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

large chaotic areas (that produced <strong>the</strong> large outflow channels) are created by this type
of mechanism.
Ow<strong>in</strong>g to <strong>the</strong> depth of <strong>the</strong> permafrost, <strong>the</strong> <strong>in</strong>vestigation of this potentially lifebear<strong>in</strong>g
environment will probably be postponed until <strong>the</strong> human exploration of Mars.
However, <strong>the</strong>re are areas at high latitudes where <strong>the</strong> permafrost is thought to exist at
very shallow depths. Moreover, steep and high cliffs occur <strong>in</strong> several areas of Mars,
and <strong>the</strong>y might expose a portion of <strong>the</strong> permafrost.
Current concepts <strong>for</strong> search<strong>in</strong>g <strong>for</strong> microbial fossils on Mars are strongly focused on
f<strong>in</strong>d<strong>in</strong>g <strong>the</strong> rema<strong>in</strong>s of <strong>for</strong>mer surface-bound microbial activity <strong>in</strong> sediments and hot
spr<strong>in</strong>g environments. While it has become accepted that subsurface life is presently
active on Earth to a depth of several km at some places (e.g. Stevens & McK<strong>in</strong>ley,
1995; Stevens 1997), barely any fossil evidence of this has been reported so far, with
<strong>the</strong> notable exception of a paper by Kretzschmar (1982), although <strong>the</strong>re have been
scattered reports <strong>in</strong> <strong>the</strong> literature s<strong>in</strong>ce 1782. Recent work has shown that subsurface
microbialites similar to those reported by Kretzschmar (1982) are common <strong>in</strong>
terrestrial samples from numerous subsurface environments (Hofmann & Farmer,
1997). This work is <strong>in</strong> progress and approximately 100 sites have so far been
identified. Geological environments conta<strong>in</strong><strong>in</strong>g such suspected fossil rema<strong>in</strong>s <strong>in</strong>clude
• sites of low-T hydro<strong>the</strong>rmal alteration of volcanic rocks (dom<strong>in</strong>ant);
• oxidation of sulphide ore deposits (quite common);
• hydro<strong>the</strong>rmal ve<strong>in</strong>-type ore deposits (rare);
• silica deposits associated with serpent<strong>in</strong>isation of ultra-basic rocks (rare).
<strong>The</strong> depth of emplacement of filamentous microstructures of probable microbial
orig<strong>in</strong> is unknown <strong>in</strong> most cases, but reaches 400-800 m <strong>in</strong> at least three of <strong>the</strong> betterstudied
sites. <strong>The</strong>re is a grow<strong>in</strong>g body of evidence that subsurface microbial activity
commonly results <strong>in</strong> macroscopic structures similar to surface microbialites
(stromatolites). Many of <strong>the</strong> hi<strong>the</strong>rto recognised possibly microbial structures <strong>in</strong> such
environments are enclosed <strong>in</strong> microcrystall<strong>in</strong>e silica varieties (chalcedony, agate).
<strong>The</strong> recent identification of agates as products of post-impact hydrous alteration <strong>in</strong> an
impact melt of a F<strong>in</strong>nish crater (K<strong>in</strong>nunen & L<strong>in</strong>dqvist, 1998) makes it likely that
similar silica varieties do exist on Mars, s<strong>in</strong>ce cool<strong>in</strong>g impact melts <strong>in</strong>teract<strong>in</strong>g with
groundwater must have been common. <strong>The</strong> likelihood of subsurface microbial fossils
on Mars is significant and a subsurface biosphere can be expected to be relatively
more important on Mars than on Earth because of <strong>the</strong> <strong>in</strong>hospitality of <strong>the</strong> martian
surface.
By consider<strong>in</strong>g not only <strong>the</strong> surface-bound but also <strong>the</strong> subsurface microbial fossils
<strong>in</strong> <strong>the</strong> process of evaluat<strong>in</strong>g land<strong>in</strong>g sites, target selection <strong>for</strong> sampl<strong>in</strong>g, imag<strong>in</strong>g etc
enlarges <strong>the</strong> selection of available possible <strong>in</strong>terest<strong>in</strong>g sites and materials. A fractured
or vesicular basalt, <strong>for</strong> example, would be considered of low <strong>in</strong>terest <strong>in</strong> exist<strong>in</strong>g
strategies focused on sediments and <strong>the</strong>rmal spr<strong>in</strong>g deposits, but may never<strong>the</strong>less
harbour rich subsurface fossil rema<strong>in</strong>s.
Reconstruct<strong>in</strong>g <strong>the</strong> climatic and environmental changes of Mars should be done
through a detailed reconstruction of its geological history. Water is <strong>the</strong> most important
factor among <strong>the</strong> variables that shaped <strong>the</strong> planet, and knowledge of <strong>the</strong> hydrological
cycle and long-term variations of <strong>the</strong> environment are based on geological analysis.
Of course, <strong>the</strong>re are serious disagreements on <strong>the</strong> reconstruction of <strong>the</strong> geologic
history but, basically, two models are proposed.
<strong>The</strong> first model proposes that <strong>the</strong> climate be<strong>for</strong>e 3.5 Gyr ago was warmer and
wetter. A thicker atmosphere was able to susta<strong>in</strong> a complex climatic and hydrological
cycle. In this scenario, ra<strong>in</strong>fall played an important role. Precipitations, <strong>in</strong> fact, were
able to degrade a large part of <strong>the</strong> geological features (mostly impact craters).
team I: exobiology and <strong>the</strong> mars surface environment/II.4
II.4.2 General Remarks
on Subsurface Microbial
Fossils
II.4.3 Climatic and
Environmental Models
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