In the past ten years renewed interest in Mars exploration, in antipation (or hope) that it will become the next big planetary goal for NASA and the worldwide space community, has shed much new light on the Red Planet. The possibilities of significant amounts of water in the martian past, and some still extant even now, excite planetologists because H2O is a necessity for life. Some evidence of microscopic life forms in meteorites known to come from Mars is claimed but this still engenders strong debate. Further exploration of Mars has become almost a crusade for the NASA milieu and other martian devotees. This is underway with the successful Mars Global Surveyor and Mars Pathfinder programs, reviewed here.
Starting with Percival Lowell's "Canali on Mars", that planet has continued to fascinate both scientists and the general public. Readers unfamiliar with what is known about ancient life on Earth may want to work through the paragraphs on that subject in the middle of page 20-11 to familiarize with characteristic of primitive life forms. The importance of those landforms
with fluvial signs looked at on the previous page is that they suggest a probability that water did (does?) exist in
sufficient volume and concentration on Mars as an essential ingredient for the
inception of organic molecules. Carbon, the other vital constituent, was certainly
present as indicated by today's atmosphere. Whether these elements came from
the interior or from meteoritic matter, during the accretion phase or later,
is still uncertain.
The absence of any viable organics in samples analyzed at
the Viking sites does not disprove the possibility of biogenic forms in modern
Mars. The sampling may not have encountered organics at these two isolated sites but they dwell elsewhere.
Both sites were in younger terrains, so the nonoccurrence could mean that any
earlier life or unorganized organic matter had perished by then. However, one of the Viking investigators, Dr. Gilbert Levin, has always contended that there is indirect evidence of organic matter extractable from the measurements made onsite and transmitted to Earth. Before (or after) reading through this subsection, you may wish to look over these three Web sites:(1), (2), and (3). For the time being, an alternative approach would be to find meteorites on Earth that can contain strong proof that they came from Mars, being ejected through impacts into Earth-crossing orbits until a few pass onto the Earth. Such meteorites have certainly now been found. These form a group known as the SNC meteorites (consisting of the Shergottite, Nakhlite, and Chassignite types). A good summary of the SNC meteorites is found at this Internet site. That they come from Mars is supported by their compositional similarities to rocks examined during the Viking and the Mars Exploration Rover missions (see below). They compositionally are similar to dunites, lherzolites, clinopyroxenites, and basalts. Some show several shock features noted in basic igneous minerals, indicating that these were close to the impact points on Mars; others without shock effects represent samples derived further from these points. At least 12 such meteorites had been found prior to the 1980s and more recent searches place the number of probably martian meteorites now being studied at more than 40. This is a thin section that shows the characteristic minerals in a Chassignite: Several lines of evidence support the martian origin postulate: One is the similarity of composition between the SNC group and inferred rock types at the two landing sites on Mars described below. Examine this plot, which shows several SNC meteorites (in orange letters) falling within a compositional field notably different from the two common terrestrial basic igneous rocks: Other evidence includes isotope compositions that do not fit terrestrial rocks or Asteroid Belt meteorite rock types. A very strong proof is the almost identical composition of gases found in a Shergotty meteorite with gases sampled on Mars, as shown in this plot: As mentioned elsewhere in this Tutorial, the number of meteorites of all types and sources being found annually went up very significantly when by the late 1970s Dr. William Cassidy of the University of Pittsburgh, and then scientists from other institutions, determined that these "stones from heaven" could remain for long periods of time on the ice surface of the Antarctic. Many such rocks have since been collected. Included in these are some 35 SNC types, found at the locations shown in this map. The majority of these martian meteorites date as younger than 1.3 billion years, implying that martian volcanism has continued at least til then. The sensation in the scientific world
during 1996 was the claim that a meteorite from Mars, found in the Antarctic, contained evidence of
life. It is known as the Allen
Hills meteorite (ALH84001), which is characterized by containing orthopyroxene and has several features interpreted as consistent with organic structures found in some ancient
terrestrial rocks. So far, only this meteorite contains enticing (but not conclusive) signs of organisms but several others contain tantallizing objects of possible biogenic origin. Below is this Allen Hills meteorite (one other has also been collected there) first as it appears before being sawed into (the black coating is glass [fusion crust) and then after the rock was sawed in half, exposing its inner materials.
