Identification of a basaltic component on the Martian surface from ThermalEmission Spectrometer data

The Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument col lected 4.8 x 10(6) spectra of Mars during the initial aerobraking and scien ce-phasing periods of the mission (September 14, 1997, through April 29, 19 98). Two previously developed atmosphere-removal models were applied to dat a from Cimmeria Terra (25 degrees S, 213 degrees W). The surface spectra de rived for these two models agree well, indicating that the surface and atmo sphere emission can be separated and that the exact atmosphere-removal mode l used has little effect on the derived surface composition. The Cimmeria s pectra do not match terrestrial high-silica igneous rocks (granite and rhyo lite), ultramafic igneous rocks, limestone, or quartz- and clay-rich sandst one and siltstone. A particulate (sand-sized) sample of terrestrial flood b asalt does provide an excellent match in both spectral shape and band depth to the Cimmeria spectrum over the entire TES spectral range. No unusual pa rticle size effects are required to account for the observed spectral shape and depth. The implied grain size is consistent with the thermal inertia a nd albedo of this region, which indicate a sand-sized surface with little d ust. The identification of basalt is consistent with previous indications o f pyroxene and basalt-like compositions from visible/near-infrared and ther mal-infrared spectral measurements. A linear spectral deconvolution model w as applied to both surface-only Cimmeria spectra using a library of 60 mine rals to determine the composition and abundance of the component minerals. Plagioclase feldspar (45%; 53%) and clinopyroxene (26%; 19%) were positivel y identified above an estimated detection threshold of 10-15% for these min erals. The TES observations provide the first identification of feldspars o n Mars. The best fit to the Mars data includes only clinopyroxene compositi ons; no orthopyroxene compositions are required to match the Cimmeria spect ra. Olivine (12%; 12%) and sheet silicate (15%; 11%) were identified with l ower confidence. Carbonates, quartz, and sulfates were not identified in Ci mmeria at detection limits of similar to 5, 5, and 10%, respectively. Their presence elsewhere, however, remains open. The Cimmeria spectra are not we ll matched by any one SNC meteorite spectrum, indicating that this region i s not characterized by a single SNC lithology. The occurrence of unweathere d feldspar and pyroxene in Cimmeria, together with the inferred presence of pyroxene and unweathered basalts in other dark regions and at the Viking a nd Pathfinder landing sites, provides evidence that extensive global chemic al weathering of materials currently exposed on the Martian surface has not occurred.