Oxygen isotopic evidence that Gale crater, Mars was home to an Early Hesperian water reservoir that underwent significant evaporation

Simultaneous measurements of HDO, H218O, and H216O in water evolved during pyrolysis of powdered rock samples acquired by the Curiosity rover within Gale crater’s clay-bearing units indicate extreme and variable heavy-isotope enrichments averaging ~4.5 times the D/H ratio and ~1.03 times the 18O/16O ratio of terrestrial seawater. These isotopic compositions are recorded in water desorbed from mineral surfaces and evolved from poorly crystalline phases, hydrated salts, jarosites, and clays. All evolved waters are deuterium-enriched relative to common terrestrial waters, reflecting hydrogen escape from the martian atmosphere. Because oxygen in structurally bound hydroxyl groups is least likely to exchange with other sources over geologic timescales, we focus on the oxygen isotopic compositions of water evolved during dehydroxylation of smectite clays. Several samples have isotopic compositions commensurate with precipitation from, or near-complete isotopic equilibration with, water moderately 18O-enriched relative to terrestrial meteoric waters—consistent with other evidence that Mars’s hydrosphere is basically like Earth’s in terms of oxygen isotopes. However, most smectite oxygen isotopic compositions require a local pronounced 18O-enrichment of their parental waters. On Earth, the most extreme D- and 18O-enrichments in surface waters are found in closed basins having undergone significant evaporative loss into a low-humidity atmosphere, and isotopic compositions of authigenic clay minerals formed in these environs reflect those enrichments. We suggest that a similar process acting on the hydrologic reservoir local to Gale at the time of clay formation and early diagenesis is a plausible explanation for the distinctive oxygen isotopic compositions of these Gale crater clays.