Biosignatures on Mars: What, Where, and How? Implications for the Search for Martian Life

International audienceThe search for traces of life is one of the principal objectives of Mars exploration. Central to this objective is theconcept of habitability, the set of conditions that allows the appearance of life and successful establishment ofmicroorganisms in any one location. While environmental conditions may have been conducive to the appearanceof life early in martian history, habitable conditions were always heterogeneous on a spatial scale andin a geological time frame. This ‘‘punctuated’’ scenario of habitability would have had important consequencesfor the evolution of martian life, as well as for the presence and preservation of traces of life at a specificlanding site. We hypothesize that, given the lack of long-term, continuous habitability, if martian life developed,it was (and may still be) chemotrophic and anaerobic. Obtaining nutrition from the same kinds of sourcesas early terrestrial chemotrophic life and living in the same kinds of environments, the fossilized traces of thelatter serve as useful proxies for understanding the potential distribution of martian chemotrophs and theirfossilized traces. Thus, comparison with analog, anaerobic, volcanic terrestrial environments (Early Archean>3.5–3.33 Ga) shows that the fossil remains of chemotrophs in such environments were common, althoughsparsely distributed, except in the vicinity of hydrothermal activity where nutrients were readily available.Moreover, the traces of these kinds of microorganisms can be well preserved, provided that they are rapidlymineralized and that the sediments in which they occur are rapidly cemented. We evaluate the biogenicity ofthese signatures by comparing them to possible abiotic features. Finally, we discuss the implications of differentscenarios for life on Mars for detection by in situ exploration, ranging from its non-appearance, throughpreserved traces of life, to the presence of living microorganisms. Key Words: Mars—Early Earth—Anaerobicchemotrophs—Biosignatures—Astrobiology missions to Mars. Astrobiology 15, 998–1029