy of a given particle, and the greatest amount of energy deposited by a particle comes at the very end of its track. Protons are generally sparsely ionizing radiations until they reach the very end of their tracks, while the heavier ions are densely ionizing at the beginning of their path and become more so as they slow down. Cells, tissues, and whole organisms have evolved a myriad of responses to the types of damage that can be caused by charged particle radiations. The early responses to charged particle exposures include recruitment of DNA repair machinery, changes in gene expression, the initiation of programmed cellular responses such as apoptosis, and alterations in the tissue microenvironment. Included among the initial types of damage that result in DNA following exposure to charged particle radiations are base damage, single-strand breaks, and double-strand breaks. Bacteria and eukaryotes have highly complex systems for the repair of these alterations, which can also result from naturally occurring errors in replication, changes that result from ongoing oxidative metabolism, and exposure to chemicals of various kinds. Following exposure to sparsely ionizing radiations, base damage and strand breaks are widely scattered throughout the cell nucleus. For densely ionizing radiations such as iron ions, the alterations are often clustered. Most of the initial damage to DNA is efficiently removed prior to the next cell division cycle by processes including nucleotide excision repair and double-strand break repair. It has been suggested that the clustered damages associated with densely ionizing radiation exposures may recruit multiple repair pathways to the site of damage. The resulting "traffic jam" may lead to an alteration of the efficiency of repair. The programmed cell death pathway may be important in maintaining tissue integrity in the space radiation environment by removing damaged cells before they can divide. Virtually all cells in multicellular organisms have evolved a system of altruistic suicide that is often referred to as programmed cell death or apoptosis. This process is critical to normal development and tissue homeostasis. Apoptosis is regulated by a series of proteins that function as either effectors or suppressors of the process. Through a complex series of heterodimeric and homodimeric interactions, these proteins act as a "rheostat" to promote or suppress apoptosis. The amounts of the various components of the "rheostat" are altered in response to exposure to both sparsely and densely ionizing radiations. One of the key players in the regulation of the apoptotic process is the p53 gene, which has been referred to as the "guardian of the genome" (Lane, D. P., Nature 358(6381):15-6). p53 is expressed in response to ionizing radiation exposures and serves to down-regulate one of the natural suppressors of apoptosis, bcl-2. Cells which have lost the ability to initiate the normal apoptotic pathway appear to accumulate mutations more readily following exposure to sparsely and densely ionizing radiation. In the context of organized tissues, there are additional factors that mediate responses to charged particle radiations. The extracellular matrix (ECM) and the cytokine environment provide signals that help maintain tissue integrity and normal development of the organism. The ECM influences cell morphology, mediates proliferative capacity, and regulates gene and protein expression. The ECM can promote cancer progression and incidence. Tissues respond to charged particle radiations by rapidly remodeling the ECM. The time course and characteristics of the remodeling are different in different tissue compartments. The long-term effects of this remodeling are fertile areas of investigation. In conclusion, biological systems have evolved a series of mechanisms at the cell and tissue level to respond to the types of damages that occur as a consequence of exposure to the various components of the space radiation environment. In large part, these responses evolved to protect the integrity of the organism. Science and Habitability Goals for Mars Exploration Michael B. Duke Lunar and Planetary Institute The rationale for human exploration of Mars is built around the concept that humans will be required to address fundamental scientific questions on Mars and that Mars will be required to address fundamental questions of humanity's future. Recently, the findings of possible evidence for ancient life on Mars has focused the science goals on the search of environments where life might have arisen. Without changing the nature of the generalized strategy for scientific exploration of Mars, the scientific questions, the needs for robotic exploration, and the potential roles for humans on Mars are coming into better focus. In order to support human scientists on Mars in a safe and productive environment, initial steps will need to be taken along the path toward a permanent martian outpost. Although addressed in large part by technology, the habitability goals also can be stated in terms of scientific questions to be addressed during the robotic and early human phases of Mars exploration. This paper describes goals and objectives for Mars science and human habitation, with emphasis on the possible contributions of humans on Mars. Artificial Gravity for Human Missions Laurence R. Young Massachusetts Institute of Technology Artificial gravity will be considered as an alternative to currently inadequate countermeasures for long-duration space flight, of the type required to explore Mars. If we are to commit to a spinning vehicle, the physiological questions get down to two types-one for a full-time rotating habitat and one for intermittent stimulation-a kind of gravity gym. For full-time rotation, the first question is what level of acceleration at foot level is the minimum required to maintain normal function? Various means of achieving the artificial gravity will be considered, but we initially just concentrate on the size and speed needed to give sufficient centripetal acceleration. We can be reasonably certain that 1 G will suffice, but is it needed? Will a half-G do? Or can we avoid deconditioning by spinning continuously at a level of 0.38 Gs to match the Martian gravity? We are likely to achieve the earliest answers with experiments on animals. We already know that rats which have been centrifuged during their space flight don't show the major deterioration in bone, muscle, and cardiovascular response seen by their free-floating brethren-but that is only at 1 G. The current International Space Station plans include a module for a centrifuge to carry up to eight modules for rodents, fish, and eggs but will not accommodate the primates which many feel are needed to adequately model human responses. Intermittent artificial gravity stimulation, on the other hand, presents a number of potential advantages. As part of our normal circadian rhythm, the very G-dependent processes which result in fluid loss and bone deconditioning are probably turned off during our normal sleeping hours. On the other hand, extended periods of bed rest produce effects on the skeleton, muscles, and cardiovascular system which are similar to those occurring in space. This simulation of space flight is made more accurate if the bed rest is conducted with a six-degree head-down tilt to accelerate the shift of fluid toward the head and if the subject lies partially immersed, although dry, in a high-tech waterbed. A combination of short-duration and long-term studies in ground centrifuges and rotating rooms can be useful in answering many of the key questions concerning the application period, frequency, and intensity of centripetal acceleration. They will also be useful in determining the seriousness of the problem of dual adaptation-to the rotating and non-rotating environments. Importantly, they may shed light on the physiological importance of a gravity gradient across the body if one is to truly proceed with rotators having a radius comparable to the subject's height. These ground gravitational physiology studies are essential for effective use and interpretation of the space