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Three quarters of Earth's near surface rock is volcanic and virtually all of it has persistent or
intermittent exposure to water. Hydration of the primary igneous silicate minerals (feldspar, pyroxene,
olivine, and amorphous glass) and chemical exchange between these minerals and water produces
secondary phyllosilicate minerals. The mineral-water interface is energy rich and supports diverse
microbial communities that take up residence along cooling cracks and fractures. Microbes bore into
minerals and leave trace fossils and organic evidence of their presence. The Martian surface is also
dominated by volcanic rocks and some of these have been exposed to water long enough for
phyllosilicates to form. These phyllosilicates are found in some Martian meteorites and a widespread
distribution of phyllosilicates is indicated by reflected infrared light from some areas of Mars. As
microbial trace fossils have been preserved for billions of years on Earth, if life ever existed at water-rock
interfaces on Mars, then evidence of this life will have been preserved in the rocks. Areas of Mars that are
likely to contain rocks that once were in contact with water can be located with orbital imagery. A rover
on the Martian surface can locate outcrops likely to contain evidence of water-rock interaction based on
the geological context and outcrop morphology. Examination of prepared surfaces of outcrops with a
microscopic imager could reveal microbial trace fossils. Chemical analysis of the same surfaces prepared
for microscopic imaging could reveal complex organic compounds. Here we report on a strategy for
evaluating landing sites on Mars for their potential for containing evidence of microbial activity in
volcanic rocks.
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