Mars regolith versus SNC meteorites: Possible evidence for abundant crustal carbonates

Viking XRF analyses of the Martian regolith are compared with typical igneous rocks of the Earth, the Moon, the eucrite parent asteroid, and especially the shergottites, nakhlites, and Chassigny (SNC) meteorites, which are suspected to be basalts and mafic cumulates from Mars. A striking feature of the Martian regolith, compared to igneous rocks with similar molar (Mg + Fe)/Si ratios, is its extraordinarily low Ca/Si ratio. The regolith's low Ca/Si ratio is probably not a result of simple mixing (isochemical weathering) of SNC-like rocks with other igneous rocks, unless the regolith contains a large component of rock with an improbable combination of extremely low Ca/Si and (Mg + Fe)/Si, and yet low K₂O and Zr. Several other models might conceivably account for the low Ca/Si ratio, but I suggest that most of the “missing” Ca was removed from the regolith as Ca-carbonate. Formation of a mass of carbonate equivalent to a global shell 20 m thick would suffice to remove 1000 mbar of CO₂ from the Martian atmosphere. Thus, the peculiar Ca/Si ratio of the Martian regolith tends to support the hypothesis that the climate of Mars was once far warmer and wetter than it is today.