variable-gravity centrifugation tests to be mentioned. Even if the main physiological issues are adequately addressed by a rotating space vehicle, important human factors issues remain. The specific constraints of artificial gravity remain at the crux of the research in terms of a practical solution to interplanetary travel. If we are right about the ability of astronauts to adapt and maintain the adaptation, it may be possible to achieve a measure of protection from an intermittent artificial g exposure with a radius as low as 4 m, rotating at 10 rpm to produce about half a G at foot level. Destination Mars: An Astronaut's Perspective Scott Parazynski, M.D. NASA Johnson Space Center The recent discovery of possible fossilized life forms within a Martian meteorite has rekindled interest in human exploration of the planet. While the global scientific community must carefully study this physical evidence as well as await the results of 4 space probes to Mars in the coming years, others are looking ahead to the next logical step of sending humans to the planet. Such a feat, while a tremendous scientific undertaking, is nearly within the grasp of current technology. The travel distance involved and the harsh Martian environment itself impose great challenges to human physiology as well as to spacecraft design. Mission planners face challenges from the long-duration microgravity environment, complex life-support systems, interplanetary radiation, and the psychological stressors involved in such an endeavor. ¥ While the human body readily adapts to weightlessness, significant deconditioning of the musculoskeletal and cardiovascular systems occur that could preclude safe landing and operations under the 1/3 G environment of Mars. Further, the long-term effects of microgravity are poorly studied, although a Russian cosmonaut colleague has spent 438 consecutive days on-orbit and has returned to normal daily activities here on Earth. This re-adaptation was due to his intensive exercise regime, including both resistive and aerobic training every day of his flight. Space travel longer than this might require "artificial gravity," but these systems might be prohibitively complex and expensive. Studies from the soon-to-be-built International Space Station will address the problem of deconditioning in-depth, and help define improved countermeasures. ¥ Interplanetary space places astronauts at significant risk of irradiation from solar particle events (SPEs) as well as low-level galactic cosmic rays. Moreover, even extravehicular activity (EVA) on the planet's surface is not without risk, since Mars has essentially no magnetic field to shield the space walking astronaut. SPE forecasting from Earth, as well as "space weather" instrumentation onboard the spacecraft will be essential during the expedition. Enroute to Mars, EVA should be minimized, and the spacecraft will need to be designed for maximum shielding from radiation. ¥ Crew selection and compatibility is a key consideration for travel to Mars. Not only must the crew have a wide skill base (scientific, engineering, medical) to enable self-reliance, but also they will need extensive compatibility screening preflight and considerable psychological support in flight. Notably, a busy training and science schedule in transit and frequent communications with family and friends back on Earth will be a formula for success. Results from long duration space crews, submariners, Antarctic scientists and Biospherians support a team approach with a well-defined command structure. Until recently, planned Mars trajectories have involved hundreds of days in interplanetary space and have relied on conventional rocket technologies. A developmental tunable exhaust plasma rocket, utilizing nuclear electric propulsion, reduces the transit time to the order of 90 days each way. Additionally, system redundancy and the power capabilities of such a vehicle would allow a powered abort capability for return to Earth. ¥ A "split-sprint" mission is favored, the first spacecraft being a payload tugboat (instrumentation, supplies, fuel, etc.) that would establish the Martian outpost and potentially generate supplies needed for the return home. Once the outpost is fully established robotically, a human-operated speedboat will transfer the crew from low-earth orbit to Mars. Depending on phasing, Martian stays could range from 30 days to 2 years. ¥ Assembly of the craft including the nuclear reactors will occur in low earth orbit for safety. The vehicle will be comprised of at least 3 redundant plasma rocket engines and 3 redundant nuclear reactors due to their criticality as well as the propulsion and power needed for such a mission. Other critical life-support systems, including environmental control and rendezvous systems must be designed to be highly reliable, modular, and fault-tolerant. ¥ Shielding from SPEs and other sources of radiation will be provided by liquid hydrogen fuel tanks surrounding the crew modules, as well as an induced magnetic field from the plasma rockets themselves. ¥ Due to the short transit times enabled with the tunable exhaust plasma rocket, complex countermeasure systems like artificial gravity will not be required. Daily exercise en route to preserve musculoskeletal and cardiovascular health will be necessary, as well as to preserve neuromuscular coordination once the crew arrives on Mars. In the not-too-distant future, the global scientific community may require human investigation of the planet Mars. Lessons learned from Skylab, the Space Shuttle Program, Mir, and the planned International Space Station, as well as technological advancements in rocketry, will take us there. Observations of Planetary System Formation W. J. Welch University of California, Berkeley At this stage, the title means observations of the formation and evolution of disks around young stars and protostars. The reality of disks is certain, with the imaging of Beta Pic and the HST pictures of ionized disks around a large percentage of stars in the Orion Nebula. Evidence about these structures around the youngest objects, especially those that are embedded, is more indirect. Infrared excesses from T Tauri stars (which are, of course, visible) imply so much dust that it must be in a flattened distribution to let out the starlight. Infrared excesses are detected at wavelengths as long as millimeter waves. Roughly half the classical T Tauri stars show this emission and have, by implication, disks. The strength of this emission implies masses as large as 0.1 Msun. The slightly more massive Ae/Be stars show similar statistics. Among the most deeply embedded stars, disks are less frequent, suggesting development of the disks as part of the emersion from the dense clouds. The gross infrared and millimeter wave excess spectra can be explained by simple disk models in which temperature and density are power laws of the radius. They can also be explained as emission from small surrounding envelopes, or a combination of both. Maps made with millimeter wavelength interferometers at a few arc-seconds resolution do not resolve them but show that at least the longest wavelength radiation must be coming from very compact structures, disks. The spectral shape of the emission at long wavelengths frequently is flatter than that of emission from dust in the interstellar medium, and this may imply that there is evolution of the grains, growth in the particle size. Spectral emission, particularly from CO, is often detected at a few arc-seconds resolution from the disks. Striking velocity gradients are seen across the disks, which may be understood as Keplerian rotation, or alternatively, continued infall-or even possibly outflow. Theoretical estimates of the likely sizes of the disks predict radii of about 100 AU, requiring resolutions of better than one arc-second to map them and sort out the various uncertainties in their structures noted above. Current developments of mm interferometers are beginning to enable this level of resolution. Recent maps of the two objects HLTau and L1551/IRS5 show interesting structures on the scales of 40 AU, a disk-like structure for the