Martian Mineralogy: Global Mapping of Solid Solution Variation from Mars Global Surveyor Thermal Emission Spectrometer Data

A major objective of the Mars Global Surveyor Thermal Emission Spectrometer (TES) investigation was to determine the composition of surface minerals and rocks. Global unit mapping based on TES data has provided a broad picture of global geologic variation and context for site-specific studies of detailed mineralogy. Studies with TES data have sought to identify which mineral groups (e.g., pyroxene, olivine, feldspar, sulfate) are present on Mars, and examined global- and local-scale variations in the absolute abundances of different mineral groups. Recently, TES data were used to show that Martian rocks exhibit a range of compositions across the forsterite-fayalite olivine series. We will show that other mineral groups also exhibit solid solution variation on Mars. For example, relative to prior global mineral mapping efforts using TES data, we find larger fractions of orthopyroxene/low Ca pyroxene and we observe spatially coherent geographic variations in Mg-Fe composition. High Ca (clino-) pyroxenes are dominated by intermediate Mg-Fe compositions (augite), and in particular, by a phase that has relatively high Ti content. Global maps of high- and low Ca pyroxene distribution and abundance are geographically correlated with geologic units derived from photogeologic mapping. For example, some low-Ca pyroxene compositions have distributions that correlate with Noachian and Hesperian terrains. Syrtis Major exhibits a combination of mineral compositions that is not shared by any other terrain, suggesting it probably should not be considered the type locality for TES Surface Type 1. We will discuss these results in detail, and also describe observed distributions of compositions within other mineral groups.