A chemical model for evaporites on early Mars: Possible sedimentary tracers of the early climate and implications for exploration
Martian geomorphology seems to indicate extensive hydrological activity during the Noachian era. Liquid water at the surface would require a large greenhouse effect that is widely hypothesized to have been caused by a high partial pressure of atmospheric carbon dioxide, PCO2. A sedimentation model driven by sequential evaporation is used to calculate the evaporite mineral sequence in a closed basin lake subject to high PCO2. The initial fluid is derived from weathered igneous rock similar to Martian meteorite basalts. Siderite (FeCO3) is always the first major carbonate to precipitate. Thus siderite is predicted to be an important facies component in ancient Martian sediments along with silica, which is also an early precipitate. These would form varves in lakes that undergo cycles of evaporation and water recharge. After silica and siderite, the sequence is magnesian calcite, nearly pure hydromagnesite, and gypsum, followed by highly soluble salts like NaCl. The presence of siderite sediments generally requires an atmospheric PCO2 level in excess of ~ 0.1 bar, otherwise iron silicates (such as greenalite) would form. This may be used in exploration as an observational test on the past atmospheric composition of Mars, subject to consideration of the depositional environment. At PCO2 of approximately several bar, gypsum precipitation could occur before calcite upon evaporation if the initial SO42-:Ca2+ ratio is high and there is no water recharge. The predicted carbonate sequence upon evaporation is generally consistent with recent hypotheses suggesting traces of evaporite carbonates in some Martian meteorites. Several mechanisms destroy or obscure carbonates at the Martian surface. Consequently, in situ analysis of the interior of ejecta from recent impact craters lying within sedimentary basins may offer the most practical approach to future exploration of ancient carbonate sediments.