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

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A list of basic exploratory features <strong>for</strong> exobiology sites has been developed by
Farmer & DesMarais:
Deposits Features
Fluvial Dendritic dra<strong>in</strong>age networks: simple; complex (higher order)
Channel morphology: widen<strong>in</strong>g down slope; meander<strong>in</strong>g;
flood pla<strong>in</strong>s; stream terraces
Accessibility: layer<strong>in</strong>g visible; impact craters
Lacustr<strong>in</strong>e Dra<strong>in</strong>age bas<strong>in</strong> fed by: simple channels; complex channels;
Shorel<strong>in</strong>e features: lake terraces; deltas
Accessibility: lava flows; aeolian cover; impact craters
<strong>The</strong>rmal spr<strong>in</strong>g Dra<strong>in</strong>age system: simple channels; po<strong>in</strong>t source
Localised heat source: surface (volcanic centre); subsurface
<strong>the</strong>rmokarst
Surface ice High latitude (>60º): lam<strong>in</strong>ated terra<strong>in</strong>
Subsurface ice Mid-latitude (30-60º): patterned ground; alases; p<strong>in</strong>gos;
fluidised crater ejecta.
(After J.D. Farmer & D.J. DesMarais (1994). In Mars Land<strong>in</strong>g Site Catalogue, (Eds.
R. Greeley & P.E. Thomas), NASA RP-1238, 2nd Edition; with permission of <strong>the</strong>
editors.)
II.2.7.2 Exploration <strong>for</strong> Ext<strong>in</strong>ct <strong>Life</strong>
If Mars and Earth did actually experience broadly comparable conditions dur<strong>in</strong>g <strong>the</strong>
first 1.5 Gyr of <strong>the</strong>ir histories, <strong>the</strong>n it is likely that life appeared dur<strong>in</strong>g that same
period. Evidence of such life on Mars may be preserved <strong>in</strong> <strong>the</strong> <strong>for</strong>m of fossils and <strong>in</strong>
various biosignatures.
In <strong>the</strong> NASA studies <strong>for</strong> site selection <strong>for</strong> exopalaeontology, priority is given to
land<strong>in</strong>g sites <strong>in</strong> ancient terra<strong>in</strong>s where hydrological systems <strong>in</strong>volv<strong>in</strong>g liquid water
appear to have been long-lived and which exhibit a high probability of hav<strong>in</strong>g
surficial aqueous m<strong>in</strong>eral deposits. Preference is given to m<strong>in</strong>eral deposits that might
have had a long residence time <strong>in</strong> <strong>the</strong> martian crust, i.e. those that are diagenetic stable
and resistant to chemical wea<strong>the</strong>r<strong>in</strong>g.
On Earth, m<strong>in</strong>eralisation occurred <strong>in</strong> many Precambrian examples very rapidly,
prior to cellular decomposition, and probably when organisms were still viable. <strong>The</strong>
best preservation is observed where organic materials were rapidly perfused with
f<strong>in</strong>e-gra<strong>in</strong>ed silica or phosphate. Evaporites <strong>for</strong>m ano<strong>the</strong>r group of potential host
phases. Whereas on Earth <strong>the</strong>se are readily dissolved <strong>in</strong> hydrologic cycles, <strong>in</strong> <strong>the</strong> very
dry martian climate <strong>the</strong>ir residence time might be prolonged.
Hydro<strong>the</strong>rmal systems are excellent targets <strong>for</strong> a search <strong>for</strong> fossil records of
biological activity. Volcanic terra<strong>in</strong>s are abundant on Mars and <strong>the</strong>re is ample
evidence <strong>for</strong> ground water or ground ice. <strong>The</strong>ir comb<strong>in</strong>ation is <strong>the</strong> basis of hydro<strong>the</strong>rmal
activity, which <strong>the</strong>re<strong>for</strong>e should also be abundant on Mars. <strong>The</strong> NASA search
<strong>for</strong> ext<strong>in</strong>ct life (fossils) strategy on Mars <strong>the</strong>re<strong>for</strong>e gives priority to <strong>the</strong>rmal spr<strong>in</strong>g
sites, which should also provide a good preservation environment <strong>for</strong> microbial
fossils and to stable lacustr<strong>in</strong>e environments and outflow channel deposits. <strong>The</strong>se
types of sites give almost a planetwide range <strong>for</strong> exploration (Greeley & Thomas
1994).
II.2.7.3 Exploration <strong>for</strong> Extant <strong>Life</strong>
Extant life, if present, probably resides <strong>in</strong> subsurface habitats where liquid water
might be present and, hence, would be most likely of a chemosyn<strong>the</strong>tic <strong>for</strong>m. It seems
possible that recent flows of subsurface water have brought such organisms <strong>in</strong>to nearsurface
environments where <strong>the</strong>y have been cyropreserved <strong>in</strong> ground ice. It has been
suggested that microorganisms may survive <strong>in</strong> ground ice <strong>for</strong> millions of years and <strong>in</strong>
evaporite deposits <strong>for</strong> hundreds of million of years. Never<strong>the</strong>less, <strong>the</strong> highest priority