This fragmental rock contains dark materials, thought to
be old martian crust, that date from 4.5 billion years ago. Golden-orange
carbonate globules, dated at about 3.6 b.y., are considered evidence of a
primitive ocean or in another interpretation weathering of surficial rock through reaction with the CO2 in the atmosphere. Here is an electron micrscope image of two Mg-Fe carbonate balls found in this meteorite:
A thin section of ALH84001 also shows the orange (iron-enriched) carbonates and orthopyroxes (clear): Fossil-like bodies consisting of the iron oxide
magnetite and iron sulphides are present in the globules, as seen at high magnification under the electron microscope. Thus, the size of the elongate tubes falls between 1/100th and 1/1000th of a millimeter. If these are truly fossils, they are about a tenth the size of nanofossils found on Earth.
Keep in mind the subroundish features seen in these images. Similar structures are seen in rocks at the Meridiani site by the Mars Rover Opportunity. They are all likely to be inorganic in origin but some that approach spheres in shape are suggestive or tiny microfossils. Associated with the globules are small amounts of polycyclic
aromatic hydrocarbons (PAHs) that, while not necessarily biogenic, are interpreted by some as indigenous to Mars rather than contamination after Earth-arrival. Still, it is hard to consider this rock itself as the primary (initial) host of tiny life forms that originated within it during its formation. The rock is a igneous cumulate - enriched in pyroxene by crystal settling during magma differentiation - and thus is a totally alien matrix for life to develop within. The carbonate mineral is likely secondary in origin, formed perhaps by subsurface weathering as water percolated downward. If these strange features are true life forms, they almost certainly were introduced somehow from an external source - perhaps when a sea, lake, or other water body covered the rock before it was heaved off Mars during an impact. The great value of ALH84001 is that it contains provocative features that offer an incentive to look at other Mars rocks, either as meteorites or by trips to Mars, for similar features. Whether it really contains evidence of once-living biogenic matter, it has served as a major inducement to expand the exploration of Mars. Another Antarctic martian meteorite, EETA79001, contains carbonate (probably an alteration product but possibly evidence of martian sediments) and organic matter.
Needless to say, many skeptics have argued that this evidence is not persuasive;
some similar features found in Earth rocks have been shown to be inorganic. A recent report
now claims that these tiny forms are indeed primitive bacteria but very similar
to types still thriving on Earth; the implication is that this is just
terrestrial contamination. Another report claims that the researcher has found very similar features in terrestrial rocks that fail to show any signs of active life during their formation. A research group at Oregon State University, led by Prof. Martin Fisk, has presented evidence of microscopic tubes or tunnels in the Nakhla meteorite that are very much like similar features in terrestrial rocks that also contained bacteria. These features have been generally accepted as produced by rock-feeding bacteria. The Nakhla meteorite is a volcanic rock - one of the defining types in the SNC group. It fell to Earth in Egypt in 1911, and was quickly collected. Here are several sets of tunnels, all emanating from a fracture. David McKay of JSC has reported finding Iddingsite, a clay mineral derived from olivine, in the Nakhla meteorite. Associated with the mineral is a dark brown to black substance rich in carbon. Although the meaning of these Nakhla discoveries is being hotly debated, their presence encourages the Mars life devotees to redouble their efforts for convincing proof. However, the possibility that they
are genuine persists: if scientists eventually certify life on Mars, the Earth would no longer
retain its unique status as the living center of the Universe; although
it is a huge leap from microscopic primitive organisms to the intelligence
that then understands them. Suffice to comment that some scientists are touting
these presumptive meteorite "life forms" as the most pressing reason to formulate and
accelerate a major space effort to return to Mars for more detailed exploration.
