Ground Truth for Mineralogic and Petrologic Mapping on Mars

Mars mineralogic and petrologic mapping depends critically on accurate mineral identifications and geochemical analyses by orbiting spacecraft. Ground truth for orbital mineralogy and geochemistry is thus a desired aspect of mapping. Martian (SNC) meteorites have sampled at least six (unidentified, so far) sites, based on clustered Mars launch ages, and rovers have provided high-quality analyses from three additional sites. Mineral assemblages in SNCs and new assessments of mineral abundances in MER rocks and soils using Mossbauer, APXS, and MiniTES data are in good agreement with mineral deconvolutions of thermal emission (TES) spectra from the Mars Global Surveyor orbiter. Minerals comprising these rocks and soils conform to basalts and consist dominantly of pigeonite, augite, plagioclase, and olivine, with lesser amounts of Fe-Ti oxides and phosphate, and sometimes sulfate and silica. Orbital geochemical measurements of Si, Fe, K, and Cl (after masking for H) by the Mars Odyssey gamma-ray spectrometer (GRS) are in close agreement with averaged APXS compositions of hundreds of rocks and soils from the Spirit, Opportunity, and Mars Pathfinder sites, especially when rover locations are compared with global element distribution maps. As with mineralogy, geochemical analyses are consistent with basalts. Martian basalts range from olivine- to quartz-normative, and SNCs have higher Fe and lower K and Cl concentrations than global GRS data. There is little, if any, evidence for calc-alkaline (andesitic) or highly alkaline igneous rocks, and evolved (granitic) rocks are uncommon. Thermal emission data are very useful in mineralogic mapping; however, bulk chemical compositions calculated from TES mineral deconvolutions differ from GRS and ground truth geochemical data, limiting their utility in petrologic classification. Instead, TES data provide insights into weathering processes on rock surfaces.