Weathering processes and soil geochemistry on Mars

In this thesis, a number of topics relevant to the issues of weathering and soil geochemistry on Mars are explored. Chapter 1 serves as an introduction Martian altered rocks and soils, which are the products of some combination of physical and/or chemical alteration, do not appear to have been affected by chemical weathering processes similar to those which are common on Earth. This dissertation presents new insights into weathering processes and soil geochemistry on Mars utilizing the results of low-pH alteration experiments performed on synthetic basalts of Martian composition, and in-situ analyses collected during landed missions to Mars (Viking, Pathfinder, and Mars Exploration Rovers). Results indicate that low-pH alteration gives rise to wholesale rock dissolution in which the relative dissolution rates of the mineral phases present in the rock dictate the chemical composition of altered materials, and the chemistry of fluids which have interacted with the rock. Comparison of our experimental results to in-situ analyses of Martian rocks and soils indicates that Martian rock surfaces have been affected primarily by the dissolution of mineral phases at low-pH and low water-to-rock ratio, and contamination by soil and Mg-sulfate salts. A significant part of the chemical variation observed in Martian soils at widely separated landing sites is consistent with a dominant alteration process similar to that observed for basaltic rock surfaces on Mars; a process strongly influenced by the dissolution of olivine at low-pH and low water-to-rock ratio, and the formation of secondary Mg (+/-Fe) sulfates and Fe-oxides. In general, alteration processes on Mars do not mimic the chemical changes produced by chemical weathering on Earth, indicating Martian rocks and soils have not interacted with large volumes of moderate pH rainfall or groundwater. This understanding of Martian rock surface alteration processes, combined with the application of an inverse modeling approach, allows for the evaluation of complex mixing and chemical weathering processes in rocks exposed in the Husband Hills of Gusev Crater. For instance, results indicate that the chemical compositions of the Wishstone and Watchtower Class lithologies of Husband Hill have been modified by interaction with an Mg-Zn-S-Cl-Br rich fluid phase, and that the chemical signature of this interaction mimics the rock surface alteration processes which are endemic to the Martian surface environment.