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

Abstract

The search for traces of life is one of the principal objectives of Mars exploration. Central to this objective is the concept of habitability, the set of conditions that allows the appearance of life and successful establishment of microorganisms in any one location. While environmental conditions may have been conducive to the appearance of life early in martian history, habitable conditions were always heterogeneous on a spatial scale and in a geological time frame. This “punctuated” scenario of habitability would have had important consequences for the evolution of martian life, as well as for the presence and preservation of traces of life at a specific landing 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 sources as early terrestrial chemotrophic life and living in the same kinds of environments, the fossilized traces of the latter serve as useful proxies for understanding the potential distribution of martian chemotrophs and their fossilized 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, although sparsely 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 rapidly mineralized and that the sediments in which they occur are rapidly cemented. We evaluate the biogenicity of these signatures by comparing them to possible abiotic features. Finally, we discuss the implications of different scenarios for life on Mars for detection by in situ exploration, ranging from its non-appearance, through preserved traces of life, to the presence of living microorganisms. Key Words: Mars—Early Earth—Anaerobic chemotrophs—Biosignatures—Astrobiology missions to Mars. Astrobiology 15, 998–1029.

Earth as a Tool for Astrobiology—A European Perspective

“…In addition, bioleaching of rocks and soils (Cockell 2010) due to metabolic activity and release of acidic substances as well as possible deposits of secondary metabolites can change mineral composition or enrich soil with organic material. The analysis of such environmental patterns as a biosignature of life can be studied in the field as well as in the laboratory, which would lead to the identification of fossilization processes on Earth and support the search for extinct life on other planets (Orange et al 2011;Westall et al , 2015aWestall et al , 2015b.…”

“…Prokaryotes (e.g. bacteria, bacterial spores, cyanobacteria) have been studied in such extreme planetary field analogue environments due to the fact that early life forms on Earth were prokaryotes and the assumption that if any extra-terrestrial life exists in the Solar System, it must be simple cellular organisms (Hansen 2007;Westall et al 2015aWestall et al , 2015b. For example, microbial cryptoendolithic communities that colonize the pore spaces of sedimentary rocks in Antarctica and other deserts to avoid stressful environmental conditions (e.g.…”

“…lichens, microcolonial fungi and tardigrades) have also been studied, and shown a high resistance against the extreme conditions Jänchen et al 2015, Onofri et al 2004, 2012, Sancho et al 2007, de la Torre et al 2010, 2014b. In addition, fossil traces of primitive prokaryotes in well-preserved rocks from the early Earth constitute ideal analogues for potential primitive life forms on the early Mars (Westall et al 2015a(Westall et al , 2015b.…”

“…Early Archaean Terrains The early Earth represents an ideal environmental and microbial analogue for early habitable bodies in the Solar System. The early Earth was a volcanically and hydrothermally active ocean planet, while Mars was basically a land-locked volcanic planet with isolated pockets of habitability (Westall 2012;Westall et al 2015a), and the now icy moons (Enceladus, possibly Titan, Europa, and Callisto) of the outer Solar System could have been or are still similar. The reason for this is that, despite the great distances of the various bodies from the early, fainter Sun, they all underwent melting, fractionation and cooling.…”

“…3). Volcanoclastic sediments deposited in shallow water-at depths similar to those of crater lakes on Mars-and hosting abundant hydrothermal vents contain the remains of early, anaerobic life forms (Westall et al , 2015a(Westall et al , 2015b. Both chemotrophic and phototrophic colonies are preserved in the strongly hydrothermally-influenced sediments.…”