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

II.4 Team I: <strong>Exobiology</strong> and <strong>the</strong> Mars Surface
Environment
<strong>The</strong> task of Team I was to def<strong>in</strong>e <strong>the</strong> environment of Mars at <strong>the</strong> surface and shallow
depth, with a view to detect<strong>in</strong>g possible signatures of past life. ‘Signature’ denotes a
large spectrum of evidence from microscopic (bacterial to nanobacterial size) to
macroscopic fossils, geochemical <strong>in</strong>dicators of biological activity, and o<strong>the</strong>r
<strong>in</strong>dicators such as superparamagnetic gra<strong>in</strong>s. <strong>The</strong> evidence of past life might be
present at <strong>the</strong> surface but it will more likely occur below <strong>the</strong> wea<strong>the</strong>red (oxidised)
upper layer. <strong>The</strong> depth of this upper ‘barren’ layer can be only guessed and <strong>the</strong> only
way to determ<strong>in</strong>e its thickness or to construct a reliable model <strong>for</strong> it is to dig <strong>in</strong>to <strong>the</strong>
martian surface and collect geochemical and petrophysical data.
<strong>The</strong> approach adopted by Team I is based on requirements concern<strong>in</strong>g two k<strong>in</strong>ds
of evidence of past life: chemical signatures and bacterial fossils. However, a large
part of <strong>the</strong> discussion is also valid <strong>for</strong> <strong>the</strong> search of extant life.
In <strong>the</strong> past few years it has become clear that Mars is an extremely diverse planet
<strong>in</strong> geological terms. <strong>The</strong> first data from Mars Global Surveyor (MGS) support <strong>the</strong>
exist<strong>in</strong>g evidence of an extensive diversification of geological units, features and
crustal structures. This geological diversity is of paramount importance <strong>in</strong> <strong>the</strong>
exobiological exploration of <strong>the</strong> planet. This new approach is matched by <strong>the</strong> f<strong>in</strong>d<strong>in</strong>g
of <strong>for</strong>ms of life <strong>in</strong> a huge variety of geological sett<strong>in</strong>gs, environments and ecosystems
on Earth. It has been well known <strong>for</strong> several decades that bacterial <strong>for</strong>ms live <strong>in</strong> <strong>the</strong>
Earth’s crust at depths of several kilometres. Many <strong>for</strong>ms of life live near <strong>the</strong>
hydro<strong>the</strong>rmal vents <strong>in</strong> <strong>the</strong> deep oceans and, recently, large liv<strong>in</strong>g communities have
been discovered <strong>in</strong> deep-sea zones around <strong>the</strong> bodies of dead whales. It is now clear
that life is a strongly colonis<strong>in</strong>g factor which, matched with <strong>the</strong> martian geological
diversity, suggests that Mars is actually a place where life was probably present and,
just possibly, might still exist <strong>in</strong> some specialised niche.
<strong>The</strong> basic requirement <strong>for</strong> <strong>the</strong> <strong>for</strong>mation and survival of life is <strong>the</strong> presence of water
and energy (nutrients). This need not be true <strong>for</strong> an exotic <strong>for</strong>m of life, but <strong>in</strong> order to
establish <strong>the</strong> environmental constra<strong>in</strong>ts <strong>for</strong> <strong>the</strong> search <strong>for</strong> life on Mars it is better to
adopt a conservative approach. <strong>The</strong>re<strong>for</strong>e, <strong>the</strong> best candidates are those environments
where water has been present <strong>for</strong> a considerable time. From this po<strong>in</strong>t of view, <strong>the</strong>
geological environments should be regarded as any geological sett<strong>in</strong>g, regardless of
dimension and position, with def<strong>in</strong>ite boundary conditions. Thus, geological
environments would be obvious features such as lakes and rivers, but we must also
<strong>in</strong>clude pores, fractures, surface roughness, <strong>for</strong> example.
II.4.1.1 Lacustr<strong>in</strong>e Environments
Lakes are ubiquitous on Earth and display a tremendous variety of sett<strong>in</strong>gs, conditions,
shapes, etc. <strong>The</strong>y have also been recognised as hav<strong>in</strong>g existed on Mars. Martian lakes
can be def<strong>in</strong>ed as localised areas where water was present as stand<strong>in</strong>g bodies. Probably,
lakes were scattered on <strong>the</strong> surface <strong>in</strong> several geological sett<strong>in</strong>gs. However, <strong>the</strong>y are
easily recognisable when <strong>the</strong>y occur as water-filled impact craters. <strong>The</strong>re are several
<strong>in</strong>stances where channels enter large craters and, as frequently, exit on <strong>the</strong> opposite side.
<strong>The</strong>se crater floors probably consist of sediments <strong>for</strong>med <strong>in</strong> ponded water. In some cases,
<strong>the</strong>re are terraces l<strong>in</strong><strong>in</strong>g <strong>the</strong> crater <strong>in</strong>terior walls and <strong>the</strong>se may be <strong>in</strong>terpreted as erosional
shorel<strong>in</strong>es. Lacustr<strong>in</strong>e deposits of this k<strong>in</strong>d may be expected to consist of bedded coarsegra<strong>in</strong>ed
sand, gravel or cobbles near <strong>the</strong> bottom, grad<strong>in</strong>g downstream and upward to
more f<strong>in</strong>e-gra<strong>in</strong>ed material as <strong>the</strong> flow slowed and particles came out of suspension.
II.4.1 Environments and
Rocks with <strong>Exobiology</strong>
Potential
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