19-45: What
would be the best proof of life on Mars? ANSWER
As the search for any hints of life on Mars continues with the 2004-6 exploration by the MER Rovers, described on the next two pages, attention back on Earth is being fixed around environments where levels of life (see page 20-12 for an overview of this topic as applies to planets in general) may be controlled by environments that may have counterparts on Mars. Extremophile environments like oceanic "black smokers" and the Antarctic both support life. The only place on Earth where almost no life whatsoever (including microbial) has been discovered is the central region of the Atacama Desert in northern Chile. This place will likely become a testing ground for life-searching robotic equipment and life form-identifying software (NASA ASTEP program) that will apply techniques for pattern recognition of morphological objects in the martian rocks of a suspicious nature as conceivable evidence of organisms. Here is a view of hills in the Atacama which certainly reminds one of some Mars scenes: As we shall see in the next pages on Mars, evidence is mounting that water existed and still exists on Mars. Thus one of the essential needs for life seems to have been present. But a study by CalTech/MIT professors of the Argon in the above meteorites indicates that the high retention rate of this radiogenic end product implies low temperatures at the surface of Mars, persisting for most of its lifetime. This would preclude any long term buildup of large bodies of water, in streams, lakes, or shallow seas. For now, the question remains open, as the more recent exploration of Mars has revealed conditions that could indicate standing and flowing water at some time in the past. After a 20-year hiatus in Mars
exploration, the quest for onsite information about our red neighbor has resumed. Several of these are described at JPL's Missions sites (Current comes up, check also Past and Future).
First up was a launch in 1993, of the Mars Observer, a billion-dollar spacecraft,
which, regrettably, failed enroute. The Mars Global Surveyor (MGS) followed on November
7, 1996, with its operation directed by JPL. The spacecraft arrived for orbital
insertion on September 12, 1997. In December of 1996 the Pathfinder mission
was underway, to land a roving vehicle capable of viewing the local terrain
and making sophisticated measurements. An orbiting spacecraft, whose task is
mainly climatic measurements, was launched in mid-December of 1998. A brief synopsis of the Mars Global Surveyor mission is given at JPL's movie site. Access through the JPL Video Site, then the pathway Format-->Video -->Search to bring up the list that includes "Mars Global Surveyor Across the Centuries", April 17, 2003 (note: this is an hour-plus long lecture). To start it, once found, click on the blue RealVideo link. If you don't visit this video site but are curious as to what the MGS looks like, here is an artist's sketch of the deployed spacecraft: From an initial orbital altitude
of 400 km (249 mi), the Global Surveyor used the aerobraking technique, which
involved progressive slowing through air friction (drag) with the thin atmosphere,
until, after six months, the spacecraft lowered to 110 km (68 mi). From there,
using thrusters to maintain this altitude, a mapping mission began in late-March
of 1998 to last a minimum of 687 Earth days. A second lowering was executed in
1999. The Mars Global Surveyor has a wide- and
narrow-angle camera system (MOC or Mars Orbiting Camera; see next page), a Thermal Emission Spectrometer (TES), a Laser Altimeter (MOLA), a Magnetometer/Electron
Reflectometer, and two other instruments. Among the first surface images, taken
from the higher altitude in early October of 1997, are these two examples of
the many views from the mission. The top image (175 km; 108 miles) is of a section of Labyrinthus Noctus, a maze of interlocking grabens. The bottom image is 12 by 12 km; it shows a canyon wall in an unidentified area of Mars. This next image is not a computer-generated perspective of the bottom (or right) image above, but was taken from MGS when its MOC was tilted 25° towards the horizon so that the view of the canyon walls is an actual scene with a 12 meter resolution, even though the target view is 1600 km (1000 miles) away. The Mars Orbiter Camera (MOC) can
take pictures with resolutions as good as 3 m (9.8 ft). This resolution will improve
slightly over time, as the orbital height lessens, because of the aerobraking
effect. A very large number of these images can now be viewed online at the Malin Space Sciences System site - this is certainly worth visiting now that they are putting up a new image each day. A number of the images shown on this and the two succeeding pages are credited to NASA/JPL/Malin Space Sciences Systems (referenced by MSSS).
