Recently, a deep-water hydrothermal environment on ancient Mars (>3.8 Ga (billion years ago)) has been interpreted for Eridania basin based on massive clay-, carbonate-, and sulfide-bearing deposits 26. The tectono-magmatic complexes Tharsis and Elysium have long been known as late Noachian–early Hesperian hydrothermal environments on Mars 21, 22. Hydrothermal settings were widespread during the early geologic history of Mars 21, 22 and continued into the late Amazonian period 23, 24, 25. Chemolithoautotrophs that thrive in geothermal springs metabolize inorganic chemicals, a source of energy that provided the most likely habitable niches for life on early Mars. Chemolithoautotrophic microorganisms employ an astonishing number of metabolic pathways to extract energy from diverse inorganic electron donors and acceptors 20, shaping global biogeochemical cycles. Chemolithoautotrophy, as the ancient metabolic form of life 15, 16, is thought to have enabled the transition of geochemistry into geobiochemistry and served as a biochemical link between the mineral world and the last universal common ancestor 15, 17, 18, 19. In this connection, a valuable source of information can be extracted from microbial fingerprints of chemolithotrophic life based on Martian materials. While a range of environments that would have been well suited to support a potential Martian chemolithoautotrophy have been proposed 1, 2, 12, 13, 14, our understanding of putative biosignatures to be targeted in Martian materials is still poor. Rovers specifically equipped and well suited to search for signs of life will traverse and explore the surface of Mars, focusing on the Noachian terrains with moisture-rich ancient geological history and mineral springs that could have been colonized by microorganisms. A primary aim of the upcoming Mars exploration missions (Mars 2020 and ExoMars) is to search for signs of ancient life.
Along with an active hydrogeological history of Mars, past chemolithoautotrophy-based habitability was suggested for lacustrine sediments at Gale crater 1, 2, many instances of phyllosilicate-containing mineral deposits have been reported 5, 6, 7, 8, 9, 10, and eventually, a thermal habitability window of the early Mars crust was also featured 11. Recent works heightened interest in the search for biologically driven alterations on Mars and its potential as a habitat for past or present life 1, 2, 3, 4.
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