Aqueous biotic and abiotic alteration of silicate rock: evaluation of landing sites on Mars for their potential of revealing evidence for life

Abstract

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.