Exobiology in the Solar System & The Search for Life on Mars - ESA

plate tectonic evidence has been observed on Mars, it is expected that legacies of such
compounds can be found <strong>in</strong> aqueous sedimentary deposits <strong>in</strong> ancient Martian cratered
terra<strong>in</strong> (NASA SP-530, 1995). Water be<strong>in</strong>g <strong>the</strong> major requirement <strong>for</strong> life, seek<strong>in</strong>g life
equates to search<strong>in</strong>g <strong>for</strong> traces of water activity (Greeley & Thomas, 1994).
As outl<strong>in</strong>ed earlier, <strong>the</strong> present conditions on Mars do not allow <strong>the</strong> presence of
liquid water on its surface. Today, liquid water may be found only beneath <strong>the</strong> surface,
where <strong>the</strong>re are higher temperatures and pressures. That means at a depth of more
than just a few metres and, hence, beyond <strong>the</strong> access of probes <strong>for</strong> some time to come.
Consequently, we could look today only <strong>for</strong> environments where ancient life might
have been preserved or <strong>for</strong> natural processes that might have brought extant
subsurface life up to more accessible depths.
II.2.7.1 Water and a Favourable Environment
With its current th<strong>in</strong> atmosphere giv<strong>in</strong>g no UV protection and a cold, dry climate,
Mars comb<strong>in</strong>es <strong>the</strong> most severe conditions <strong>for</strong> <strong>the</strong> <strong>in</strong>ception of life. However,
conditions were different <strong>in</strong> <strong>the</strong> past, and evidence <strong>for</strong> water activity and aqueous
sedimentary bas<strong>in</strong>s have been demonstrated (Cabrol & Gr<strong>in</strong>, 1995; Carr, 1996). <strong>The</strong>
duration of water activity and liquid habitats is also a critical issue (McKay & Davis,
1991). Though <strong>the</strong> conditions might have been more favourable dur<strong>in</strong>g <strong>the</strong> first
15 Gyr of martian history, <strong>the</strong>re is no evidence that large bas<strong>in</strong>s such as Elysium were
active dur<strong>in</strong>g <strong>the</strong> Amazonian era (Scott, 1995; Scott & Chapman, 1991). Even more
<strong>in</strong>trigu<strong>in</strong>g is <strong>the</strong> evidence of <strong>the</strong> activation of large runoff valleys, and development
of smaller-scale lakes <strong>in</strong> highland craters, such as <strong>in</strong> <strong>the</strong> Gusev and Gale craters
(Cabrol et al., 1996; Gr<strong>in</strong> & Cabrol, 1997) dur<strong>in</strong>g <strong>the</strong> same geological period.
Morphologic evidence show <strong>the</strong>se lakes to have existed over long periods and <strong>the</strong>y
may have provided oases <strong>for</strong> life more recently.
Fluvial valley networks and channels <strong>in</strong> <strong>the</strong> highlands and on volcano slopes,
ancient bas<strong>in</strong>s and lakes are, <strong>the</strong>n, <strong>the</strong> best evidence of a water-rich past. This
evidence, coupled with <strong>the</strong> discovery on Earth of microbiota able to survive <strong>in</strong> a
Mars-analogue environment (Friedman & Ocampo, 1976) and <strong>the</strong> better understand<strong>in</strong>g
of <strong>the</strong> early evolution of life on Earth, have allowed researchers to identify
candidate sites <strong>for</strong> exobiological exploration on Mars (Greeley & Thomas, 1994). <strong>The</strong>
selection of sites <strong>for</strong> exobiology is thus based on geologic and geomorphic evidence
of paleowaterflows, ponds, ice and hydro<strong>the</strong>rmal activity.
<strong>the</strong> planet mars/II.2
Fig. II.2.7/2. Frost deposit at <strong>the</strong> Vik<strong>in</strong>g-2
Lander location (48ºN/226ºW). This May 1979
image shows a coat<strong>in</strong>g of ice a few microns
thick. (NASA)
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