Sulfate minerals are significant components of the martian surface and provide clues about the martian geochemical environment. One unusual Fe-sulfate phase has been intriguing Mars scientists for over a decade due to its unique spectral bands that are distinct from any known minerals and its occurrence in layered sedimentary rocks. We describe here detection of ferric hydroxysulfate (Fe3+SO4OH) and its implications for the geochemical history of Mars. Crystalline ferric hydroxysulfate is formed by heating hydrous Fe2+ sulfates to 100 °C or above and has a strong spectral band at 2.236 µm, similar to the spectral feature observed on Mars at Aram Chaos and on the plateau above Juventae Chasma. Hydrated sulfates at these locations likely formed through evaporative processes or low-temperature alteration. In contrast, Fe3+SO4OH is more consistent with heating and oxidation of hydrated ferrous sulfates, potentially through deposition of lava, ash, or through hydrothermal processes.