The next view (supplied at the MSSS web site), taken after the orbit was lowered to the altitude chosen
for the first major scientific measurements, shows details of the terrain
surrounding an elongated volcanic caldera, 2 km in length in the Tempe-Marcotis
Fossae region. Almost touching it is a round crater that may be impact in
origin. The surface surrounding these two features is marked by small dunelike
features probably built up by wind action For life to have ever existed on Mars, the best environment to expect evidence or trace of primitive life forms would be sedimentary rocks, primarily in layers, which would suggest some type of water action (although layering from volcanism or from wind deposition of loose surficial grains could also produce depositional layers). MGS's MOC has been especially valuable in imaging layering of the bedrock of Mars (presumably either lake sediments or dust beds or
volcanic flows) that has been found at many martian locations. This figure shows eight widely separated localities where layering is observed:
19-46: What
can you say about the layering seen along the left facing cliff in the right
center of the image? ANSWER
Many Mars investigators, including Dr. Michael Malin, assert views such as above as unequivocal evidence for depositional layering. Examine this view (MSSS) of another part of Candor Chasma: The Valles Marineris canyon and trough system is very well suited to the recognition and characterization of thick sequences of martian layering because, like the Grand Canyon of Arizona, it has cut so deeply into the upper martian crust. Thick layers are especially well-exposed in the tributary canyons of Valles Marineris:
Layering can show cross-bedding even when viewed from above, as evident in this example from a Mars Reconnaissance Orbiter (MRO) image of a Valles Marineris tributary valley: High resolution MRO images are providing new details about martian layering:
Here is another example from MGS MOC imagery, showing both (presumed sedimentary) layering and linear features that are likely dune deposits, here in the walls of Melas Chasma, a tributary canyon to Valles Marineris: Note that the so-called layers are arranged stepwise in terrace or benchlike patterns in part of the image. Such offset "layering", if this it be, is sometimes found associated with mesas in the horizontal rocks of the Colorado Plateau. In the butte-like prominence at the center right of the image, the layers are well exposed along the steep sides. Two more MOC examples further support the presumption that this layering is sedimentary in nature; the precise mode of deposition is still being debated, with the two favored choices being either lake beds or volcanic ash deposits from cyclic eruptions. This image occurs in the Juventae Chasma region:
The next pair of images show layering in the wall and the floor of Becquerel Crater in West Terra Arabia: Units that are almost certainly layers have been exposed in craters located in the West Arabia Terra as seen in these MOC images. The top image shows layers that appear to have filled an earlier-formed crater. The layers have since been partially sculpted out as an erosional depression has formed.
In rare instances, the layers can show visible offsets, indicating faulting occurs within them. This image of layers in an impact crater in Arabia Terra reveals subtle but detectable fault lines: In the color images below showing part of a larger crater in West Arabia Terra, the layers appear inclined or dipping; but in an image from the crater center they look more horizontal, as expected if an impact crater hits on a sequence of non-inclined beds, with the dips then caused by the cratering action (on Earth craters [e.g., Meteor Crater in Arizona] in horizontal bedrock will show strongly inclined and even overturned layers outward towards the rim). The left image is near the rim top. The right image is further down the crater walls and contains black deposits which could be shock-melted rock or black basalt sand brought in by the wind.
Eroded terrain, with horizontal beds, very similar to the WTA crater has been imaged by MOC in the floor of West Candor Chasma, as seen in this black and white image: The West Candor Chasma is a subsidiary canyon tied to Valles Marineris. This Viking image shows the rugged nature of the interior terrain: One of the largest features on Mars is the Hellas Basin. It too contains areas where prominent layering is observed, as at Aeull Valley, shown here: As seen from the above images, direct visual signs of layering are widespread on Mars. Some is caused by deposition of dust/fine sand layers in annual ice deposits in the polar regions, and is therefore presumed young. However, much older layers of varied nature - mainly basalt and sedimentary lake-or-ocean deposits - occur over wide areas of the martian terrains. In the MOC image below, layers are standing on end - nearly vertical - within the Oudemans crater near Valles Marineris. In appearance these look similar to those found at the MERS Opportunity Meridiani site (page 19-13b) and are probably sedimentary units that were shoved up in the central peak that developed during the actual cratering. An area in the Meridiani region is interesting because the light sedimentary units are topped by a dark unit (basalt?) which is now partially removed. The general setting is shown first; below it the light units are displayed in an enlargement to show the ripple and polygon features that may be primary sedimentary structures: The MOC onboard the Mars Global Surveyor has been returning some provocative high resolution images that, after much interpretive debating, many geoscientists now believe offers supporting evidence of either water escape in the recent past or even indications of water still emerging. This MOC view below (5 m resolution) indicates channels coming from material in or below the edge of the southern ice cap. The stream outflow, if that is a valid identification, in each rill ends up with a small fan of deposits at its terminus - again, a feature consistent with fluid transport. Gullies, or rills, also are noted in the walls of large craters, such as seen below. These may be caused by extended outflow of warmed subsurface ice which upon melting flows out at the walls. Other examples (second image) are considered by some to be