Assessing habitability of aqueous environments on Mars

A source of metabolic energy is a requirement for life. One possible source of energy that may have supported potential organisms on Mars was geochemical energy from chemical disequilibrium. We evaluated the habitability of aqueous environments on Mars by quantifying the amount of available geochemical energy from chemosynthetic reactions from a range of martian environments. By determining the overall Gibbs energy yields for redox reactions in the H-O-C-S-Fe system, the amount of geochemical energy that was available for potential chemolithoautotrophic microorganisms was quantified and the amount of biomass that could have been sustained was estimated. Biomass estimates show that Fe and S redox reactions in basalt aquifers may have supported the production of 1012 cells of biomass per kg of altered basalt. Additionally, a putative martian subsurface hydrothermal system would have had the potential to support a maximum of 109 cells per kilogram of vent fluid. The geochemical models indicate that aqueous environments on Mars would have had the potential to generate chemical energy sources to allow for habitable environments, and potential populations of organisms at subsurface hydrothermal systems on Mars would have been approximately two orders of magnitude less than what was modeled for terrestrial hydrothermal systems. Furthermore, the results were applied to four of the Mars Science Laboratory potential landing sites in order to assess which sites may have had the most biological potential. The most habitable sites are considered here to be Gale Crater and Marwth Vallis based on mineralogical evidence with various oxidation states of Fe and S.