former and a binary system for the latter. Prospects for the next few years are for maps with resolutions as good as 15 AU. Gas kinematics should then become more clear. Temperature distributions should help clarify the local chemistry. There may be some evidence of the formation of major planets and cometary belts, and comparative studies should tell us about the course of evolution. A most important ground-based development will be the Millimeter-Wave Array planned by the NRAO. It will have a resolution of about 7 milli-arc seconds at a wavelength of 0.5 millimeter. This corresponds to a linear scale of about 1 AU in the Taurus Clouds, and it may detect the formation of important structures in protostellar disks. Ultimately, the best sensitivity and resolution for this work would come from the development of an imaging interferometer which could operate at wavelengths between 50 and 100 microns. This is where the peak of the emission occurs for these planetary (or pre-planetary) systems. This part of the spectrum is totally inaccessible from the ground. Indeed, it is not too early to contemplate the construction of an imaging interferometer to be placed in space to operate at these wavelengths. Theory of Planetary System Formation Patrick Cassen NASA-Ames Research Center Observations and theoretical considerations support the idea that the Solar System formed by the collapse of tenuous interstellar matter to a disk of gas and dust (the primitive solar nebula), from which the Sun and other components separated under the action of dissipative forces and by the coagulation of solid material. Thus, planets are understood to be contemporaneous by-products of star formation. Because the circumstellar disks of new stars are easier to observe than mature planetary systems, the possibility arises that the nature and variety of planets might be studied from observations of the conditions of their birth. A useful theory of planetary system formation would therefore relate the properties of circumstellar disks both to the initial conditions of star formation and to the consequent properties of planets to those of the disk. Although the broad outlines of such a theory are in place, many aspects are either untested, controversial, or otherwise unresolved; even the degree to which such a comprehensive theory is possible remains unknown. The main features of the theory adopted by most researchers are as follows. A small fraction of the material in cold, interstellar clouds becomes gravitationally unstable to collapse, either by gradual evolution or by a sudden compression induced by some disturbance. Collapse occurs rapidly enough (within a million years) to preclude the loss of much of the cloud's angular momentum, so the collapsed configuration takes the form of a centrifugally supported disk, in which most of the original angular momentum is retained. Dissipative forces within the disk promote the accretion of most of the material to its center to form a star; lesser fractions of material spread out in the remaining disk or are ejected back into space by energetic processes near the star. Solid objects grow in the disk due to the coagulation of dust and ice grains (both interstellar survivors and newly condensed). Although gasdynamic forces may cause most of these objects to be accreted into the growing star, or even ejected in the wind, some become large enough for gravitational forces to dominate their motions, at which point they become potential planetary system survivors. Growth to planetary size occurs through collisions made possible by the perturbations of orbits induced by mutual gravitational scattering and perhaps by collective interactions with nebular gas. In the case of the Solar System, the outer planets apparently grew fast enough to capture and retain substantial amounts of hydrogen and helium from the solar nebula. The final planetary masses were probably attained within 100 million years, although the cleanup of debris persisted longer. In the context of this theory, the asteroid belt is the result of a failed planet, the accumulation of which was frustrated by the gravitational influence of Jupiter. Comets are believed to be planetesimals accumulated in the Uranus-Neptune region, scattered to great distances by encounters with the outer planets. Several components of the theory enjoy solid support from observations and/or rigorous theoretical analysis. For instance, the basic tenets of star formation theory have been verified by observations which have recently detected direct evidence for collapsing gas in star-forming regions and established the prevalence of circumstellar disks around young stars. The idea that disk evolution proceeds through the accretion of material onto the young star is confirmed by measurements of the accretion luminosity. The fact that dissipative angular momentum transport in a disk leads to distributions of mass and angular momentum characteristic of a star and planetary system has an unequivocal physical basis. Numerical simulations have demonstrated that the characteristics of the terrestrial planets are typical of the configurations ultimately attained by a swarm of dynamically evolving planetesimals possessing the total mass and angular momentum of the planets they eventually form. Nevertheless, there are major gaps at every step in the theory, and recent developments have emphasized the need to remain undogmatic about even the most entrenched assumptions. For instance, it has been commonly supposed that the distribution of mass in the present Solar System is not a perverse distortion of that of the primitive solar nebula in the planet-forming era. Although it was acknowledged that the distribution of sedimented solids need not be exactly the same as that of the gas (the latter being the predominant component), the discovery of Jupiter-mass planets very close to their parent stars has focused attention on the theoretically identified possibility of extensive planetary migration: the radial motion of a planet due to its gravitational interaction with its parent disk. A consequence of this phenomenon could be the effective obliteration of evidence of the original mass distribution in the disk. (In a similar vein, the recent suggestion that meteoritic material was thermally processed near the Sun and subsequently ejected to great distances in the nebula contains the implicit and radical notion that even the distribution of coagulated solids had little to do with the original distribution of mass in the nebula.) Thus, understanding the precise way in which planetary material becomes decoupled from nebular gas is central to interpretations of disk properties in terms of potential planetary systems. In addition to these issues, one can identify several other specific, unresolved questions of the most fundamental order, relevant to the characteristics of planetary systems: (1) How are the conditions that lead to the birth of a single star surrounded by a disk distinguished from those that lead to the formation of a multiple star system? (2) What are the specific mechanisms responsible for the transport of angular momentum in circumstellar disks, and how efficiently do they act? (3) What processes determine the mass of a gas-rich planet? (4) What determines the ultimate fate of nebular gas that is not incorporated into planets? (5) What processes govern the final distributions of volatile substances among the planets? For most of these questions, definitive observational tests will be difficult to identify. Well-designed, computationally intense studies, which incorporate dissipative processes in a quantitatively realistic way, will be required to address them. (This abstract is based in part on the article "Origin of the Solar System," by P. Cassen and D. S. Woolum.) Primitive Materials D.E. Brownlee University of Washington During formation of the solar system, most of the mass of the solar nebula was incorporated into the Sun and planets or was ejected back into the interstellar medium. Only a tiny fraction survived as small bodies capable of preserving "primitive materials", solids not severely altered since the early