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Since the first photogeologic exploration of Mars, vast mounds of layered sediments found within the Valles Marineris troughs (interior layered deposits, ILDs) have remained unex- plained. Recent spectroscopic results showing that these materials contain coarse-grained hematite and sulfate suggest that they are fundamentally similar to layered sulfate deposits seen elsewhere on Mars, and are therefore a key piece of the global aqueous history of Mars. In this work we constrain the origin of the ILDs by considering mass balance equations. One model involving formation of the ILDs by groundwater upwelling requires that a significant fraction of the global Martian sulfur budget was concentrated in the Valles Marineris at the time when the ILDs formed. It also necessitates high deposition and erosion rates in the Hesperian. We favor an alternative model in which the ILDs formed in a configuration similar to what is observed today through atmospherically driven deposition of ice, dust, and volcanogenic sulfuric acid. Such a model is easily compatible with the global sulfur budget, and does not require significant erosion rates or large volumes of liquid water. We propose that formation of sulfate-rich layered sediments on Mars was governed through time by volcanogenic SO2 and H2 O emission rates and dust production against a backdrop of obliquity variation in a largely cold and dry climate. [ABSTRACT FROM AUTHOR]

Clay minerals detected spectroscopically in the Mawrth Vallis region of Mars correspond to a complex, layered, thick (>600 m) stratigraphic section of ancient bedrock. Because the lighttoned, clay-bearing rocks are lithologically diverse over a broad area (>80,000 km2), have significant internal layering and complexity, and contain buried impact craters within the section (demonstrating that they were deposited over a geologically significant duration of time), we interpret the host rocks as sedimentary or possibly pyroclastic. Crater counts date the claybearing rocks to early-middle Noachian time (estimated as 3.8-4.1 Ga). Geomorphic observations suggest that the rocks were lithified early and deeply eroded during the late Noachian-early Hesperian. The combination of a probable ancient sedimentary context of the clays and a moderate pH formation environment implied by the occurrence of smectites clearly places these rocks among the most important targets for future astrobiological exploration. [ABSTRACT FROM AUTHOR]