In Situ Identification of Hydrated Carbonates on Mars

Hydrated carbonates uniquely record low-temperature interactions between CO₂-rich fluids and crustal rocks in water-rich conditions. Hydrated carbonates had not been previously detected in situ on the surface of Mars, but the Mars 2020 Perseverance rover reveals a local ~3.7-billion-year-old sediment preserving grains of hydrated Mg-carbonate only ~1 cm below the surface. This sediment, located at the ancient lake margin of Jezero Crater, was analyzed by the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument aboard Perseverance. The hydrated magnesium carbonate, spatially associated with amorphous silica, formed through olivine alteration by aqueous CO₂ – a process known as mineral carbonation – indicating early-stage alteration in an alkaline, water-rich environment. This carbonate is closely associated with hydrous and anhydrous sulfates, anhydrous carbonates, phosphate, and perchlorate, reflecting formation in multiple, temporally distinct aqueous environments. A very localized long-term preservation of this hydrated carbonate indicates exceptional local boundary conditions at the bedrock, including water-rich, alkaline chemistry and effective shallow-subsurface thermal and diagenetic shielding over geologic timescales. Discovery of hydrated carbonate also offers new in situ evidence of Mars’s long-missing CO2 sink.