A Field Study of Thermal Infrared Spectra of Carbonates , with Implications for Studies of Mars

Data recorded of the Mormon Mesa by the airborne hyperspectral imaging spectrometer SEBASS show that some massive carbonates exhibit dramatically reduced spectral contrast for the strong carbonate bands at 6.5 and 11.25 μm. If carbonates are present on Mars, this type of reduced spectral contrast in massive carbonates could explain why they have not been detected using remotely sensed thermal infrared spectra, including data returned by the Global Surveyor Thermal Emission Spectrometer (TES). A goal of this study is to examine spectra recorded of weathered carbonates in the field, and to appraise what the results indicate for the search for carbonates on Mars. We conclude that the observed reduction in band contrast may be sufficient to preclude TES from detecting massive, well exposed carbonate deposits such as those at the Mormon Mesa. However, SEBASS data demonstrates that these deposits can be detected if the spectra are recorded with sufficient signal to noise ratio (SNR). Introduction. If Mars had a denser CO2 atmosphere in its past and if liquid water had been present on its surface, then large deposits of carbonate materials likely formed [1]. Critical to the validation of this warm and wet climate model is the determination of whether carbonates are present on Mars. In addition, carbonates typically mark regions of hydrothermal activity, and provide a suitable environment for the preservation of biomarkers. Identifying carbonate deposits on Mars would thus provide valuable input for landing site selection [2]. Currently, there is no strong spectral evidence for the presence of carbonates on Mars. Three spectrometers have returned thermal infrared spectra from Mars: the 1969 Mariner 7 Infrared Spectrometer (IRS, 1.9 14.4 μm); the 1971 Mariner 9 Infrared Interferometer Spectrometer (IRIS, 5-50 μm); and the 1996 Thermal Emission Spectrometer (TES, 6 50 μm). These spectral data sets are available to the planetary community and can be used to search for the presence of carbonates on Mars. Calcite has strong infrared bands centered near 6.5, 11.25, and 35 μm. However, weathering and surface roughness dramatically reduce the band contrast of most materials, including the carbonate bands listed above. To better understand the real-world signatures of carbonates deposits, and how they might be detected remotely using thermal infrared spectroscopy, we studied the spectral signature of weathered carbonates at the Mormon Mesa, Nevada. Field site and data. Mormon Mesa is near Mesquite, Nevada. It has a cap rock of massive, strongly indurated calcite (calcrete), overlain by loamy soil rich in carbonate and quartz, and significant coverage by fragments of calcrete [3] (Figs. 1 and 2). In addition, there are localized regions that contain limestone in a conglomerate (Fig. 3)