history of the solar system. Solar system repositories of primitive materials include the comets, and some asteroids and small satellites. Information on primitive materials can be obtained by telescopes and by close-up study with spacecraft. Primitive materials are very fine grained and the most detailed information is obtained by laboratory analyses of meteoritic samples, either meteorites or collected samples of interplanetary dust. In the future, missions like STARDUST will directly return samples of known comets and asteroids. The solar system's small bodies carry records on the materials, conditions, time scales and processes that occurred during planetary formation. They also contain the best clues on the form and processing of the key biogenic elements in the stages that preceded planet formation and the development of life. While primitive materials provide a direct window of historical information on the formation of planets and early evolution of the biogenic elements, the window is some-what obscured and confusing. In all cases primitive materials have been stored inside bodies of substantial size and processes within these bodies, to various degrees, overprint original material properties. Common alterations include thermal metamorphism, aqueous alteration, shock and compaction. In many cases it is difficult to distinguish original properties from secondary ones. Outstanding exceptions are presolar grains such as SiC and graphite that survived parent body, nebular, and interstellar environments with many of their properties intact. Primitive materials in the solar system have a considerable range of diversity. Primitive asteroidal meteorites show a wide range in mineralogy, grain size, oxidation state, and systematic deviations from solar elemental composition. Reflection spectra indicate a strong radial gradient of material types within the asteroid belt. This is in part due to spatial variations of particle composition in the solar nebula, consistent in principle with differences in different meteorite types. The radial variation is also influenced by mysterious, strong heating of bodies in the asteroid region that had variation with solar distance. Wide variation of spectral reflectance also exist in the Kuiper and Centaur objects although it is unknown how much if this due to intrinsic material properties and how much is due to space weathering effects. Accounting for parent body alteration effects, it is clear that broad pre-accretion variations occurred in particle types in various regions of the solar nebula. In part these are due to mixing of presolar grains, nebula condensates and solids modified by various nebular processes. These studies of preserved solar system solids can be directly compared to analogous solids observed in circumstellar regions surrounding other stars. The role of primitive materials in the development of life on Earth is unknown and perhaps unknowable. The bulk of the Earth was constructed from materials that did not have primitive elemental composition but it is likely that much of the Earth's carbon, nitrogen and water were delivered by primitive bodies from the outer solar system. These bodies carried a wealth of complex organic material produced in the interstellar medium, in the solar nebular and inside parent bodies. For large bodies this molecular material is destroyed upon impact but for dust and meteorite sized objects much of it survives accretion to play possible roles in the origin and early evolution life. Planetary Perspective on Life on Early Mars and the Early Earth Norman H. Sleep Stanford University Kevin Zahnle NASA-Ames Research Center Impacts of asteroids and comets posed a major hazard to the continuous existence of early life on Mars as on the Earth. The chief danger was presented by globally distributed ejecta, which for very large impacts takes the form of transient thick rock vapor atmospheres; both planets suffered such impacts repeatedly. The exposed surface on both planets was sterilized when it was quickly heated to the temperature of condensed rock vapor by radiation and rock rain. Shallow water bodies were quickly evaporated and sterilized. Any surviving life must have been either in deep water or well below the surface. On Earth, global thermal excursions are buffered by the heat capacity of the oceans, especially the latent heat of condensation. Such buffering does not occur on Mars; therefore, relatively small (70 km diameter), relatively frequent impacts heat the surface everywhere to the melting point. But for impacts large enough to vaporize the oceans (400 km diameter), thermal buffering serves only to prolong the disaster. Very high surface temperatures are maintained for thousands of years while the oceans recondense and rain out. Obviously survival in open water is more likely on Earth, where the energy required to boil massive oceans is enormous. But survival in deep subsurface environments is more likely on Mars because (i) Mars' lower background heat flow and lower gravity (the latter permitting open porosity at greater depths) allow deeper colonies, and (ii) the thermal heat pulse from a major impact is briefer and so penetrates less deeply. A third factor that favors ultimate survival on Mars rather than Earth is that the bigger planet statistically experiences more and bigger impacts. Impacts in the 200 km diameter range are known to have occurred on the Moon and Mars, and impacts in the 400 km range are likely on Earth but may not have happened on Mars. Even the largest bodies that may have hit Mars-400-km diameter-would have sterilized only the upper few hundred meters of the subsurface. A similar impact on Earth would boil the oceans and sterilize the subsurface down to 1 km depth. Only thermophile organisms could have survived impacts of asteroids large enough to leave heat or boil the entire terrestrial ocean (around 300-km diameter) on the Earth. Studies of terrestrial microorganisms indicate that the last common ancestor may have been a thermophile and therefore the survivor of such a catastrophe. This organism needs to be distinguished from first common ancestor, which may well have originated in a different environment and adaptively radiated to occupy thermophile niches. There was probably time for this, as the intervals between dangerous impacts are millions of years on both the Earth and Mars. An additional refugium that is poorly understood is ejection of weakly shocked material to space by impacts and its return to its home planet or transfer to a neighboring planet. Overall, early Mars was safer from impacts than the early Earth and probably was habitable before the Earth-Moon system formed. Extant martian life in the subsurface is possible. Exploration for martian life and interpretation of martian fossils needs to be planned with the possibility that martian and terrestrial life are related by the space transfer of organisms. Origin of Protocells David W. Deamer University of California, Santa Cruz An important question that must be answered if we are to understand life's origin fully is whether the living state arose a priori from pre-existing cellular structures. The alternative is that living molecular systems were first present as solutions or adsorbed films, with cellular life developing only at a later evolutionary stage. If the self-assembly of amphiphilic molecules into membranes preceded the origin of life, it is plausible that the earliest living systems may have had access to encapsulated environments. The following concepts and experimental results support this conjecture. Bilayers assemble from a variety of amphiphilic compounds. Although contemporary cell membranes incorporate phospholipids as the primary component of the lipid bilayer, it is not necessary to think that complex lipids were required for early cellular life. In fact, simpler amphiphilic molecules can also assemble into bilayer membranes (1). These include single-chain amphiphiles such as soap molecules, glycerol monooleate, oxidized cholesterol, and even detergents like dodecyl sulfate mixed with dodecyl alcohol. It seems likely that primitive cells incorporated lipid-like