streaks of dust avalanches, in which surface dust is mobilized, perhaps aided by water flow: This Mars Global Surveyor image shows something unusual - what appears on the right to be a wedge of frozen ice cascading downslope. This has been interpreted to indicate that water moves at shallow subsurface depths and here extrudes at a canyon wall where is freezes (much like the ice sheets seen in roadcuts during winter). If so, this further supports the argument that the rills or gullies also seen in this image are water-generated. A MGS MOC image analysis team has accumulated visual evidence that some active process - the best "guess" favors running water - is taking place today on Mars. Before-and-after images show light-toned deposits along gullied crater walls that were definitely not there on the earlier date: A color view of the Centauri Montes crater deposit indicates that it is much like similar deposits caused by water on Earth: Early surmise held these deposits to have formed by release of liquid water. However, modeling has shown a much likelier explanation is formation by landslides that release dry powdered soil whose tone is less dark than the thin surface layer. Water seems to exist on Mars in four plausible settings: 1) in small amounts in the atmosphere; 2) in polar ice caps; 3) in subsurface ice; and 4) as yet unproved, liquid water probably derived from melting of that ice along crater walls; even if so released, this water does not survive long in the fluid state but evaporates and/or freezes. The next three images (NASA JPL and Malin Space Science Systems [MSSS]) demonstrate how planetary geologists go about interpreting and reasoning to conclusions using imagery returned from martian satellites and probes. The argument to be devised uses Martian Global Surveyor data to examine the region illustrated in this topographic map: The Cerberus Fossa (channel) includes this 100 m wide, 10 m deep straight feature that, by comparison to similar areas on Mars, is almost certainly a tectonic graben, bounded by faults on either side expressed as steep walls. Emanating from the upper wall is what appears to be lava flows spread over both the flat land beyond the fault and into the graben floor. Water is presumed to have excaped from the Cerberus Fossa during, or at different times from, periods of volcanic activity. The amount of water needed to develop the valley is estimated to be about that found in Lake Erie today. Based on crater counts and other data, geologists believe that the principal valley-forming events may be as recent as 10 million years ago. 19-48: What
causes the numerous indentations found in several of the dark layers?
ANSWER The Athabasca Valles extends southwest of the Fossa. It resembles a stream-cut valley similar to some on Earth. Within it is strong evidence of stream flow in the form of the streamlined, tear-drop shaped flatlands within it (see above on this page), such as shown here: The Ma'adim Vallis (white arrow) is a 2.1 km (6900 ft) deep cut into the Southern Highlands that runs from a topographically lower area to the Gusev crater (see illustration on previous page). H. Irwin III, G. Franz of the National Air and Space Museum (NASM) and others have published evidence that the low area was once filled with water in several lakes whose extent, if on Earth, would extend across Texas and New Mexico. Their starting point is the low dark areas present in the MGS MOC image of this region. Using MOLA data to determine elevations, they have reconstructed these lakes (not now existing) by coloring the topographic lows blue against a color image of the region: In their model, at some stage in Mars' past, the water from the central lake burst from its confines and rushed through the already forming valley, thus deepening it rapidly (analogous to the Scablands of the State of Washington; see page 17-4). A new line of evidence for water and/or ice on Mars has come from MGS images of what is considered to be analogs to phreatomagmatic "rootless" volcanoes on Earth (especially in Iceland). Consider this image (MSSS) of a swarm of cones in the Cerberus Plains near the martian equator: The origin of these miniature cones (typically, about 50 meters at their base) requires water to have been released at earlier times that becomes trapped near the surface (perhaps at depths of 3-5 meters). At the cold martian temperatures, this water is preserved as ice. Then, later eruption(s) of volcanic flows heat the water to steam, causing a pressure buildup that ejects material locally to build up the cones. The implication of the presence of these cones on what may be a young surface is that ice entrapment may occur within suitable martian terrains and could thus be available as a source of water during human exploration of this planet. Some Mars scientists are arguing for the presence of water as ice and/or "snow" deposits that form both varying and persistent deposits in areas of lower latitudes. This is related to the migration of water vapor and CO2 during the changing martian seasons. Perhaps the most provocative indicator of subsurface water that outflows in non-polar regions of Mars is shown in this next illustration: This feature has been given two interpretations: 1) it is an ice glacier (note the parallel ridges); or 2) it is a rock avalanche. Its geomorphic expression as seen from above seems better fitted to the first explanation. The Laser Altimeter (MOLA) on the Mars Global Surveyor yields
maps and profiles that show martian relief. The southern hemisphere is, on average,
10 km (6 miles) higher than the younger northern hemisphere. This is brought home by this block diagram that shows variations in topography alone the 0° latitude running from the North Pole (left) to the South Pole:
The maximum relief
on Mars ranges to 30 km (19 miles), determined by Valles Marineris as the high point and the Hellas Basin the low point, about 1.5 times greater than that on Earth. The three white round areas are the Tharsis volcanoes and the offset white one is to the left. The brown rounded area above these is Alba Patera. Valles Marineris is obvious. The blue-green circular feature below is Argyre Planitia. The large elliptical blue region
in the red southern hemisphere is a huge topographic depression about 2100 kilometers (1300 miles) wide known as the Hellas Basin.
