Evaporites at Meridiani Planum and Implications for Surficial Processes on Mars

Results obtained from the Opportunity Rover have significantly changed the current views of surficial processes on Mars. After one martian year of exploration, it is clear that the sediments which continue to be characterized preserve an intricate record of depositional and geochemical processes at Meridiani Planum. The saline mineral assemblages identified from Opportunity data imply a distinct sulfate-rich and carbonate-poor acidic environment which produced jarosite, Mg- and Ca-sulfates, and other phases. The major processes controlling mineralogy and chemistry at Meridiani Planum were chemical weathering of basaltic materials, evaporation, sedimentary processing, and diagenesis. In addition, new data returned from the OMEGA instrument aboard Mars Express have revealed localized regions of sulfate-rich lithologies with a lack of carbonates. However, in the broader context of chemical weathering and evaporation processes at the martian surface, the saline mineral assemblages inferred from MER and OMEGA results differ considerably in character from those identified in SNC-type meteorites. For example, Nakhlite-type evaporite assemblages include carbonates such as siderite, gypsum (+ anhydrite), Mg-sulfates and halite. Here, we discuss saline mineral production at the martian surface by re-visiting the well established "chemical divide" concept, which has been used successfully to predict evaporite mineralogy and brine evolution on Earth. The chemical divide concept states that the precipitation of a salt mineral causes fractionation of chemical components in solution, depending on the ratio of the components in solution compared to that of the saline mineral. A new system of chemical divides is built for martian evaporative systems which shows that the uniqueness of evaporite mineralogy at the martian surface is controlled by at least three factors fundamental to surficial processes on Mars: (1) acidic environments controlled largely by SO4, HCO3 and Cl input, (2) increased mobility and concentration of Fe in aqueous systems, and (3) dilute water chemistry controlled by the weathering of basalt. The chemical divide system shows that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among martian evaporite assemblages evident from MER, OMEGA, and SNC results.