Composition of Amazonian volcanic materials in Tharsis and Elysium, Mars, from MRO/CRISM reflectance spectra

Abstract

Compositions of the Amazonian-aged Tharsis and Elysium volcanic provinces of Mars have been largely obscured in Mars-orbital remote sensing data by windblown dust. Fresh impact craters formed within the last few years have disturbed surface dust, providing unique windows to explore these regions' relatively dust-free mineralogic compositions. Such fresh craters, plus other small exposures of less dusty materials, are resolved spatially in high-resolution targeted observations of visible/short-wave infrared spectral reflectance by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Analysis of CRISM observations of small, relatively dust-free locations in the Tharsis and Elysium regions shows that these provinces are dominated by high-Ca pyroxene and olivine, similar to volcanic materials of Hesperian age. Thus, the mafic mineral compositions of Hesperian and Amazonian volcanic materials appear similar to each other, but distinct from the olivine- and low-Ca pyroxene-rich compositions that dominate Noachian rocks. In the core regions of both provinces, where thermal infrared data indicate the thickest dust cover, the dust is sufficiently thick that few fresh craters penetrate to expose the underlying volcanics. These results may be consistent with low Si contents of surface materials in Elysium and western Tharsis measured by gamma-ray spectroscopy resulting from a thick cover of dust depleted in Si-rich phases, possibly due to eolian sorting.