One of the most dramatic of these
MOLA data plots presents an exaggerated profile across Olympus Mons and Arsia
Mons/Alba Patera (which is actually higher than Olympus).
Life on Mars?
Resumption of Martian Exploration
Here is an MOC view at 7 meters resolution of the wall of Candor Chasma in Valles Marineris that shows distinct layers. 
."> 
19-47: Compare the relief (difference in elevation) between Alba Patera and Olympus Mons. ANSWER
Still another example of a MOLA-based topographic map is that centered on the Herschel crater east of Hesperia Planum:
The topographic map of Mars, combined with gravity and other data, allow calculation of the general thickness of the martian crust, as shown in this global map:
Thicknesses of the crust at major locations are estimated as shown in this Table: Note the fairly strong correlation between topography and thickness, emphasized here by using the same scheme of reds/orange for higher elevation/greater thickness and blues for lowest elevations/minimal thickness in the two relevant maps. Another exciting and informative
scene taken by the Global Surveyor combines a color view of the North Pole
Ice Cap with topographic data obtained from a series of Laser Altimeter passes
to produce this three-dimensional view: The size and amount of ice (which
the spectrometer confirmed was largely water ice) was less than anticipated
from pre-Surveyor observations. The cap is roughly 1,200 km (746 mi) across
(crudely circular) and as much as 3 km (1.9 mi) thick. With an average thickness
of 1 km (0.62 miles), the volume of ice is about 1.2 million cubic kilometers
(288,000 cubic miles), roughly half that of the Greenland Ice Cap on Earth.
The cap's general surface is quite smooth, but the ice cap is cut by large,
deep (up to 1 km; 3,280 ft) steep-walled canyons and troughs, which may have
come from cracks enlarged by wind and possibly meltwater. The amount of water
located today at the North Pole, and lesser quantities now at the South Pole, does
not seem sufficient to have once made an ocean over parts of Mars. It instead
may be the remnants of any seawater that could have existed in the Mars past. Any relationship of this surviving water to the river-like channels on Mars is still speculative. Water may be widespread even now but would be subsurface. Detecting that water is one of the objectives of future Mars missions. MGS's Thermal Emission Spectrometer (TES) can produce images similar to those made using visible light (as an example, check the image of Ma'adim Vallis on the previous page). Other THEMIS (longer) IR wavelengths form images which clearly differentiate hotter (whitish) from cooler (dark) surface materials. This image shows a thermal pattern in which the blackish part indicates a smooth, dust-covered surface and the light blotches represent blocks and debris in some cases associated with craters: When TES surveys the same scene during martian daylight and again at martian nighttime, the differences in tone reveal information about the thermal inertia of the materials involved. Here are two view of a martian crater. The ejecta blanket held more heat during the day and lost more during the night, owing to its porous nature. The surrounding rock/soil remained warmer during the night. TES can identify some individual minerals and determine in a semi-quantitative way their proportions in the surface rocks (believed to be primarily basalt and andesites). The mineral Pyroxene (an iron-magnesium silicate) occurs in rocks found in Syrtis Major. Note its variable distribution, with the largest amounts
present in the darker areas of this region.
19-49: Account for the area that shows the highest pyroxene content; why? ANSWER
MGS sheds new light on the famous "face" on Mars first noted in a Viking image and seized upon by zealots favoring a lost civilization on the Red Planet as an artifact carved (like the Sphinx) into bedrock. The left view is the Viking image; the center (MGS) view shows the feature to have several mini-mountain peaks partly responsible for the fortuitous shadowing that highlights the face; the right view is a negative of the center view that recreates the shadowing.
