Starting with Percival Lowell's popularization of "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.
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.
A short-lived 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.Even more lifelike in appearance is the elongated chain of tapering magnetite crystals seen below, an arrangement known to be produced on Earth solely by certain bacteria. Some scientists hail this as convincing proof of martian life, formed very early in its history. David McKay and his cohorts at JSC have studied magnetite organisms on Earth and cite 6 criteria that associated with an organic mode of origin. They claim that all six are involved with the Allen Hills magnetite bodies.
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.
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, except for the dark skys, certainly reminds one of some Mars scenes:
In January of 2009, NASA confirmed and augmented evidence of methane (CH4) in the atmosphere over several martian regions. This gas was detected by the NASA Infrared Telescope on Mauna Kea in Hawaii. Methane could indicate organisms that provide it as a by-product of their metabolic action. But methane is also released by volcanoes. Thus, this result for the moment is ambiguous. Here is the methane map that shows the distribution of this gas:
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. Another argument against the presumption that life may have once existed on Mars is the discovery of parts of Mars containing layers of evaporites that formed from very acid waters - some scientists contend this acidity is too extreme for life to have even staarted. But parts of the martian surface (specifically, around the north polar region) have alkaline soils (page 20-13b). 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 left image (175 km; 108 miles) is of a section of Labyrinthus Noctus, a maze of interlocking grabens. The right 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.Here is an MOC view at 7 meters resolution of the wall of Candor Chasma in Valles Marineris that shows distinct layers.
The next pair of images present solid visual evidence for what appears to be significant stratification of bedrock units. The scene just below is so striking - almost artificial - that one might believe it to be a hoax. But cross-checking on the Internet verified its authenticity; it was imaged by the HiRISE camera on the Mars Reconnaissance Orbiter. The hills shown with inclined layers are in the Arabia Terra region of Mars. A close-up of the layers appears below the perspective view:
Many Mars investigators, including Dr. Michael Malin, assert views such as above as unequivocal indications for depositional layering from an aqueous medium such as water. Examine this view (MSSS) of another part of Candor Chasma:
The layers can be expressed in offset ledges, referred to as "stairs-and-mounts", as exemplified here:
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 two favored choices being either lake beds or volcanic ash deposits from cyclic eruptions. They 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:
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: