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Sulfate-rich mineral deposits have been discovered in many locations on Mars through observations by orbiters, landers, and roving spacecraft. It appears that in most cases, these minerals are produced by acid-sulfate weathering of igneous rocks, which may have been a widespread process for the first billion years on Mars. The origin of life on Earth may have occurred in iron-sulfur hydrothermal settings, and early Mars likely had similar environmental conditions. An excellent terrestrial analog for acid-sulfate weathering of Mars-like basalts exists at Cerro Negro, Nicaragua, where acidic sulfur-bearing gases interact with recently erupted basaltic ash in numerous active fumaroles. We investigated the chemistry and mineralogy of the pristine basalts and their chemically weathered products, and we studied the associated microbiological communities as an analog for potential early life on Mars. Measured pH values of condensed volcanic vapors range from −1 to 5, and near-surface temperatures in the fumaroles range from 40 to 400 °C. In a few years, fresh basalt can weather to amorphous silica and gypsum, along with lesser amounts of other sulfates (natroalunite and jarosite), Fe-hydroxides, and clays. Altered rocks have up to 35 wt% SO3 equivalent, similar to the amounts of sulfur reported for Meridiani Planum bedrocks and inferred in other sulfate-bearing bedrock on Mars. Heavily weathered rocks have silica contents up to 80 wt%, similar to silica-rich soils at Gusev crater that possibly formed in hydrothermal environments. Here, we provide preliminary results of our studies and outline the logistics for a field excursion to this Mars analog system.

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