molecules from the environment, almost as a nutrient, rather than undertaking the more difficult process of synthesizing complex lipids de novo. Amphiphilic molecules are present in carbonaceous meteorites. The presence of amino acids in carbonaceous meteorites supports the possibility that amino acids were available in the prebiotic environment. Amphiphilic molecules have also been demonstrated in such meteorites and furthermore have the ability to self-assemble into barrier membranes (2, 3). This observation suggests that amphiphiles were present on the early Earth and available for incorporation into boundary membranes of primitive cellular organisms. Bilayer permeability strongly depends on chain length of the component amphiphilic molecules. We tend to think of the lipid bilayer as being a nearly impenetrable barrier to ionic solutes and other large, polar molecules. This leads to a conundrum when we try to imagine how early forms of cellular life could have functioned in the absence of highly evolved transport enzymes that translocate ionic nutrients and metabolites across the bilayer barrier. It is true that lipid bilayers of contemporary cell membranes present a significant permeability barrier that is necessary for normal cell functions, particularly those related to bioenergetics of ion transport and chemiosmotic ATP synthesis. However, recent results show that shortening lipid chain length from 18 to 14 carbons increases the permeation of ionic solutes by several orders of magnitude (4). This level of permeability is sufficient to encapsulate large molecules such as proteins and polynucleotides, yet still allow external substrate to reach an encapsulated enzyme (5). It follows that early cell membranes could have been composed of shorter chain lipids that provided access to nutrients for macromolecules undergoing growth and replication in an encapsulated microenvironment. Macromolecules can be encapsulated in bilayer vesicles under simulated prebiotic conditions. Another conceptual problem has been to imagine how vesicular lipid bilayers could capture macromolecules, given that the bilayer must present a nearly impenetrable barrier if the macromolecules are to be maintained within the membrane-bounded volume. There is a reasonably straightforward answer to this question. If dispersed lipids are first mixed with macromolecules such as proteins or nucleic acids, then subjected to drying and wetting cycles to simulate a tide pool environment, the macromolecules are readily captured in membrane-bounded vesicles (6). Lipid bilayers grow by addition of amphiphilic compounds present in the bulk phase medium. It is not sufficient for a primitive cell to replicate its macromolecular components unless the boundary membrane can also increase in area to accommodate the internal growth. Recent experimental results from liposome model systems have provided a useful perspective on primitive membrane growth processes (7). Encapsulated polymerases can synthesize nucleic acids. Polynucleotide phosphorylase (5, 8), and the enzymes required for the PCR reaction (9) have now been encapsulated in liposomes and shown to synthesize nucleic acids. What would be required for the next step, to a true replicating system of encapsulated polynucleotides that can undergo cell-like growth? In concept, the answer is straightforward. Externally added substrates must be supplied to the captured polymerase, and a mechanism for simultaneous addition of membrane components to the vesicles must also be available. Furthermore, there should be a regulatory coupling between growth of the internal molecules and growth of the membrane. Last, protein enzymes cannot be used as catalysts because they are not reproduced in such a system. Instead molecules resembling ribozymes should be components of the system, incorporating both genetic information and the polymerase activity. In practice, there is still no way to deal simultaneously with all of these requirements. However, a few years ago it would have been inconceivable that we would soon reach a point at which intravesicular synthesis of nucleic acids became a reality. It seems likely that a laboratory version of an encapsulated replicating system of molecules capable of growth and perhaps evolution will be achieved in the next decade. References 1. Hargreaves, W. R., and Deamer, D. W. 1978. Liposomes from ionic single-chain amphiphiles. Biochemistry 17:3759-3768. 2. Deamer, D. W. 1985. Boundary structures are formed by organic components of the Murchison carbonaceous chondrite. Nature 317:792-794. 3. Deamer, D. W., and Pashley, R. M. 1989. Amphiphilic components of carbonaceous meteorites. Orig. Life Evol. Biosphere 19:21-33. 4. Paula, S., Volkov, A. G., Van Hoek, A. N., Haines, T. H., and Deamer, D. W. 1996. Permeation of protons, potassium ions and small polar molecules through phospholipid bilayers as a function of membrane thickness. Biophys. J. 70:339-348. 5. Chakrabarti, A., Breaker, R. R., Joyce, G. F., and Deamer, D. W. 1994. Production of RNA by a polymerase protein encapsulated within phospholipid vesicles. J. Mol. Evol. 39:555-559. 6. Deamer, D. W., and Barchfeld, G. L. 1982. Encapsulation of macromolecules by lipid vesicles under simulated prebiotic conditions. J. Mol. Evol. 18:203-206. 7. Walde, P., Wick, R., Fresta, M., Mangone, A., and Luisi, P. L. 1994a. Autopoietic self-reproduction of fatty acid vesicles. J. Am. Chem. Soc. 116:11649-11654. 8. Walde, P., Goto, A., Monnard, P.-A., Wessicken, M., and Luisi, P. L. 1994b. Oparin's reactions revisited: Enzymatic synthesis of poly(adenylic acid) in micelles and self-reproducing vesicles. J. Am. Chem. Soc. 116:7541-7547. 9. Oberholzer, T., Albrizio, M. and Luisi, P. L. 1995. Polymerase chain reaction in liposomes. Current Biology 2:677-682. Biological Perspective on the Early Earth and the Chemistry that Spawned Life Norman R. Pace University of California Our perspective on the nature of the geochemical processes that gave rise to life on Earth has improved enormously over the past decade. Astronomical and geochemical studies provide an increasingly clear view of the character of the early Earth, at the time of the origin of life. Microfossil work is increasingly sophisticated in its interpretations and has provided solid evidence for life at 3.5 billion years ago or before. Perhaps most dramatically, we can now infer with increasing confidence the biochemical nature of the earliest life, which also provides information on the environment in which that life came to be. The record that we interpret for the nature of the earliest life is a quantitative one, phylogenetic trees based on molecules that are found in all of modern-day life and so must have been present in the earliest genetic cell. Together, the biological and physical information paint a harsh cradle for life, a violent world of volcanism and hydrothermal interaction with the molten bones of the forming planet. We must couch our speculation on the origin of life in terms of these findings. The Phylogenetic Record The geophysical record tells us that the early Earth was hot and volcanic, but it tells us nothing about the nature of the earliest organisms. By analysis of molecular sequences, we can infer maps of the course of evolution. In this analysis, "homologous" (of common ancestry) genes from different organisms are compared, and the number of differences in their nucleotide sequences is counted. The greater the number of differences between the genes, the more evolutionarily separated are the pairs of organisms. The "evolutionary distance," the fraction of sequence differences between pairs in a collection of sequences, can be used to infer "phylogenetic trees," maps of evolutionary diversification. Because of their slow rates of evolutionary change, small-subunit (16S or 18S) ribosomal RNA (rRNA) sequences have been used most extensively for phylogenetic classification of all life. The molecular phylogenetic studies provided, for the first time, a clear view of the evolutionary scope of life on the grand scale. The figure shows a phylogenetic tree based on rRNA sequence comparisons. It can be viewed as a