IN SITU EVIDENCE OF AN ACTIVE CARBON CYCLE ON ANCIENT MARS

The apparently limited volumes of carbonate minerals in the sedimentary strata of Mars represent a key paradox in our understanding of the planet’s habitability. Climate models suggest hundreds of millibars of atmospheric CO2 (and likely additional, reducing greenhouse gases such as H2) would have been required to stabilize liquid water. Yet, orbiter- and rover-based data acquired to date have yielded limited evidence for carbonate minerals, which commonly form in lacustrine and marine sediments on Earth. The Tapo Caparo and Ubajara drill samples, acquired by the Mars Science Laboratory Curiosity rover in sedimentary strata of Mount Sharp in Gale crater, add important, in situ evidence to our growing understanding of an active carbon cycle on ancient Mars. Indeed, quantitative analyses by the onboard CheMin X-ray diffractometer (XRD) confirm that siderite (FeCO3) is an abundant, rock-forming mineral in these samples, adding to limited carbonate identifications in the previous Kilmarie, Glen Etive, Mary Anning, Groken, Nontron, and Bardou drill samples. The siderite co-occurs with typical detrital mafic minerals (e.g., pyroxene and plagioclase), sulfate minerals suggestive of evaporative concentration of Gale Crater waters, and elevated trace element concentrations. Together, these observations suggest that an active carbon cycle, in which aqueous weathering of silicates contributed to water-mediated carbonate mineral formation, was active on ancient Mars during Gale crater sedimentation and/or subsequent diagenesis. Because siderite formation requires specific pH, alkalinity, pCO2, and Fe/Ca ratio, its identification in these samples will help to constrain the habitability of Gale crater waters at the time of its formation. Ultimately, refinement of the specific mechanism by which the observed carbonate minerals formed will permit increasingly quantitative constraints on the contribution of the evolving carbon cycle to the evolution of Martian habitability. Additional substantial contributions to this work were made by David Blake, Steve Chipera, Richard Morris, and the MSL CheMin Team.