A more recent image of the "face", taken by Mars Odyssey (see page 19-13a) reveals an even more uncanny likeness to a face:
In this image, the rock promontory that looks like a nose even has a "nostril". Surrounding the "face" is a blanket of material that resembles hair. Judge for yourself what this feature might really be.
Perhaps another view will help in your surmise. This is a perspective image made from Mars Express data:
This view indicates the face to be an eroded mountain. The term "massif" applies to this and similar features. When seen as a vertical image, the height of such a massif is not particularly impressive. But when seen in perspective, this topographic type comes across as a dominating landmark. Consider these two Mars Express images of Ausonia Massif; the first looking straight down; the second as a perspective view:
The MGS on Odyssey had an instrument capable of detecting weak magnetic fields. Earlier spacecraft suggested that one exists. MGS provided this data set shown as three maps of the field strengths in different directions:
It is still somewhat uncertain whether this is an active magnetic field (implying some surviving liquidity of the core, within which magnetic fields are generated) or is a fossil (remanent) field. Opinion currently favors the latter; there are strong negative fields associated with the Argyre and Hellas Basins, which suggests an already frozen field lost strength from the impacts that caused these two structures. If true, then the protective cover afforded by a magnetic field that captures solar wind and other radiation did not last long. But prior to that an active field would help to maintain a thicker atmosphere which in turn might have supported water vapor that could produce rain, streams, and primitive life.
An April 1999 report by Mars Global Surveyor investigators has created a flurry of excitement about an aspect of the martian crust which may be a counterpart to the mobile segments of terrestrial crust that are involved in the concept of Plate Tectonics - a cornerstone in our understanding of the operation of the Earth's outer layers embodied in the idea of "continental drift". The Mars Global Surveyor magnetometer has picked out a series of magnetic stripes on Mars that systematically reverse their polarities. Thus:
As more data accumulated, a nearly complete map of magnetic strengths has been made. Again, the stripes are the most obvious feature. One interpretation is that they are related to a time in the past when Mars had some mode of plate tectonics, with crust forming, rising, and subducting.
These martian strips are found mainly in the older terrains of the Southern Hemisphere. They trend at high angles to the polar axis. The implication is that this crust was formed by outpourings of lava from analogs to the terrestrial oceanic ridges that push the plates towards subduction zones (no such features have yet been recognized on Mars nor are there changes of mountains that result above these zones). This also suggests a much stronger magnetic field during early martian times; this would indicate a (partially) molten core. The full meaning of this new discovery is yet to be assessed.
Martian exploration reached a new plateau when landers allowed on site close-up inspection of martian terrain and rocks. Launched on December 4, 1996, JPL's Pathfinder landed on July 4, 1997, in the Ares Valle near the earlier Viking 1 site . This shorter transit time (seven months) resulted from a better alignment of Mars and Earth in their orbits The primary scientific purpose of Pathfinder was to release the small Sojourner Rover, guided from Earth through the lander's communication system. The Rover moved independently (after departing the lander down a ramp) up to 20 m away on its six, flexible, rocker wheels, to take pictures and bring its spectrometer against rock surfaces for analysis.
This microrover, which weighed 11.5 kg (25.4 lbs), measured the following: length = 630 mm (24.8 in), width = 480 mm (18.9 in), and height = 280 mm (11.0 in). Using power from a solar panel, Sojourner moved at a maximum speed of 40 mm/min (0.13 ft/min) during the martian day. Its front and back wheels moved independently to control its steering. The microrover used a stereo camera, mounted at its front, which imaged objects in its path. Each excursion was intermittent, because operators on Earth determined direction changes to reach target rocks and avoid obstacles. This took extended time periods because of the long distances radio and video signals traveled back and forth. When it encountered small rocks, the wheels raised up to glide over the obstruction.
The stereo camera system on the Pathfinder took images of the surface around the lander, obtaining black and white views and colored ones with color filters. Sojourner's camera also collected images (mainly rock close-ups). In the three images below, the first shows part of the scene viewed by the camera, the second is a montage of part of the surrounding site, in which we identify (using friendly names that speak a personal touch) some of the rocks that it visited, and the third is a panorama of the 360° view around Sojourner.
