WEATHERING OF ROCKS AND SOILS ON MARS

Studying weathering processes is a critical first step to understanding the sedimentary history of Mars, and Hesperian to Amazonian weathering processes appear to differ fundamentally from those typically seen on Earth. Rock analyses from Gusev crater indicate that rock surface weathering relationships at the Spirit site are well described by the chemical fractionations observed during experimental alteration of synthetic Martian basalts at pH=0-1. Comparison of experimental data to in-situ rock analyses by the Alpha Particle X-ray Spectrometer (APXS) onboard Spirit indicates that a leached alteration zone, developed under acidic conditions, is present on rock surfaces at Gusev. This zone is not chemically fractionated to a large degree from the underlying rock interior, indicating that the rock surface alteration process has occurred at low fluid-to-rock ratio. The geochemistry of natural rock surfaces analyzed by the APXS is consistent with a mixture between adhering soil/dust and the leached alteration zone. The chemistry of rock surfaces analyzed after brushing with the Rock Abrasion Tool (RAT) is representative of the leached alteration zone. The chemistry of rock surfaces analyzed after grinding with the RAT is representative of the interior of the rock, relatively unaffected by the alteration process occurring at the rock surface. The RAT penetration depths range from ~2-9 mm, indicating that the leached alteration zones are not of significant thickness. Elemental measurements on soils from the Spirit, Opportunity, Pathfinder and Viking 1 landing sites indicate that soil chemistry from widely separated locations is consistent with the low-pH, low fluid to rock ratio alteration relationships developed for Gusev Plains basalts. Soils are affected principally by mobility of FeO and MgO, consistent with alteration of olivine-bearing basalt and subsequent precipitation of FeO and MgO bearing secondary minerals as the primary control on soil geochemistry. The surface weathering relationships apparent for rocks at Gusev crater, and soils analyzed at multiple landing sites, do not require that alteration occurred under conditions similar to those in the terrestrial environment where significant volumes of moderate pH water are typically involved.