Evidence from Chemical Modeling for Small Amounts of Ephemeral Water during Alteration at Meridiani Planum, Mars

Introduction: Thanks to the instruments onboard recent ground-based and orbital missions, an unprecedented amount of high quality data exists relevant to the secondary minerals characteristics of the altered martian surface. One of the most striking results is that sulfates are among the most abundant secondary phases and are concentrated in geological units intermediate in age between older Noachian terrains where clays are observed and younger Amazonian terrains dominated by the presence of nanophase iron oxides [1]. These observations are of interest as they are relevant to important questions, such as the abundance and lifetime of liquid water over geological time. In this respect the sulfate-rich bedrocks analyzed by the rover Opportunity in Eagle and Endurance craters at Meridiani Planum provide important constraints on the physical and thermodynamic conditions at the time of sulfate deposition. Among the different interpretations proposed in literature [2-4], the models involving in-situ alteration of basaltic sands have the advantage that they alleviate the need for large unidentified external sources of altered grains and/or brines. With these issues in mind we have used a geochemical simulator (JChess) to model basalt alteration as a function of time, with the aim of identifying self-consistent scenarios for rock-formation at Meridiani. Model parameters: The initial silicate material modeled is unaltered olivine-rich Martian basalt Adi-rondack from Gusev crater, described in [5]. The initial fluid phase is assumed to be pure H 2 O at 0°C, produced by melting of near-surface ground ice. One of the originalities of our model is that the source of sulfur , as well as the driving force of alteration, is assumed to be gaseous SO 3 , the most soluble and most acidic of the potential sour gases assumed to be derived from oxidation volcanic SO 2 , the aqueous phase simply acting as a medium allowing the chemical reactions to proceed. Dissolution of SO 3 into water at 0°C is considered to be instantaneous, and simulations were performed assuming partial pressures for O 2 and CO 2 close to values observed in the current Martian atmosphere. For kinetic reasons the precipitation of quartz (replaced by chalcedony), hematite (replaced by goethite) and secondary mafic minerals is suppressed in the calculations. Previous calculations [6] assuming congruent dissolution and corresponding to an embedded dune situation (W/R=6), showed that at low SO 3 /basalt the solu