Investigating Sources of Spectral Olivine Enrichments in Martian Bedrock Plains Using Diurnal Emissivity Changes in THEMIS Multispectral Images

We investigate spectral olivine detections in the Noachian cratered highlands that are spatially associated with flat bedrock exposures. Strong olivine detections were previously attributed to relatively high (∼10%–15%) modal abundances of olivine in the bedrock, and then later hypothesized to instead originate from a patchy cover of olivine-rich sediment formed from erosional lag deposits and mineral-specific comminution/transport. We test these hypotheses by developing a new technique to partially separate the spectral signals of bedrock and unconsolidated surficial sediment. It uses the differing thermophysical properties of sand and indurated materials, which change the relative contributions of these materials to the emitted spectrum over the diurnal cycle. We use multispectral Mars Odyssey Thermal Emission Imaging System (THEMIS) observations collected in the mid-afternoon and early evening to apply this technique to eight Terra Sabaea bedrock plains and the Nili Fossae olivine-carbonate plain. Significant changes in spectral shape arising from compositional differences between bedrock and sediment cover are observed in four bedrock plains. Where these differences are observed, the spectral changes are consistent decreased olivine abundance in the sediment fraction, rather than increased as previously hypothesized. Our results indicate that bedrock materials in the Terra Sabaea region are olivine-rich in composition, likely indicating a pyroclastic origin for these materials.