Aqueous environmental history of Mars revealed by mineralogy and geochemistry of outcrop exposures of sedimentary rocks

概要

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Recent high-resolution spectral images and in-situ observations have revealed more than 1000 outcrops of diverse aqueous mineral deposits on Mars. Layered deposits of phyllosilicates (and carbonates) have been found in the oldest terrains, suggesting active interactions between liquid water and basalts in the subsurface and/or on the surface of Mars at around 4.3–4.0 billion years ago. Hydrated sulfates and silica deposits formed by evaporation of acidic and oxidizing surface water under arid conditions have been found in the second period (∼4.0–3.2 billion years ago). After this period, liquid water does not seem to have played a major role for the formation of sediments on Mars. Here, we describe characteristics and possible origins of these aqueous sediments, review hypothesis for the causative mechanisms for Martian environmental change, and discuss new questions posed by the measurements.