History/Background
How did the Earth and life arise? Is there life elsewhere? The tools and knowledge required for humans to answer these fundamental questions are various and complex. Our approach to bringing NASA closer to answering these questions uses molecules as historical records of the conditions in space when our Solar System formed, and as signs of life beyond Earth. Using organic chemistry as our unifying theme, this investigation spans the cosmochemical context for life, from the origin of chemical components in interstellar space, through their delivery to Earth (and other habitable worlds) by meteorites and cometary and asteroidal dust (IDPs), to the self assembly of the first membranes on the early Earth. We do this with lab work that includes measuring spectra for comparison to astronomical observations, studying chemical reactions under low temperature space conditions, and exploring the biophysics of simple meteoritic molecules that may have made up the first membranes. Our lab spectra will allow the identification of species in astronomical objects, and our chemical studies will advance our ability to detect potential biomarkers in extraterrestrial samples or on other planets and judge their significance.
Carbon forms in stars and is ejected at the end of the star's life. Organic compounds including polycyclic aromatic hydrocarbons (PAHs) form as a by-product of this stellar death. Following ejection, carbon-containing material disperses into the surrounding diffuse interstellar medium (ISM), where it is modified by different physical and chemical processes. Eventually, much of this material becomes concentrated in dense molecular clouds where (as described in the previous investigation) new stars and planetary systems form. In these dense molecular clouds these molecules are modified and new organic compounds form, some of which are of potential prebiotic interest. Thus, to study the molecules in these dense molecular clouds is to study the basic materials from which planetary systems are made. Through laboratory spectra we can interpret astronomical observations and identify materials from which planets and perhaps life itself was constructed.
The production of prebiotic molecules in the interstellar medium is of little consequence to the origin of, and search for, life unless these molecules can be delivered intact to habitable planets. This requires that they survive the transition from the dense cloud to protostellar nebula and subsequent incorporation into planetesimals, followed by delivery to a planetary surface (See Figure 1).
Figure 1 - We trace our chemical heritage, from the interstellar cloud that made the Solar System to the start of life on Earth. The molecules from which planets and life are composed originate in stellar atmospheres and the interstellar medium. They are modified upon incorporation into the solar nebula, and delivered to the surface of planets by meteorites and dust from asteroids and comets.
The early Earth swept up vast quantities of extraterrestrial organic matter, some of which was created in the interstellar medium. These exogenous molecules include many of prebiotic importance, including functionalized PAHs and amino acids, as well as simple membrane forming compounds. Perhaps exogenous materials did more than simply deliver carbon as a starting ingredient for the primordial soup. Rather, the specific molecules may matter, having properties that were of relevance to the rise of life on Earth (Figure 2).
Figure 2 - Delivery and potential prebiotic importance of amphiphiles. From Popular science article by Shawna Williams www.kcwrites.com/scicom/0301/origin/index.html
Detection of PAHs in Absorption
Polycyclic aromatic hydrocarbons (PAHs) and related aromatic materials are thought to be present in virtually all phases of the interstellar medium (ISM; Allamandola et al. 1999). Interstellar PAHs are most easily seen under conditions where gas-phase PAHs (both neutrals and ions) are excited by UV and visible light and cool through the emission of infrared (IR) photons, giving rise to the well-known emission spectra. Weak mid-infrared absorption features have been attributed to PAHs along lines of sight to a limited number of objects embedded in dense clouds. These include bands near 3030 cm -1 (3.3 microns; Smith et al. 1989; Sellgren et al. 1995; Brooke et al. 1999), and 1600 cm -1 (6.2 microns; Chiar et al. 2000), and 890 cm -1 (11.2 microns; Bregman et al. 2000). The PAHs in dense clouds are expected to be condensed onto dust grains, largely as neutral molecules frozen in H2O-rich ice mantles. Under these conditions, the PAHs will interact with each other and with other molecules. Because these interactions will perturb their infrared spectral properties (band positions, widths, profiles, and strengths), analyses of aromatic absorption bands in the spectra of dense clouds will require laboratory absorption spectra of appropriate analog materials taken under realistic astrophysical conditions.
