Production of Phyllosilicates and Sulfates on Mars Through Acidic Weathering: The Rio Tinto Mars Analog Model

摘要

Understanding the paleoclimate of Mars and the nature and ex-tent of the interaction of water with crustal materials is essential to evaluate if life emerged once on Mars. Using recent results obtained by orbiters and surface rovers we propose that mildly acidic aqueous conditions produced phyl-losilicate minerals during the Noachian period on Mars related to a CO_2 -rich atmosphere and an active hydrosphere. Underground neutralization of these mild acidic meteoric solutions with crustal materials would produce subsurface carbonates inside the crust after. Moreover, hydrothermalism provides miner-alization to favor subsurface sulfide orebodies, but also secondary geochemical processes would also favor its formation. After cessation of the internal magnetic dynamo, the CO_2 -rich atmosphere was eroded and decreased by interactions with the solar wind and the hydrologic cycle that induced a climatic change to dry conditions where hydrological processes had subsurface dominance. Un-der this thin atmospheric aridic conditions photochemistry played an essential role in generating oxidizing and acidifying compounds that after entering the Mars crust promoted carbonate dissolution and sulfide oxidation. As a conse-quence, acid-sulfate evaporite deposits were precipitated in areas where acidic subsurface solutions emerged. Analogous processes have been observed in the underground fluids that feed the Rio Tinto Mars analog. Seasonal subsurface of sulfur continued by a rapid oxidation induced by rainwaters, which induces a strong acidification through proton releasing (FeS_2 + 3. O_2 + 2 H_2O→2. 50_4~(2-)+ Fe~(2+) + 4 H~+). This stage is followed by a neutralization and reduction of the ferric and sulfate rich acidified waters, which favors the subsurface carbonate precipitation. In addition, phyllosillicate sedimentation under the strong acidic conditions of Rio Tinto, supports preservation of same minerals under early Noachian mildly acidic conditions.