Elevated olivine weathering rates and sulfate formation at cryogenic temperatures on Mars

Freeze-thaw cycles drove chemical weathering and enriched sulfates in the Burns formation at Meridiani, Mars

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.

High resolution Raman microscopy and imaging to propose a jarosite formation process in the MIL 090030 Martian Meteorite

Jarosite is an iron sulphate that has been found in different Martian meteorites as well as on the Martian surface. In most of the cases, this mineral was detected related to olivine grains. In this work the Miller Range 090,030 Martian Nakhlite (MIL 090030) has been analysed by high resolution Raman microscopy and imaging in areas where jarosite was present. In the case of MIL 090030 meteorite, jarosite was found beside olivine and rasvumite (KFe2S3). There are different hypotheses for the formation of jarosite on Martian related samples, but so far, no explanation for the formation of jarosite from olivine and rasvumite has been proposed. In this study, such hypothesis for this transformation is proposed based on the obtained Raman image results where jarosite appears surrounding primary compounds. The proposed alteration mechanism consists on the formation of magnetite and jarosite from rasvumite and Fe-rich olivine, with the subsequent enrichment of olivine in forsterite and the formation of quartz. With the ions released in the different chemical reactions, jarosite can precipitate as a stable phase. Taking this into account, the jarosite detected in the MIL 090030 Martian Meteorite can be considered as an original Martian mineral.

Intense subaerial weathering of eolian sediments in Gale crater, Mars

After over 8 years of successful surface operations on Mars, the Curiosity rover has revealed much about the environment in Gale crater. Despite early observations of a lacustrine environment, few of the subsequent deposits exhibit demonstrable lacustrine character. We suggest instead that most of the stratigraphic section explored to date can be best explained as eolian and/or volcaniclastic sediments subaerially chemically weathered by acidic precipitation in a reduced atmosphere. Most of the deposits in Gale crater seemingly did not form in an ancient lake, but the results nonetheless shed considerable light on ancient climate, environmental change, and the astrobiology of Mars. Discoveries by Curiosity provide a critical piece to Mars' global alteration puzzle.

Jarosite formation in deep Antarctic ice provides a window into acidic, water-limited weathering on Mars

Many interpretations have been proposed to explain the presence of jarosite within Martian surficial sediments, including the possibility that it precipitated within paleo-ice deposits owing to englacial weathering of dust. However, until now a similar geochemical process was not observed on Earth nor in other planetary settings. We report a multi-analytical indication of jarosite formation within deep ice. Below 1000 m depth, jarosite crystals adhering on residual silica-rich particles have been identified in the Talos Dome ice core (East Antarctica) and interpreted as products of weathering involving aeolian dust and acidic atmospheric aerosols. The progressive increase of ice metamorphism and re-crystallization with depth, favours the relocation and concentration of dust and the formation of acidic brines in isolated environments, allowing chemical reactions and mineral neo-formation to occur. This is the first described englacial diagenetic mechanism occurring in deep Antarctic ice and supports the ice-weathering model for jarosite formation on Mars, highlighting the geologic importance of paleo ice-related processes on this planet. Additional implications concern the preservation of dust-related signals in deep ice cores with respect to paleoclimatic reconstructions and the englacial history of meteorites from Antarctic blue ice fields.

The authors report in-situ formation of jarosite witin the Talos Dome ice core (East Antarctica) and show that this ferric-potassium sulfate mineral is present in ice deeper than 1000 meters and progressively increases with depth. This has implications for the presence and formation mechanisms of jarosite observed on Mars.