Surficial materials, including soil and dust, are abundant in the upper tens of centimeters of the Martian surface sensed by the Mars Odyssey Gamma Ray Spectrometer (GRS). Seven large areas (14% of the Martian surface) that represent possible compositional end‐members were selected, including three regions heavily mantled with surficial materials. The selection process included mapping the ratio of exposed rocky terrain to surficial materials using high‐resolution imagery. GRS data for H, Cl, Fe, Si, K, and Th were obtained for each area. The areas are chemically homogeneous within each area, given the spatial resolution and analytical uncertainty of the GRS data. However, substantial chemical differences exist among the areas, including the different mantled terrains, contrary to earlier assumptions that surficial materials are globally homogeneous due to aeolian mixing. The observed chemical differences among the areas may be due to variations in the protolith compositions, extent of alteration of the protolith regions, or post soil formation processes. The abundances of Cl, K, and Th in rockier (but still soil‐rich) areas such as Syrtis Major Planum can be explained by mixing between a soil with higher concentrations of Cl, K, and Th, similar to the abundances in the mantled terrains (and some of the landing sites), and crustal rocks containing lower abundances of these elements, similar to Martian meteorites.

Abstract-Drill-core samples from the Bosumtwi impact structure (1.07 Myr old and 10.5 km in diameter) in Ghana exhibit mineralogical evidence for post-impact hydrothermal alteration. Nine samples of drill core obtained through the 2004 International Continental Scientific Drilling Project (ICDP) were studied, including an uppermost fallback layer overlying impactite breccias, and partly deformed massive meta-graywacke bedrock. The petrographic study revealed alteration veins containing secondary sericitic muscovite (comparable to 2M 1 -muscovite) crosscutting original bedding in meta-graywacke and forming a matrix between clasts in impactite breccias. X-ray diffraction (XRD) shows that these impactite samples are rich in 2M 1 -muscovite, consistent with post-impact fluid deposition and alteration. Optical analysis indicates the presence of a pre-impact stratiform chlorite in meta-graywacke samples and a secondary alteration chlorite occurring in all samples. Secondary illite was detected in upper impactites of drill core LB-08A and samples containing accretionary lapilli. The lower temperature constraint for the hydrothermal event is given by 2M 1 -muscovite, secondary chlorite, and illite, all of which form at temperatures greater than 280°C. An absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 °C. The presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores LB-07A and LB-08A indicates that a post-impact hydrothermal system was present in and adjacent to the central uplift portion of the Bosumtwi impact structure. A sample containing accretionary lapilli obtained from drill core LB-05A exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor.