An interval of high salinity in ancient Gale crater lake on Mars
Résumé
Precipitated minerals, including salts, are primary tracers of atmospheric conditions and water chemistry in lake basins. Ongoing in situ exploration by the Curiosity rover of a thick section of Hesperian (~3.3-3.7 Ga) sedimentary rocks within Gale crater has revealed clay-bearing fluvio-lacustrine deposits with sporadic sulfate occurrences primarily as late-stage diagenetic veins and concretions. Here, we report the discovery of bulk enrichments, disseminated in the bedrock, of Ca-sulfate (30-50 wt%) that occur intermittently over ~150 m of stratigraphy and hydrated Mg-sulfate (26-36 wt%) within a thinner section of strata. We use geochemical analysis, primarily from the ChemCam laser-induced breakdown spectrometer (LIBS) and combined with results from other rover instruments, to characterize the enrichments and their lithology. The deposits are consistent with early diagenetic, pre-compaction, salt precipitation from brines concentrated by evaporation, including Mg-sulfate rich brines indicating extreme evaporative concentration. This saline interval represents a significant hydrologic perturbation affecting the lake basin, and possibly reflecting variations in obliquity and orbital parameters of the planet. Our findings support a stepwise change in Mars’ Hesperian climate towards more arid and sulfate-dominated environments inferred from orbital observations.
Domaines
Planétologie
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