PAHs Spectroscopy
In 2004 we published spectra of the smalled PAH, naphthalene in H2O at various concentrations and temperatures - Sandford, Bernstein, and Allamandola (2004) The mid-infrared laboratory spectra of naphthalene (C10H8) in solid H2O ApJ, 607, 346-360. We recently published an article presenting IR spectra of a number of other neutral polycyclic aromatic hydrocarbons (PAHs) in solid H2O at low temperature to facilitate the detection of PAH neutrals in dense clouds and on outer Solar System objects. The citation is: Bernstein, Sandford, and Allamandola, L. J. (2005) The Mid-Infrared Absorption Spectra of Neutral Polycyclic Aromatic Hydrocarbons in Conditions Relevant to Dense Interstellar Clouds. The Astrophysical Journal Supplement Series, Volume 161, Issue 1, pp. 53-64.
PANHs
In collaboration with Andrew Mattioda of the SETI institute and Charles Bauschlicher, Jr. of Ames we have made great progress on our work on aromatic nitrogen heterocycles (PANHs) - PAHs bearing a nitrogen in the ring structure in place of a carbon atom. These compounds are central to modern biology and are found in porphryns, nucleobases and other key biomolecules. They are present in meteorites (Stoks & Schwartz 1982; Basile et al. 1984; Pizzarello 2001; Sephton 2002), are predicted to be a component of Titan's haze (Ricca et al. 2001) and, as a class, are present in the interstellar medium.
We have just published a major astronomy paper on the presence of PANHs in space (Hudgins, Bauschlicher, and Allamandola (2005) The Peak Position of the 6.2 um PAH Emission Feature: A Tracer of N in the Interstellar PAH Population, The Astrophysical Journal, 632, 316-332, and we have published two experimental papers on PANHs: (1) Bernstein, M. P. ; Mattioda, A. L. ; Sandford, S. A. ; Hudgins, D. M. (2005) Laboratory Infrared Spectra of Polycyclic Aromatic Nitrogen Heterocycles: Quinoline and Phenanthridine in Solid Argon and H2O. The Astrophysical Journal, 626, 909-918, and (2) Bernstein, M. P. ; Sandford, S. A. ; Walker, R. L. , (2005) Laboratory IR spectra of 4-azachrysene in solid H2O Advances in Space Research Volume 36, 166-172. In addition, we have also published mid & near IR spectra of ionized PANHs in inert gas matrix: Mattioda, A. L.; Hudgins, D. M. ; Bauschlicher C. W. ; Allamandola, L. J. ; (2005) Infrared spectroscopy of matrix-isolated polycyclic aromatic compounds and their ions. 7. Phenazine, a dual substituted polycyclic aromatic nitrogen heterocycle. Advances in Space Research 36, 156-165.
Optical spectroscopy is a new tool to study processes occurring within H2O ice and have found the unexpected result that PAH ions are stable in cosmic ices at temperatures between 100 and 120 K, a very important regime for cosmic ices. (Gudipati, M. S. , and Allamandola, L. J. , (2003) Facile Generation and Storage of Polycyclic Aromatic Hydrocarbon Ions in Astrophysical Ices, ApJ. 596, L195-L198). We have since measured IR spectra of PAH captions in solid H2O at various temperatures, and are working on papers on that subject.
We have applied the IR lab spectra to the analysis of new data from NASA's Spitzer Space telescope. i.e. , we showed, in a letter to the Astrophysical Journal last year (E. Peeters, A. L. Mattioda, D. M. Hudgins, and L. J. Allamandola (2004) Polycyclic Aromatic Hydrocarbon Emission in the 15-21 Micron Region The Astrophysical Journal, 617: L66-L68. ), that the new complex of emission features near 17 microns in many objects arise from PAHs and the variations give unique PAH size and structure information.
Detection and Stability of Amino Acids
Nitriles (or cyanide compounds) are among the most commonly reported interstellar gas-phase organic molecules. Over a dozen different interstellar nitriles have been identified in dozens of sources through the detection of characteristic rotational transitions. For example, nitriles have been observed toward protostars (Kalenskii et al. 2000; Pankonin et al. 2001) and ultracompact H ii regions (Akeson & Carlstrom 1996), and have been used as indicators of gas temperature (Schwortz & Mangum 1999). Nitriles are commonly identified, in part, because of the relatively strong dipole moment imparted to these molecules by their CN group, making these species amenable to detection through their rotational spectra.
