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Mineral Mapping of Gale Crater Using Orbital Data: Results From Visible-Near Infrared Reflectance Spectroscopy


Over the past decade, high spatial and spectral resolution visible-near infrared spectrometers have revolutionized our understanding of Martian mineralogy. Hyperspectral imaging data returned by the OMEGA and CRISM instruments (onboard the European Space Agency's Mars Express mission and NASA's Mars Reconnaissance Orbiter mission, respectively) spanning the 0.35 – 5 μm wavelength region have revealed numerous exposures of hydrated sulfates, clay minerals, zeolites, opaline silica, Fe-oxides and carbonates in addition to mafic minerals such as pyroxene and olivine. When used in conjunction with high-resolution orbital images (∼0.25 m/pixel), the highest spatial resolution reflectance data (∼18 m/pixel) allow for the assessment of mineralogical variations with stratigraphic position. In Gale Crater, analysis of these data have revealed that the ∼5 km tall central mound (Mt. Sharp) contains Fe-oxides, hydrated sulfates, clay minerals, olivine, and pyroxene. The lowermost strata of Mt. Sharp contain weak signatures consistent with sulfates, which are overlain by meter-scale clay-bearing strata, which in turn are overlain by sulfate and Fe-oxide bearing units. Sulfate compositions are most consistent with Mg-varieties of variable hydration state, and both monohydrated phases such as kieserite as well as more hydrated phases such as hexahydrite may be present. Reflectance spectra of clay-bearing rocks are best matched by Fe-rich smectites such as nontronite or possibly Fe-rich saponite. Visible imagery indicates that some of these mineral detections may correspond to sulfate and/or clay cemented sandstones. In contrast to the preponderance of hydrous minerals in the lowermost strata, the uppermost strata in Mt. Sharp exhibit spectral signatures similar to ubiquitous Martian dust and lack evidence for hydrated sulfates and clays. The upsection transition from mineral assemblages dominated by hydrated clays/sulfates to ones dominated by apparently anhydrous phases and Fe-oxides suggest that the stratigraphy in Mt. Sharp records the ‘drying out’ of Mars approximately 3-3.5 Ga. Integration of mineralogical and stratigraphic variations also indicate that Mt. Sharp records a variety of depositional environments and post-depositional processes, providing testable hypotheses for environmental conditions, water-rock interaction, and diagenesis that can be tested in situ with Curiosity's payload.