(This section is based on Mars Landing Site Catalog, p. 11-16 by J.D. Farmer, D.J. DesMarais and H.P. Klein. The catalog is NASA Reference Publication 1238, 2nd edition.)
Liquid water is fundamental for all forms of life on Earth and, we expect, on other worlds also. Thus the selection of martian landing sites best suited to seek evidence of past (or present) life draws heavily on interpretations by geologists of images and other information acquired from Mars orbit -- the identification of water-associated landforms and sedimentary deposits. For life to have developed on Mars, liquid water must have existed at or near the surface for a long enough time to have allowed complex biochemical systems to originate and evolve. Consequently, sites suitable for exobiology exploration all revolve around evidence of former aqueous environments.Exploring for extant life
The martian environment in the past may have been more Earth-like than today. The atmosphere may have been denser and warmer, and there may have been abundant surface water. Under such conditions, just as on our own planet, biological systems could have arisen. As the martian environment shifted toward the dry, cold conditions of today, life forms may have been unable to adapt and, thus, would have become extinct or would have been restricted to any oases that survived (within the data available to us today we do not see evidence of such oases but our data are still relatively crude in resolution). Thus, the search for extant life on Mars involves exploration for environmental oases where life may yet survive, i.e places were liquid water may still be present.
Given that liquid water is, as we believe, an essential requirement for life the past presence of liquid water on Mars becomes the central issue. One focus then is on the possible origin of life under conditions of an early clement climate during Noachian period before 3.8 billion years ago and its possible survival under the degrading conditions during the Hesperian and Amazonian periods.
Deposits Features
Fluvial Dendritic drainage networks : Simple Complex (higher order) Channel morphology : Widening downslope Meandering Flood plains Stream terraces Accessibility ; Layering visible Impact craters
Lacustrine Drainage basin fed by : Simple channels Complex channels Shoreline features : Lake terraces Deltas Accessibility : Lava flows Aeolian cover Impact craters
Thermal Spring Drainage system : Simple channels Point source Localized heat source : Surface (volcanic center) Subsurface (thermokarst)
Ground ice High latitude (>60∞) : (Surface ice) Laminated terrain Mid-latitude (30-60∞) : (Subsurface ice) Patterned ground Alases Pingos Fluidized crater ejecta
Future exploration of the martian subsurface by drilling may provide direct access to deep aquifers and hydrothermal systems (a real technology challenge!) allowing us to examine these environments directly. In the relatively near term, subsurface sampling and seismic profiling can provide the stratigraphic information needed to reconstruct a near-surface geological history of Mars, and to aid in the search for a fossil record.
Exploring for a fossil record
The oldest fossils on Earth are found in rocks dated at 3.5 billion years, a time not long after the intensive asteroidal bombardment phase of our planet's history was over. It is perhaps remarkable that life could evolve so quickly once conditions had become relatively benign and, in fact, suggests that life might have evolved in the same time period on Mars. Since we cannot explore all of Mars, we need to evolve a strategy that maximizes our chances of finding fossil evidence of past martian life. This is a task that the exobiology community has been carefully developing and documentingAs a particular example, hypothesized deposits from ancient martian thermal springs are high priority targets because of their potential 1) as locales for life to originate and 2) for preserving microbial fossils by silicification or calcification. In addition, in many arid lacustrine (lake-related) environments on Earth, microbial fossils are preserved in carbonates precipitated from sublacustrine springs, in intergranular cements, or in shoreline tufas.
In general, sedimentary deposits have the highest priority for martian fossil exploration, especially those with low permeability which have been neither deeply buried nor subjected to high thermal gradients. The best rocks for the retention of organics compounds are fine-grained lacustrine sediments such as shales and mudstones. Deposits from outflow channels have a lower priority in the search for fossils because of inferred rapid sedimentation rates. On the other hand, mature fluvial-lacustrine basins may represent relatively stable, long lived hydrological systems capable of preserving long-term sedimentary and fossil records.
Search strategies for detecting morphologic or chemical fossils on Mars must consider methods able to sample the subsurface. Many sedimentary deposits are probably covered by lava flows or other materials. Although drill cores may eventually sample such deposits, drilling to great depth will not be possible in the immediate future. Simple models for impact crater formation indicate that ejecta deposits provide surface access to former subsurface materials. Although ejecta may have been intensely shocked by the impact and/or weathered, the analysis of such ejecta offers an excellent opportunity to sample material excavated from significant depth.
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How do planetary scientists select interesting sites ?. How is orbital imaging able to differentiate between sites? Some of the tools they use are summarized in the tables available below. You can use them too when you study the Viking Images contained in the atlas and, soon, in new much higher resolution data returned by Mars Global Surveyor will send back to us.
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