近年の火星隕石研究・火星探査から得られた新しい火星の描像(企画総説「地球化学の最前線」)

概要

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Recent Mars exploration missions have revealed that Martian meteorites do not represent typical Noachian/Hesperian (>3Ga) basaltic crusts. Martian meteorites (young igneous rocks, typically <1.3Ga except for ALH 84001) are distinctly depleted in aluminum relative to the old Martian crust. Numbers of petrologic and geochemical studies suggest that the Martian meteorites should have sampled geochemical source reservoirs formed by deep Martian magma ocean processes in the earliest history of Mars (〜4.5Ga). These meteorite source reservoirs probably would not represent geochemical source reservoir(s) of the typical old Martian basaltic crusts. Mars exploration missions have also revealed that Mars was once warm enough to maintain persistent liquid water on its surface. Such a warm and wet environment implies an effective greenhouse gas (CO_2 and/or other volatiles) that was dominantly supplied by early Martian magmatism. Recent findings of carbonate outcrops and bedrocks buried in the Martian crust may account for the early, thick CO_2-rich atmosphere. Furthermore, the enrichment of some volatile elements (e.g., halogens and sulfur), which would have dissolved in surface water, could have influenced water chemistry (e.g., acidity) and, thus likely played a significant role in weathering and aqueous alteration processes on the surface of early Mars.
2011-09-05