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Sulfate-rich sedimentary rocks explored by the Opportunity rover during its 14-year surface mission at Meridiani Planum provide an invaluable window into the thousands of sulfate deposits detected on Mars via remote sensing. Existing models explaining the formation of martian sulfates can be generally described as either bottom-up, groundwater-driven playa settings or top-down icy chemical weathering environments. Here, we propose a hybrid model involving both bottom-up and top-down processes driven by freeze-thaw cycles. Freezing leads to cryo-concentration of acidic fluids from precipitations at the surface, facilitating rapid chemical weathering despite low temperatures. Cryosuction causes the upward migration of vadose water and even groundwater with dissolved ions, resulting in the accumulation of ions in near-surface environments. Evaporation precipitates salts, but leaching separates chlorides from sulfates during the thawing period. Freeze-thaw cycles, therefore, can enrich sulfates at the surface. While freeze-thaw is more commonly understood as a mechanism of physical weathering, we suggest that it is a fundamental aspect of chemical weathering on Mars. [ABSTRACT FROM AUTHOR]

The article presents a research report on intense subaerial weathering of eolian sediments in Gale crater, Mars. Topics include early observations of a lacustrine environment, few of the subsequent deposits exhibit demonstrable lacustrine character; and discoveries by Curiosity provide a critical piece to Mars' global alteration puzzle.

Observations by the Mars Reconnaissance Orbiter/Compact Reconnaissance Imaging Spectrometer for Mars in the Mawrth Vallis region show several phyllosilicate species, indicating a wide range of past aqueous activity. Iron/magnesium (Fe/Mg)-smectite is observed in light-toned outcrops that probably formed via aqueous alteration of basalt of the ancient cratered terrain. This unit is overlain by rocks rich in hydrated silica, montmorillonite, and kaolinite that may have formed via subsequent leaching of Fe and Mg through extended aqueous events or a change in aqueous chemistry. A spectral feature attributed to an Fe2+ phase is present in many locations in the Mawrth Vallis region at the transition from Fe/Mg-smectite to aluminum/silicon (Al/Si)-rich units. Fe2+-bearing materials in terrestrial sediments are typically associated with microorganisms or changes in pH or cations and could be explained here by hydrothermal activity. The stratigraphy of Fe/Mg-smectite overlain by a ferrous phase, hydrated silica, and then Al-phyllosilicates implies a complex aqueous history. [ABSTRACT FROM AUTHOR]