Soil mineralogy and chemistry on Mars: Possible clues from salts and clays in SNC meteorites

Abstract

All subgroups of the shergottite, nakhlite, and chassignite (SNC) meteorites contain traces of water-precipitated minerals that include various combinations of carbonates, sulfates, halides, ferric oxides, and aluminosilicate clays of preterrestrial origin. Traces of sulfur- and chlorine-bearing aluminosilicates, which suggest possible affinities with scapolite minerals, occur in one shergottite but might be products of shock-metamorphic reactions rather than aqueous precipitation; even so, they are subordinate to discrete sulfate and carbonate minerals of clearly aqueous origin. If the SNC parent planet is Mars, as previously inferred from independent evidence, the aqueous precipitates indicate that oxidizing, water-based solutions probably have been chemically active on Mars for at least the time interval represented by the radiometric ages of the meteorites, namely, the past 200–1300 million years (myr). Those solutions included the chemical elements H, C, O, P, and S (N has not yet been found among the salts), although inorganic precipitates apparently predominated over organic products. The 13C/ 12C ratios in carbonates are satisfactorily explained by inorganic chemical reactions without requiring oxidation of 12C-rich organic matter. Further detailed mineralogical and stable-isotopic studies of the secondary minerals might help establish limits for biological activity in water-based Martian chemistry over the past 1300 myr. A mixture of aqueous precipitates found in the SNCs, comprising smectite, illite, and, gypsum (with minor halite ± calcite and hematite), provides a self-consistent, though not unique, model for the bulk elemental composition of surface sediments at the Viking Lander sites. Among other implications, the smectite-illite model lends support to a previous hypothesis that catalytic action of clay minerals was responsible for the surprising chemical reactivity discovered by the Viking Lander biology experiments. The smectite-ellite-salt model is also compatible with visible and infrared spectrophotometry of Martian dust while neither requiring nor excluding scapolite minerals as minor components.