road map of evolution. Evolutionary distances between organisms (i.e., the number of sequence-changes separating them) are represented by the lengths of line-segments (see scale bar in the figure). The phylogenetic results show that known biodiversity falls into three primary groupings, or 'Domains': Archaea (formerly archaebacteria), Bacteria (eubacteria), and Eucarya (eukaryotes). Although both are prokaryotic (i.e., lack a nuclear membrane), Archaea and Bacteria are phylogenetically distinct from one another. Indeed, there is strong evidence indicating that Archaea and Eucarya are more closely related to each other than either is to Bacteria: the origin of the extant evolutionary lineages, the root of the tree of all life (indicated as "origin" in the figure), appears to separate the Bacteria from the other two forms. Some of the lessons gleaned from the universal tree of life contradict long-held beliefs. For instance, the recognition of the deep evolutionary divergence between Archaea and Bacteria shattered the entrenched notion of evolutionary unity among prokaryotes. The phylogenetic results also show that the eukaryotic nuclear line of descent, Eucarya, is as old as the prokaryotic lines, Archaea and Bacteria. The idea that the eukaryotic cell arose relatively recently (1-1.5 billion years ago was a common assumption based on no credible data), from the fusion of two prokaryotes, proved essentially incorrect. To be sure, the rRNA-based data confirm that mitochondria and chloroplasts are of endosymbiotic bacterial origin, as postulated long ago (note in the figure the association of the mitochondrial and chloroplast lineages with Bacteria). The organellar lineages emerge from subgroups of the Bacteria, however, so their evolution must have occurred relatively late in the history of life. Some microbial eukaryotic lineages seem never to have had mitochondria and chloroplasts so may have diverged from the eukaryotic line of descent prior to the incorporation of the organelles. Although phylogenetic trees are based on modern-day organisms, we can, in principle, infer properties of ancestral organisms by "parsimony": if two organisms have a common trait, then it is likely that their common ancestor had the same trait. With that perspective, we can infer that the last common ancestor of all life possessed DNA, a rudimentary transcription system, ribosomes similar to the modern ones, and many other traits-all the commonalities of today's life. More specialized traits of the earliest organisms also are suggested by the phylogenetic results. In the figure, organisms shown in bold are high-temperature organisms, extreme thermophiles. It is noteworthy that all of the most deeply diverging lineages in both Bacteria and Archaea are thermophiles. This suggests that their common ancestor, the common ancestor of all life, was thermophilic as well. Additionally, all of these types of organisms have physiologies consistent with a geothermal setting. All thrive by metabolizing compounds such as hydrogen, sulfur, and iron. The Setting for the Origin of Life We can now imagine, based on solid results, a fairly credible scenario for the terrestrial events that set the stage for the origin of life. It seems fairly clear, now, that the earliest Earth was, in essence, a molten ball with an atmosphere of high-pressure steam, carbon dioxide, nitrogen, and the other products of volcanic emission from the differentiating planet. The cooling curve of the molten Earth would have reached the critical point of water by roughly 4.2-4.4 billion years ago; then it would have rained, depositing during an unknown time period 3-4 kilometers of water over the hot crust. It seems unlikely that any landmass would have reached above the waves, to form the "tidal pools" invoked by some theories for the origin of life. This is because the early crust would have been dynamic and thin and consequently probably incapable of supporting a continental mass sufficient to reach the surface of the overlying ocean. Instead, the conspicuous feature of that early Earth would have been the zone of interaction between the water and the cooling crust. Water would have taken the main role of conducting heat from the interior of the Earth, as it does now along the Mid-Ocean Ridge; the hydrosphere would have been in dynamic interaction with the forming crust through convective hydrothermal circulation. We can imagine that this early hydrothermal circulation would have stirred up a thick sludge of volcanic elements, probably highly enriched with reactive kerogens and organic tars extracted by convection from the differentiating planetary mass. It was in this sludge, a perfect setting for complex organic surface-chemistry supported by pyrite and basaltic glasses, that life probably came to be. This physical scenario is probably a common one in the formation of rocky, iron-rich planets. If so, then the origin of life may be a common consequence of planetary origins. Perhaps the question is not the probability of the origin of life but rather the probability that life, having arisen, survives and comes to dominate a planet. The Chemistry of the Origin of Life We know very little about the course of chemical events that gave rise to life's molecules. Since the setting was geothermal, the most fruitful arena for experimentation would seem to be the interactions of iron, sulfur, and hydrogen with carbon compounds. This notion is also indicated by the biological record: iron and sulfur are the basis for many universally conserved biological catalysts which, because of their universality, probably were employed by the earliest life. Since evolution builds upon the pre-existing, iron and sulfur are indicated as main proto-physiological catalysts. For the same reason, I think it unlikely that clays or like minerals played much role in the origin of life. If they had, we might expect materials such as aluminum and silicate to be widely distributed in biology. Instead, these compounds are rare. Considerable attention has focused on RNA as the first catalyst with genetic relevance, but the evidence for this is hypothetical: RNA is the only (known) biological molecule that is capable of serving as both catalyst and genetic information. As argued previously, however, the fragile chemical nature of RNA is inconsistent with the hydrothermal fluids that bathed early life. There probably was no "first self-replicating catalyst." Rather, following the tenets of biology, it seems much more likely that RNA, DNA, and the other molecules of cells co-evolved, as generally is the case with interacting organisms today. Intelligent Life in the Universe Frank Drake SETI Institute The recent discoveries of other planetary systems and inconclusive evidence for ancient life on Mars have supported the long-held view that intelligent life on Earth was the result of common processes which should have occurred in many planetary systems. The most substantial controversies remaining concern the frequency with which biotas produce intelligent, technology-using species, and the lengths of time that technological civilizations produce detectable manifestations of their existence. It would appear that these two controversies can only be resolved through the detection of extraterrestrial intelligent life. In any case, the planetary and Martian discoveries encourage further searches for extraterrestrial intelligent life and have served to motivate increased contributions of resources to searches and the magnitude of the searches themselves. Analyses of possible methods to detect extraterrestrial technology still tend to favor radio searches, and thus most current searches continue to search for radio transmissions, primarily at microwave frequencies. However, modest searches at optical and infrared wavelengths are in progress and may grow in ability in the future. It is clear that there is no logic which can conclusively identify the search method most likely to succeed, and thus all reasonable searches are worthy of support. Progress in radio searches has been enormous over the 36 years that such searches have been carried out. The search capabilities of