Abstract
Clay minerals, nanoscale clay precursors, and other hydrated secondary minerals have been observed on Mars, providing evidence of water-rock interaction in the planet’s ancient past. Mineralogical and stratigraphic evidence suggests that some of these minerals may have formed under conditions of pedogenic weathering. We review pedogenic weathering processes, climatic factors, and rates in terrestrial systems and consider their possible implications for ancient Mars. We also review the proposal that some Martian clay minerals may have precipitated directly from magmatic fluids, and discuss the possible implications for aqueous processes on Mars.