However, interstellar and circumstellar environments cannot be too inhospitable to these compounds, or they would not be detected so often. Nitriles are of relevance to astrobiology because they are putative intermediates from which prebiologically important organic acids (such as amino acids) are proposed to form, whether by a Strecker synthesis or by Michael addition (see Ehrenfreund et al. 2001b). Nitriles are thought to be the parents of many of the acids extracted from primitive meteorites. It is interesting to note that while nitriles are common in space and organic acids are scarce, the opposite is true in primitive meteorites.
In a paper in 2004 we presented laboratory showing that the photostability of acids is very much less than nitriles on exposure to ultraviolet (UV) photolysis. Bernstein, Ashbourn, Sandford, and Allamandola, (2004) The Lifetimes of Nitriles (CN) and Acids (COOH) during Ultraviolet Photolysis and their Survival in Space, ApJ 601, 365-370. We compared two structurally related acid-nitrile pairs: acetic acid (CH3COOH) versus acetonitrile (CH3CN) and glycine - the smallest amino acid (H2NCH2COOH) versus aminoacetonitrile (H2NCH2CN). The laboratory experiments showed that organic nitriles (cyanide compounds) survive 5 to 10 times longer on exposure to UV photolysis than do the corresponding acids. This is true in both solid H2O and argon matrices. Acids are therefore expected to be much less stable than nitriles to UV in space environments and, all else being equal, nitriles should be much more abundant in space than their associated acids.
This is interesting because there was a recent (if somewhat controversial) detection of glycine in the ISM (Kuan et al). Perhaps we should be looking for the nitrile precursor to glycine, amino aceto nitrile, it should be more abundant if destruction by radiation is a deciding factor.
The paucity and instability of organic acids in space relative to nitriles is at odds with their prevalence in extracts of carbonaceous meteorites (Cronin & Chang 1993). Indeed, not only do meteorites contain a fair proportion of amino acids, but these amino acids (or their precursors) are thought to have an interstellar heritage because they contain substantial deuterium enrichments (Kerridge 1999). However, nitriles can be easily converted to acids by heating in liquid H2O (hydrolysis). Liquid H2O is thought to have been at least briefly present in the asteroidal parent bodies of certain meteorites (Robert & Epstein 1982), and this hydrolysis is a step in various proposed parent body reactions that may lead to amino acids (Ehrenfreund et al 2001b). For example, the classic Strecker synthesis of the amino acid glycine is such a reaction, and this is perhaps the most popularly proposed pathway to meteoritic glycine. In this pathway to glycine aminoacetonitrile (one of the nitriles we have studied in this paper) forms first and then reacts with liquid H2O to form glycine. Rather than forming in the parent body by a Strecker-type reaction, perhaps some aminoacetonitrile was already present in space and was incorporated into the forming parent body. Thus, presolar nitriles may have been converted to acids by reacting with liquid H2O in asteroids or perhaps even comets. This is of relevance not only to amino acids, but also to other acids such as glyceric acid, found in meteorites.
See also Bernstein, Bauschlicher & Sandford (2004) The infrared spectrum of matrix isolated aminoacetonitrile, a precursor to the amino acid glycine Ad. Sp. Res. , 33, 40-43.
Astrobiology Roadmap Goals Served by this Investigation
GOAL 2 - Explore for past or present habitable environments, prebiotic chemistry and signs of life elsewhere in our Solar System. Determine any chemical precursors of life and any ancient habitable climates in the Solar System, and characterize any extinct life, potential habitats, and any extant life on Mars and in the outer Solar System.
GOAL 3 - Understand how life emerges from cosmic and planetary precursors. Perform observational, experimental and theoretical investigations to understand the general physical and chemical principles underlying the origins of life.
GOAL 7 - Determine how to recognize signatures of life on other worlds and on early Earth. Identify biosignatures that can reveal and characterize past or present life in ancient samples from Earth, extraterrestrial samples measured in situ, samples returned to Earth, remotely measured planetary atmospheres and surfaces, and other cosmic phenomena.
Ames Team Members Participating in this Investigation:
For additional information on this group's research, visit the following website: http://www.astrochem.org/
See the following Ames Team research pages:
Formation and Evolution of Habitable Planets
Prebiotic Organics from Space
Origin and Early Evolution of Proteins and Metabolism
Biosignatures in Chemosynthetic and Photosynthetic Systems
Modeling Ecosystems and Biospheres
Hind-Casting Past Environments
Interplanetary Pioneers
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