SETI systems have grown very nearly exponentially over that time, with a capability-doubling time of about 2/3 of a year. The search systems of today are about 100 trillion times more powerful than those of 1960. This improvement has benefited greatly by the construction of large radio telescopes. More important has been the application of modern computer technology to produce affordable multichannel radio receivers, with contemporary search systems being capable of monitoring more than 100 million channels simultaneously. Of great importance has been the development of computer hardware and software which can search for a variety of intelligent signal forms in the data produced by the multichannel system. Also of great importance has been the implementation of search systems in which two radio telescopes, widely separated, are used in approaches which allow radio frequency interference to be quickly and reliably identified. This is costly but very effective in dealing with the major impediment met in current searches, terrestrial radio interference. A recent unfortunate development in this field has been the total withdrawal of financial support by U.S. federal agencies. This withdrawal has ignored the strong support within the scientific community for such research and appears to be driven by politics outside the scientific community. However, a positive development has been the development of massive financial support by private individuals and foundations; this support has been adequate to continue existing programs and even to improve them. The search for manifestations of extraterrestrial technology still appears to be one of the more promising and much less costly methods to detect extraterrestrial life. If successful, it obviously will provide much more interesting data than will just the discovery that life exists on another planet. There is still room to expand existing search systems greatly without requiring new technological developments. This can come about through the purchase of still more systems similar to those already in place. One of the most important steps which could be taken to enhance search capability would be the provision of large radio telescopes dedicated to SETI searches. These could be relatively inexpensive, since they would not need the range of capabilities normally sought in radio telescopes. Their construction would lead to faster searches, and higher quality searches, since there would be adequate opportunity to optimize all aspects of the telescope and its receiver system for SETI. It would also make searches much more cost effective by eliminating the costs involved in moving complicated search systems, and their operators, to remote sites. In the long run, a goal should be to build and operate large SETI systems on the far side of the moon, the only place in the solar system which is shielded from human radio transmissions at all times. Exploring Mars for Evidence of Past or Present Life: Roles of Robotic and Human Missions Jack D. Farmer NASA-Ames Research Center During the coming decade, robotic field science will play a fundamental role in exploring Mars for evidence of past life and/or prebiotic chemistry. To create a context for such exploration, we especially need to understand the mineralogy and chemistry of the Martian surface. We have learned that the preservation of biological signatures in rocks on Earth is favored by rapid mineralization processes that are restricted to a comparatively small number of geological settings. Thus, a detailed knowledge of surface mineralogy will provide valuable clues about past Martian environments as a necessary context for future exobiological exploration. Information about past climate and volatile history resides in mineralogy, and specifically the mineralogy associated with aqueous sedimentary processes. The targets of choice in exploring for past life on Mars are therefore aqueous sedimentary deposits, particularly those in ancient terranes that date to the early clement period in Mars' history when liquid water was present at the surface. Site selection for landed missions will be critical because we cannot expect to land just anywhere on Mars and find the kinds of deposits that are likely to preserve evidence of past Martian life or climatic history. Even if life never developed on Mars, the same kinds of target deposits are likely to harbor a prebiotic chemical record that would be crucial in understanding the origin of terrestrial life. Achieving a scientific consensus on the question of past Martian life is likely to require multiple sample returns, followed by extensive interdisciplinary studies carried out in labs on Earth. To find the best places to explore, we will need to optimize site selection by carrying out high spatial resolution mineralogical mapping from orbit, with ground truthing of surface mineralogy using well-placed and appropriately instrumented robotic rovers. Remote sensing studies of geological terranes on Earth suggest that spatial resolutions of < 30 m/pixel for visible range imaging and < 100 m/pixel for multispectral imaging may be required to accurately identify high-priority targets on Mars for ancient climate and life studies. Increased landing precision and highly mobile rovers are technological priorities that will ensure we reach high-priority targets during nominal mission times. Precise mobility requirements will vary with the science goals of each mission, reflecting a balance between landing precision and the size of target deposits. Landing targets will need to be pre-selected from orbital imaging, and the higher the spatial resolution the better for precisely locating the best landing sites. To optimize for a high science return, it is likely that rovers will need to be able to traverse distances exceeding their own landing errors (perhaps multiple kilometers or more during nominal mission times). In order to target samples for remote analysis by Earth-based investigators and for pre-planning traverses using virtual terrain models, rovers will also require instrumentation for analyzing the composition of rocks at a distance (e.g., "spot" spectrometers). Considering the small amount of material that will be brought back to Earth, in situ mineralogical analysis will be crucial for selecting the best materials for sample return. Rock surfaces are likely to be covered by dust or weathering rinds, and accurate compositional analysis will require access to freshly exposed interior rock surfaces. Microscopic imaging of rock surfaces will provide valuable microtextural and mineralogical information to assist in targeting smaller areas on rock surfaces for more detailed compositional analyses. Given the minimal sample preparation required, spectral analysis (e.g., by infrared or laser Raman spectroscopy), especially in combination with reflectance techniques for elemental analysis (e.g., alpha proton X-ray spectroscopy or X-ray fluorescence), provides an especially favorable approach to in situ mineralogical and organic analysis for early missions. The exploration for extant Martian life will require a fundamentally different approach from that used for ancient climate studies and Exopaleontology. The Viking missions revealed the surface of Mars to be inhospitable for life as we know it, owing primarily to the absence of liquid water. However, it has also been suggested that life could exist in the subsurface of Mars (perhaps tens to hundreds of kilometers depth), where an extensive hydrosphere could be present. It is also possible that areas of rising ground water may provide shallow subsurface oases capable of sustaining life. We know that on Earth, subsurface environments are a haven for a wide variety of heterotrophic microorganisms that do not necessarily require a direct connection to the surface environment for their survival. During the upcoming decade of exploration, we could initiate systematic orbital searches for such "oases" using high spatial resolution multispectral remote sensing to explore for anomalous concentrations of water vapor, methane, or other reduced gases, as well as thermal anomalies suggestive of near-surface