Many of the rocks are grayish on fresher surfaces. These surfaces appear weathered and pitted, and often are, at least partly, dust-covered. A typical large rock is Yogi (shown below), which appears to have a reddish dust blanket on one side and a sharp boundary with fresher rock (right side).
The primary instrument on the microrover is the alpha-proton-x-ray spectrometer (APXS). This instrument uses high speed alpha particles to bombard soil or rock surfaces. These particles generate a back scattering of alpha particles, alpha-proton particles, and x-rays, whose energies as determined at the detectors are indicative of a wide number of elements. It thus detects, quantitatively, elements such as those in rocks, i.e., Si, Al, Ca, Fe, Mg, K, Na, Ti, Mn, Cl, S, P, O, and C. It can report these elements in their elemental form or as oxides. The diagram below is a histogram of many of these elements from Barnacle Bill, Scooby Doo, Yogi, and some soil sites. Barnacle Bill's composition is close to that determined earlier at the Viking site but the other rocks and soils are different.
The next graph (top), is a plot of values of elemental ratios of Fe/Si and Ca/Si from the Pathfinder site, compared with certain Earth rocks. The Mars rocks at this site are definitely not basalt. They plot closer to but not within the compositional field of the terrestrial volcanic rocks known as Andesites. When we recalculate the elemental composition of the Pathfinder rocks into what petrologists term normative rock compositions, we see that the mineralogy is similar to andesites (bottom diagram).
But the presence of normative quartz (leftover silica, after we assign all other SiO2 to the other minerals), technically makes this rock type a dacite. Andesites and dacites are the common volcanic rocks emitted from continental volcanoes, such as those along the Ring of Fire (e.g., the Pacific Coast Cascades). The presence of these rock types at this martian site, if typical of other sites, suggests that the Red Planet had melted and differentiated, so that at least part of its crust is andesitic. Paul Lowman (author of Section 12 of this Tutorial) had postulated more than a decade ago that an andesitic primordial crust probably developed early in Earth history and may be the norm for other inner planets.
19-50: Which rock should be darker: Barnacle Bill or Yogi? ANSWER
Most of the rocks plot in the basaltic and andesitic fields of a diagram which shows the classification of basic to silicic igneous (volcanic) rocks found on Earth in terms of NasO + K2O versus Sio2:
The global map below uses MGS Thermal Emission Spectrometer data to determine the distribution of lavas of more andesitic composition on Mars. These tend to be most prominent in the younger surfaces of the martian northern hemisphere, at higher latitudes.
Interpretation of MGS and Pathfinder data, supplemented by that acquired later by Mars Odyssey (see below), has led to this general map of the surface rocks and sediment cover on Mars. Basalts, overall the most common of the martian surface volcanic rocks, are shown in green and yellow; andesites in blue, and regions where dust cover is so thick that data on underlying bedrock could not be recorded appear in medium tan:
Some individual volcanoes appear to produce lava outpourings of different compositions from time to time, suggesting differentiation in the magma chamber. A good example is Nili Patera, in Syrtis Major, which produces both basaltic (blue) and andesitic (red) lavas as determined from MGS data; some of the latter may actually be dacites (andesitic but with quartz); one or more small peaks within the scene may have been further enriched in silica to gain a granitic composition:
The occurrence of andesites leads to several postulates: 1) these rocks are evidence of some differentiation in the martian mantle; 2) they may represent rocks involved in subcrustal release of water; or 3) they are anomalous local thin veneers on the dominant basalts caused by surface altering processes. No definitive conclusion has been agreed upon.
So far, martian Landers have gathered compositional data at five widely-separated martian sites (Pathfinder; the the two Vikings; the two MERS rovers.). In general, bedrock at the sites are similar in their chemical makeup. This may simply mean that the martian crust tends to be homogeneous, but even more likely is the explanation that martian wind circulation has scattered and homogenized compositional differences across the planet. The distribution of rocks and boulders at the Pathfinder site further supports earlier ideas that at one time in the Mars' past there was considerable water cover and resultant flow currents, causing a pattern sometimes observed on Earth as flood-related.
The last Pathfinder image we show below is a classic sunset picture, taken through the thin, dusty, martian atmosphere that produced a fan-shaped glow around the distant Sun.
The martian exploration program since the opening of the next millenium (we accept that time start as January 1, 2000) is presented on the next pages (19-13a and 19-13b).
Primary Author: Nicholas M.
Short, Sr.