The role of aqueous alteration in the formation of martian soils

Martian equatorial dark regions are dominated by unweathered materials and it has often been assumed that they have not been significantly altered from their source lithology. The suite of minerals present is consistent with a basaltic composition and there has been no need to invoke additional processes to explain the origin of these compositions. We have begun to question this result based on detailed observations using a variety of datasets. Locally derived dark soils have a mineralogy distinct from that of adjacent rocky surfaces; most notably a lower olivine content. This pattern is common for many surfaces across the planet. Previous work using detailed measurements acquired within the Gusev Plains has shown that olivine dissolution via acidic weathering may explain chemical trends observed between rock rinds and interiors. Mineralogical trends obtained from rocks and soils within the Gusev Plains are more prominent than the elemental trends and support previous results that indicate that dissolution of olivine has occurred. However, clear differences are also present in elemental abundances that indicate a variety of inputs and processes are likely responsible for the formation of martian dark soils. Despite the potential complexity of source materials and processes, it appears that most martian dark regions have likely experienced aqueous alteration and chemical weathering appears to be closely linked to the mechanical breakdown of materials. Regardless of the responsible mechanism, there appears to be a general, though not perfect, correlation between elevated olivine abundance and high-thermal inertia surfaces on Mars. (C) 2010 Elsevier Inc. All rights reserved.