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

• voids <strong>in</strong> volcanic rocks (vesicles);
• fractured rocks of any type.
<strong>The</strong> chances will be best <strong>in</strong> rocks that were buried to a considerable depth (several
hundred metres or more) and have resurfaced ow<strong>in</strong>g to impact ejection, on canyon
walls etc. <strong>The</strong> follow<strong>in</strong>g land<strong>in</strong>g sites belong to <strong>the</strong> second type and appear most
promis<strong>in</strong>g <strong>for</strong> subsurface fossil microbes:
Chaotic terra<strong>in</strong>s: material broken up at a large scale provides <strong>in</strong>sight <strong>in</strong>to <strong>the</strong>
subsurface geology of <strong>for</strong>merly water-rich environments. Imag<strong>in</strong>g of nearby
steep mounta<strong>in</strong> slopes would allow <strong>in</strong>sight <strong>in</strong>to geological context.
Steep canyon walls: excellent <strong>in</strong>sight <strong>in</strong>to geological context but virtually
<strong>in</strong>accessible. Distal debris aprons may be good sites <strong>for</strong> sampl<strong>in</strong>g.
Channel floors (outflow from chaotic terra<strong>in</strong>s): possible fragments from microbebear<strong>in</strong>g
subsurface environments, but with limited geological context.
Impact crater ejecta: can provide access to material hidden <strong>in</strong> <strong>the</strong> subsurface.
Limited geological context.
<strong>The</strong> selection of <strong>the</strong> land<strong>in</strong>g site should be <strong>the</strong> responsibility of a restricted mission
team, but <strong>the</strong> scientific community should be closely <strong>in</strong>volved through meet<strong>in</strong>gs and
discussions. Of course, <strong>the</strong> safety of <strong>the</strong> land<strong>in</strong>g site is of paramount importance.
Ano<strong>the</strong>r task is to understand <strong>the</strong> types of <strong>in</strong>struments <strong>in</strong>volved, <strong>the</strong>ir operation
capabilities and, most important, <strong>the</strong> scientific background. <strong>The</strong> mission concept will
<strong>in</strong>fluence <strong>the</strong> type of land<strong>in</strong>g sites. <strong>The</strong> most challeng<strong>in</strong>g – and promis<strong>in</strong>g – sites are
those with environments such as sebkha, lakes and hydro<strong>the</strong>rmal vents capable of
susta<strong>in</strong><strong>in</strong>g life <strong>for</strong>ms. <strong>The</strong>se k<strong>in</strong>d of sites are identified at present on <strong>the</strong> basis of <strong>the</strong>ir
sedimentological and geomorphologic features. Some knowledge about <strong>the</strong>
m<strong>in</strong>eralogical composition of <strong>the</strong> surface will be extremely useful <strong>in</strong> ref<strong>in</strong><strong>in</strong>g <strong>the</strong>
<strong>in</strong>terpretation. However, <strong>the</strong>se type of data are lack<strong>in</strong>g and it is difficult to <strong>for</strong>esee <strong>the</strong>ir
full availability be<strong>for</strong>e 2004, even with <strong>the</strong> <strong>The</strong>rmal Emission Spectrometer return<strong>in</strong>g
data from <strong>the</strong> current Mars Global Surveyor mission.
Several basic requirements <strong>for</strong> a proper land<strong>in</strong>g site can be summarised as:
Age: accord<strong>in</strong>g to conventional wisdom, which sees <strong>the</strong> Early Noachian as <strong>the</strong> only
geological epoch with climatic conditions suitable <strong>for</strong> <strong>the</strong> development of life, <strong>the</strong>
best targets are geological units of that age. This hypo<strong>the</strong>sis is, however, not fully
supported by <strong>the</strong> geological evidence (see above). A large number of lakes,
channels and o<strong>the</strong>r environments suitable <strong>for</strong> life are as young as middle
Amazonian. Thus, <strong>the</strong> range of ages of rock units with biological potential
actually span a large part of <strong>the</strong> planet’s history;
Concentration: it is quite important that <strong>the</strong> material <strong>in</strong>dicat<strong>in</strong>g <strong>the</strong> past presence of
biota, ei<strong>the</strong>r microbial fossils or biomolecules, is concentrated <strong>in</strong> appreciable
amounts. This concentration occurs <strong>in</strong> several environments (see above) and depends,
<strong>in</strong> geological terms, on <strong>the</strong> ability of <strong>the</strong> lithological substratum to susta<strong>in</strong> biota;
Preservation: <strong>the</strong> preservation of <strong>the</strong> evidence of past biota is of paramount
importance <strong>in</strong> <strong>the</strong> exploration. Microbial material should fossilise rapidly.
Environments with crystallisation potential, such has hydro<strong>the</strong>rmal spr<strong>in</strong>gs or
sebkha, are quite efficient. Organic molecules and o<strong>the</strong>r chemical evidence may
require rapid burial to avoid alteration at <strong>the</strong> surface;
Th<strong>in</strong> aeolian cover: <strong>the</strong> Mars Global Surveyor images have confirmed that <strong>the</strong>
martian surface is extensively affected by aeolian deposition. <strong>The</strong> w<strong>in</strong>d-blown
detritus can cover large parts of <strong>the</strong> planetary surface and <strong>the</strong> selected land<strong>in</strong>g site
should show <strong>in</strong>dications of th<strong>in</strong> aeolian coverage (that is, high <strong>the</strong>rmal <strong>in</strong>ertia and
low albedo);
team I: exobiology and <strong>the</strong> mars surface environment/II.4
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