[M-IS14] Aqua planetology

[MIS14-P10] Formation conditions of magnetite and hematite in Sheepbed mudstone and bioavailable energies in lake water-groundwater mixing zone in Gale crater, Mars

*Sakiko Kikuchi¹, Takazo Shibuya¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:early Mars, saponite, bioavailable energy, water-rock reactions

The Sheepbed mudstone of Yellowknife Bay, Gale crater was interpreted as an ancient habitable environment, based on its detailed mineralogical and geochemical composition analyses. However, the energetic availability for metabolic reactions is poorly constrained. Here, we propose the possible mixing of surface water and groundwater that (i) explains the formation of magnetite and hematite detected in Sheepbed mudstone and (ii) works as a potential habitable zone for aerobic Fe²⁺-oxidizing microbes. Our thermodynamic modeling of water–rock reactions revealed that the formation of abundant Fe-rich saponite in Sheepbed mudstone may occur under various conditions of water-to-rock mass ratios, temperatures (5–300ºC) and initial fluid compositions (O₂/H₂+CO₂-bearing and gas-free fluids). In contrast, the formation of magnetite and hematite in the mudstone can be explained only by the mixing of Fe²⁺-rich groundwater and oxygenated surface water, where the Fe²⁺-rich groundwater can be generated by the low-temperature water–rock reactions with a CO₂-bearing initial fluid. The calculated free energy yield of aerobic Fe²⁺ oxidation in the fluid-mixing zone on Mars is similar to that estimated for a fluid-mixing zone on Earth actually inhabited by aerobic Fe²⁺-oxidizing microbes. The findings will contribute to a better understanding of potential habitability on Mars.