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

morphological and biochemical signatures of extraterrestrial life: utility of terrestrial analogues/I.4
and radiation environment, as well as organic molecules <strong>in</strong> sediments. <strong>The</strong> search <strong>for</strong>
possible biological oases will be connected with <strong>the</strong> detection of areas where liquid
water still exists under <strong>the</strong> current conditions on that planet. By analogy with
terrestrial ecosystems, potential protected niches have been postulated <strong>for</strong> Mars, such
as sulphur-rich subsurface areas <strong>for</strong> chemoautotrophic communities, rocks <strong>for</strong>
endolithic communities, permafrost regions, hydro<strong>the</strong>rmal vents, soil and evaporite
crystals.
Much <strong>in</strong><strong>for</strong>mation can be obta<strong>in</strong>ed from remote-sens<strong>in</strong>g global measurements,
such as <strong>the</strong> seasonal atmospheric and surface water distribution, <strong>the</strong> m<strong>in</strong>eralogical
<strong>in</strong>ventory and distribution, geomorphologic features obta<strong>in</strong>ed with high spatial
resolution, <strong>the</strong>rmal mapp<strong>in</strong>g of potential volcanic regions to determ<strong>in</strong>e possible
geo<strong>the</strong>rmally active sites, and trace gases such as H 2, H 2S, CH 4, SOx, and NOx.
It is only after identify<strong>in</strong>g sites that are suitable environments <strong>for</strong> a potential
martian biota that landers are required <strong>for</strong> more detailed characterisations. <strong>The</strong>n
analyses of <strong>the</strong> elemental and isotopic compositions, of m<strong>in</strong>eral composition and
structure, and of organic compounds can follow. Concern<strong>in</strong>g Mars, it is also very
important to explore and understand <strong>the</strong> strong oxidative processes on <strong>the</strong> surface.
If <strong>the</strong> <strong>in</strong> situ <strong>in</strong>vestigations confirm that <strong>the</strong> sites constitute potential biological
oases, detailed searches <strong>for</strong> direct and <strong>in</strong>direct biomarkers should follow. <strong>The</strong> search
protocol, i.e. what methods to apply, depend largely on <strong>the</strong> environmental conditions
offered by <strong>the</strong> sites. Alternatively, <strong>the</strong> analysis of samples collected and returned to
Earth should be considered. This would allow much more crucial tests <strong>for</strong> extant
biology <strong>in</strong> ground-based laboratories. However, <strong>in</strong> <strong>the</strong> latter case, a str<strong>in</strong>gent
planetary protection programme is required.
I.4.3.1 Paleontological Evidence
Given <strong>the</strong> validity of terrestrial analogues, <strong>the</strong> stratigraphic (sedimentary) record of
any planet should serve as a potential store <strong>for</strong> both bodily remnants of <strong>for</strong>mer
organisms and o<strong>the</strong>r possible traces of <strong>the</strong>ir life activities. This should also apply to
Mars dur<strong>in</strong>g <strong>the</strong> early stages of its history, when <strong>the</strong> planet may have been ba<strong>the</strong>d <strong>in</strong>
abundant water and environmental conditions on <strong>the</strong> surface did not differ much from
those on <strong>the</strong> early Earth (Carr & Wänke, 1992; Carr, 1996). If all terrestrial planets –
and notably Mars and Earth – had occupied comparable start<strong>in</strong>g positions <strong>in</strong> terms of
solar distance, condensation history and primary endowment with matter from <strong>the</strong>
parent solar nebula (Moroz & Mukh<strong>in</strong>, 1978), <strong>the</strong>n <strong>the</strong> surface conditions on both
planets should have been very similar <strong>in</strong> <strong>the</strong>ir juvenile states.
Specifically, with evidence at hand <strong>for</strong> a denser atmosphere and extensive aqueous
activity on <strong>the</strong> martian surface dur<strong>in</strong>g <strong>the</strong> planet’s early history, a conv<strong>in</strong>c<strong>in</strong>g case can
be made that <strong>the</strong> primitive martian environment was no less conducive to <strong>the</strong>
<strong>in</strong>itiation of life processes and <strong>the</strong> subsequent emplacement of prolific microbial
ecosystems, than <strong>the</strong> surface of <strong>the</strong> ancient Earth (McKay, 1991; McKay & Stoker,
1989). Even if <strong>the</strong> evolutionary pathways of both planets diverged dur<strong>in</strong>g <strong>the</strong>ir later
histories, so that life became ext<strong>in</strong>ct on Mars as <strong>the</strong> gradual deterioration of surface
conditions rendered <strong>the</strong> planet <strong>in</strong>hospitable to prote<strong>in</strong> chemistry <strong>in</strong> <strong>the</strong> widest sense,
Mars could still have started off with a veneer of microbial (prokaryotic) life
comparable to that exist<strong>in</strong>g on <strong>the</strong> Archaean Earth (Schopf, 1983; Schidlowski,
1993a). Given <strong>the</strong> apparent failure of <strong>the</strong> Vik<strong>in</strong>g life-detection experiment <strong>for</strong> <strong>the</strong>
present martian regolith (Biemann et al., 1977), <strong>the</strong> prime objective of a search <strong>for</strong> life
on Mars should be to seek evidence of ext<strong>in</strong>ct (fossil) life. <strong>The</strong> oldest Martian
sediments would be appropriate targets <strong>for</strong> such ef<strong>for</strong>ts.
In any search <strong>for</strong> ext<strong>in</strong>ct life on Mars, <strong>the</strong> basic problem would not rest with <strong>the</strong>
cognitive aspects of <strong>the</strong> identification of <strong>the</strong> fossil evidence, but ra<strong>the</strong>r with <strong>the</strong>
serendipity <strong>in</strong>evitably <strong>in</strong>volved <strong>in</strong> <strong>the</strong> selection and recovery of suitable sampl<strong>in</strong>g
material from <strong>the</strong> vast stretches of potential host rocks exposed on <strong>the</strong> planetary
surface. Among <strong>the</strong> two-thirds of <strong>the</strong> martian surface deemed to be covered by rocks
older than 3.8 Gyr (McKay, 1986), <strong>the</strong>re are several occurrences of well-bedded
I.4.3 Evidence of Ext<strong>in</strong>ct
(Fossil) Extraterrestrial
<strong>Life</strong>
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