Modeling Ferrous/ferric Iron Chemistry with Application to Martian Surface

Introduction: The Mars Exploration Rover (MER) missions have stimulated considerable thinking about the surficial geochemical evolution of Mars [1-4]. Among the major MER findings is the presence of the mineral, jarosite (a ferric sulfate salt), which suggests formation from an acid-sulfate brine. The objectives of this work were to (1) add ferric iron chemistry to an existing ferrous iron model (FREZCHEM), (2) extend ferrous/ferric iron geochemical models to lower temperatures (< 0 °C), and (3) use the reformulated model to explore ferrous/ferric iron geochemistries on Mars. Methods and Materials: The FREZCHEM model is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer equations [5] for the temperature range from <–70 to 25°C and the pressure range from 1 to 1000 bars [6-10]. The model is parameterized for the Na-K-Mg-Ca-Fe-H-Cl-SO4-NO3-OH-HCO3-CO3-CO2CH4-H2O system, which includes 81 solid phases. Results: One of our prime objectives was to extend existing Pitzer-approach ferric iron parameterizations to lower temperatures. Freezing-pointdepression data for FeCl3 solutions [11] were used to extend Fe(III)-Cl parameterization to the eutectic temperature at 238 K (-35°C). A similar approach was used to extend Fe(III)-SO4 parameterization to 261 K (-12°C). But, in this particular case, we used Al2(SO4)3 data as an analogue for Fe2(SO4)3 data because the latter cold temperature data are lacking. Table 1 is a listing of ferrous and ferric iron solid phases that are currently in the model. Extending the solubility products to temperatures < 25°C was based on experimental measurements whenever possible. Lacking such experimental data, we relied on the van’t Hoff equation Ln(KT2 ) = Ln(KT1 ) + ΔHr R 1