In order to simulate the weathering of primary phases likely to occur on the Martian surface, metallic iron α-Fe, magnetite, and pyrrhotite were aged in CO2 + H2O or CO2 + H2O2 atmospheres at room temperature for 1 yr. Only the magnetite remained stable during experiments; thus any magnetite on Mars is likely to be inherited from primary bedrock, whereas any metallic and most sulfide iron minerals are provided by meteoritic accretion. Metastable siderite, neoformed from α-Fe, as well as sulfates and sulfur from pyrrhotite, account for various Martian in situ observations. Stepwise color changes are related either to changes in the relative proportions of neoformed phases or to atmosphere-related changes in crystallinity, rather than to fundamental mineralogical variations of iron phases. Goethite is the main crystalline iron-bearing end product, eventually associated with ferrihydrite. If hematite is the actual dominant iron oxide that colors the Red Planet, our results imply strong changes in water activities of the primary CO2 and H2O rich atmosphere (i.e., evolution toward anhydrous conditions), or long-term evolution, for goethite to further convert into hematite. Our experiments suggest that iron weathering may have been active until recent times and would not have required bodies of liquid water.

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