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

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II.4.5 <strong>The</strong> Rationale <strong>for</strong>
Land<strong>in</strong>g Site Selection
Several attempts have been made to model <strong>the</strong> early planetary climates tak<strong>in</strong>g <strong>in</strong>to
consideration <strong>the</strong> lower solar lum<strong>in</strong>osity and putative early atmospheric conditions.
On Mars, <strong>the</strong> valley networks and erosion rates provide conv<strong>in</strong>c<strong>in</strong>g evidence of liquid
water on <strong>the</strong> early surface. This implies elevated surface temperatures, to allow liquid
water to flow across <strong>the</strong> surface <strong>for</strong> enough time to cut <strong>the</strong> valley networks. Most
models focus on atmospheres composed ma<strong>in</strong>ly of CO 2 and H 2O, which are efficient
greenhouse gases. O<strong>the</strong>r gases, such as NH 3, CH 4, and SO 2 are too sensitive to photodissociation
<strong>in</strong>duced by solar UV radiation to be relevant <strong>for</strong> greenhouse warm<strong>in</strong>g.
However, so far, no satisfactory model has been developed to expla<strong>in</strong> <strong>the</strong> rise of
surface temperature to above freez<strong>in</strong>g by greenhouse warm<strong>in</strong>g. This is especially
complicated by <strong>the</strong> lower solar constant predicted from stellar models.
<strong>The</strong> selection of land<strong>in</strong>g sites is a ra<strong>the</strong>r complex procedure and it must take <strong>in</strong>to
account several variables. What k<strong>in</strong>d of evidence are we seek<strong>in</strong>g: present life, past
life, microbial fossils, geochemical signatures? Where are we look<strong>in</strong>g <strong>for</strong> evidence of
life: at <strong>the</strong> surface, <strong>in</strong> <strong>the</strong> subsurface? Which k<strong>in</strong>ds of <strong>in</strong>struments are we go<strong>in</strong>g to
use? At present, we are consider<strong>in</strong>g a lander deploy<strong>in</strong>g a rover with drill<strong>in</strong>g
equipment. Instrumentation <strong>in</strong>cludes cameras and geochemical analytical tools to
look <strong>for</strong> chemical <strong>in</strong>dicators of past (present) life.
Two different k<strong>in</strong>ds of land<strong>in</strong>g sites can be envisaged. <strong>The</strong> first is a particular
environment where life could have been present. Several were summarised above.
<strong>The</strong> second is an area where <strong>the</strong>re are a large number of lithotypes represent<strong>in</strong>g a
random collection of rocks. <strong>The</strong> first type is based on determ<strong>in</strong>istic thoughts, whereas
<strong>the</strong> second is based on more stochastic considerations. Sites of <strong>the</strong> first type are
required <strong>for</strong> geochemical <strong>in</strong>vestigation and drill<strong>in</strong>g (as well as fossil imag<strong>in</strong>g). <strong>The</strong>
second type is good <strong>for</strong> a random and extensive search <strong>for</strong> imag<strong>in</strong>g microbial fossils,
but not <strong>for</strong> chemical <strong>in</strong>vestigation of <strong>in</strong>dicators of life. This type of site, similar to <strong>the</strong>
Mars Pathf<strong>in</strong>der one, is valid <strong>for</strong> petrological and m<strong>in</strong>eralogical studies.
<strong>The</strong> first type of land<strong>in</strong>g sites was discussed above, and <strong>the</strong>re is a clear picture
of <strong>the</strong> exobiological potential. Of course, with <strong>the</strong> current state of our knowledge of
<strong>the</strong> martian environments, <strong>the</strong> selection of a site of this k<strong>in</strong>d will be based mostly
on <strong>the</strong>oretical work and educated guesses. <strong>The</strong> Mars Land<strong>in</strong>g Site Catalogue
(Greeley & Thomas, 1994) lists 150 possible land<strong>in</strong>g sites, only a fraction of which
are related to exobiology, however. Land<strong>in</strong>g sites with exobiology <strong>in</strong>terest were
selected on <strong>the</strong> basis of: <strong>in</strong>ferred presence of sediments, often <strong>in</strong> craters; partial
excavation of sediments by younger impacts to allow access to suspected buried
sediments; stratified canyon walls (possible sediments); chaotic terra<strong>in</strong>s <strong>in</strong>dicat<strong>in</strong>g
<strong>the</strong> presence of water and possibly of hydro<strong>the</strong>rmal activity; high-albedo features <strong>in</strong>
craters (<strong>in</strong>ferred evaporites); ground ice (cratered); polygonal features (sediments/ice?);
terrac<strong>in</strong>g (water flow). Similar criteria can be applied dur<strong>in</strong>g <strong>the</strong>
selection of land<strong>in</strong>g sites <strong>for</strong> ESA landers. However, all of <strong>the</strong> above criteria are
based on <strong>the</strong> expectation that evidence of <strong>for</strong>mer surface-bound life is most likely
to be found.
Apart from <strong>the</strong> possibility of <strong>for</strong>mer surface-bound life on Mars, land<strong>in</strong>g site
selection should also give consideration to <strong>the</strong> possibility of fossils of microbes that
lived <strong>in</strong> <strong>the</strong> subsurface. <strong>The</strong> geological structures where such fossils might be
expected are fundamentally different from those possibly conta<strong>in</strong><strong>in</strong>g surface-bound
microbial rema<strong>in</strong>s. Such environments have, so far, not been considered <strong>in</strong> published
land<strong>in</strong>g site catalogues, but <strong>the</strong>y should be <strong>in</strong>cluded <strong>in</strong> <strong>the</strong> overall evaluation. <strong>The</strong>
likely presence of a variety of geological materials at many sites makes it possible that
both surface and subsurface fossils might occur at a s<strong>in</strong>gle land<strong>in</strong>g site.
Subsurface environments possibly conta<strong>in</strong><strong>in</strong>g ext<strong>in</strong>ct microbial life on Mars are:
• highly porous or diagenetically cemented sedimentary rocks (e.g. conglomerate
with low amounts of f<strong>in</strong>e-gra<strong>in</strong>ed material);
• voids <strong>in</strong> impact breccias and impact melts;