hydrothermal systems or fumeroles. This kind of exploration would be especially interesting if targeted at low elevation areas where atmospheric density is higher and where crustal thinning is likely to have occurred, increasing the heat flow to the surface (e.g., the floors of chasmata). Given the present technological challenges of deep drilling, robotic platforms are likely to provide very limited access to the Martian subsurface. As presently envisioned, rover-based drilling systems are unlikely to penetrate much deeper than a few meters. The implementation of a subsurface exploration program to search for extant Martian life could require drilling to kilometer depths. Drilling to such depths will quite likely require a human presence. It follows that extensive exploration of the Martian subsurface to search for deep subsurface water and an extant microbiota may provide the most compelling reasons for carrying out human missions to Mars. A fundamental issue facing the scientific community and public at large is planetary protection. We are embarking upon a new decade of Mars exploration with a clearly identified objective of sample return. Aside from the concerns associated with forward contamination of Mars, particularly in connection with missions aimed at detecting extant life, the threat of back-contamination raises issues of broader concern. These issues have yet to be fully addressed, and concerns over such things as the reliability of sample containment technologies, the effects of sterilization on the science return of missions, and the added costs to missions must be clearly understood and transformed into an effective policy. Planetary protection issues could be a sleeping giant that, once awakened, could dictate the future of Mars exploration. Astrobiology Workshop Attendees Acevedo, Sara SETI Institute Allamandola, Louis NASA Ames Research Center Anderson, Phillip Princeton University Arnold, James NASA Ames Research Center Bada, Jeffrey Scripps Institute of Oceanography Baldwin, Betty NASA Ames Research Center Baldwin, Ken University of California, Irvine Berry, William NASA Ames Research Center Bielitzki, Joseph NASA Ames Research Center Billingham, John SETI Institute Borucki, William NASA Ames Research Center Boyd, Jack NASA Ames Research Center Briggs, Geoff NASA Ames Research Center Brinton, Henry NASA Headquarters Brownlee, Donald University of Washington Bullock, Mark University of Colorado Bunch, Ted NASA Ames Research Center Cabrol, Nathalia National Research Council Carr, Michael US Geological Survey Cassen, Patrick NASA Ames Research Center Chahine, Mous Jet Propulsion Laboratory Chan, Roland NASA Ames Research Center Chandler, David Boston Globe Chang, Polly Stanford University Chang, Sherwood NASA Ames Research Center Chatfield, Robert NASA Ames Research Center Christiansen, Marvin Lockheed-Martin Engineering and Sciences Chyba, Christopher Princeton University Clark, Benjamin Lockheed-Martin Cohen, Malcolm NASA Ames Research Center Coleman, Paul Universities Space Research Associates Condon, Estelle NASA Ames Research Center Connell, Kathleen Aerospace States Association Corbin, Barbara NASA Ames Research Center Cordova, France University of California, Santa Barbara Coulter, Gary Colorado State University Cowing, Keith American Institute of Biological Sciences Cronin, John Arizona State University Cudaback, David University of California, Berkeley Cullers, Kent SETI Institute Cuzzi, Jeff NASA Ames Research Center D'Antoni, Hector NASA Ames Research Center David, Leonard Space News Davidson, Keay San Francisco Chronicle Deamer, David University of California, Santa Cruz Dean, William NASA Ames Research Center DesMarais, David NASA Ames Research Center DeVincenzi, Donald NASA Ames Research Center DeVore, Edna SETI Institute Doyle, Lawrence SETI Institute Drake, Frank SETI Institute Dressler, Alan Carnegie Observatories Duke, Michael Lunar and Planetary Institute Ehrlich, Anne Stanford University Elachi, Charles Jet Propulsion Laboratory Estes, Jack University of California, Santa Barbara Farmer, Jack NASA Ames Research Center Feldman, Lewis University of California, Berkeley Franklin, Lewis Stanford University Fuller, Charles University of California, Davis Ganapol, Barry University of Arizona Ganong, W. Francis University of California, San Francisco Gibson, Everett NASA Johnson Space Center Gore, Warren NASA Ames Research Center Greeley, Ronald Arizona State University Greenwood, MRC University of California, Santa Cruz Haberle, Robert NASA Ames Research Center Hargens, Alan NASA Ames Research Center Harper, Lynn NASA Ames Research Center Haynes, Norman Jet Propulsion Laboratory Hines, Michael NASA Ames Research Center Hollenbach, David NASA Ames Research Center Holley, Daniel San Jose State University Holton, Emily NASA Ames Research Center Hubbard, Scott NASA Ames Research Center Hutchison, Anne NASA Ames Research Center Kasting, James Penn State University Kauffman, Stuart Santa Fe Institute Kronenberg, Amy Lawrence Berkeley Laboratory Langhoff, Stephen NASA Ames Research Center Lederberg, Joshua Rockefeller University Levine, Ben Institute for Exercise and Environmental Medicine Levinthal, Elliot Stanford University Levison, Harold Southwest Research Institute Liang, Shoudan Penn State Unviersity Lissauer, Jack NASA Ames Research Center Lomax, Terri Oregon State University Luhmann, Janet University of California, Berkeley Lusignan, Bruce Stanford University Lyons, Richard Lockheed-Martin Engineering Services MacElroy, Robert NASA Ames Research Center Marcy, Geoffrey University of California, San Francisco Marlaire, Michael NASA Ames Research Center Martin, Frank Lockheed Martin May, James Center for Science, Technology and Information Resources McDonald, Henry NASA Ames Research Center McDonald, Ian Penn State University McKay, Christopher NASA Ames Research Center McKay, David NASA Johnson Space Center Mead, Susan NASA Ames Research Center Meyer, Michael NASA Headquarters Morrison, David NASA Ames Research Center Munechika, Ken Moffett Federal Air Field Nealson, Ken Center for Great Lakes Studies Nicogossian, Arnauld NASA Headquarters Nuth, Joseph Goddard Space Flight Center Pace, Norman University of California, Berkeley Parazynski, Scott NASA Johnson Space Center Parkinson, Bradford Stanford University Pendleton, Yvonne NASA Ames Research Center Penwarden, Michael Computer Life Magazine Pepin, Robert O. University of Minnesota Peterson, David NASA Ames Research Center Pierson, Tom SETI Institute Pilcher, Carl NASA Headquarters Pohorille, Andrew University of California, San Francisco Pool, Sam NASA Johnson Space Center Potter, Christopher NASA Ames Research Center Reightler, Ken Lockheed-Martin Rosen, Robert NASA Ames Research Center Ross, Muriel NASA Ames Research Center Rothschild, Lynn NASA Ames Research Center Rummel, John Marine Biological Laboratory, Woods Hole Russell, Phillip NASA Ames Research Center Sadeh, Willie Colorado State University Sandford, Scott NASA Ames Research Center Sargent, Anneila California Institute of Technology Schild, David Lawrence Berkeley National Laboratory Schulte, Mitch Washington University Shostak, Seth SETI Institute Shu, Frank University of California, Berkeley Skiles, Jay NASA Ames Research Center Sleep, Norman Stanford University Soffen, Jerry Goddard Space Flight Center Souza, Ken NASA Ames Research Center Sprigg, William National Research Council Stofan, Andrew Lockheed Martin Strawa, Tony NASA Ames Research Center Supulver, Kim University of California, Santa Cruz Tarter, Jill SETI Institute Toon, Brian University of Colorado Townsend, William NASA Headquarters Vandonzel, Judy NASA Ames Research Center Wade, Charles NASA Ames Research Center Weiler, Edward NASA Headquarters Welch, Jack University of California, Berkeley Wetherill, George Carnegie DTW Wiersema, Juliet SETI Institute Wolgemuth, Debra Columbia University Wong, Carla NASA Ames Research Center Woolum, Dotty California Institute of Technology Young, Lawrence Massachusetts Institute of Technology Zabor, Susan NASA Ames Research Center Zahnle, Kevin NASA Ames Research Center Zubrin, Robert Pioneer Astronautics