A Field Study of Thermal Infrared Spectral Signatures , with Implications for Studies of Mars

Data recorded of indurated, weathered carbonates 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 massive carbonates are present on Mars, this type of reduced spectral contrast could explain why they have not been detected using thermal infrared data sets, including the Global Surveyor Thermal Emission Spectrometer (TES). It could also cause similarly rough carbonates to be missed by the planned 2001 nine-band radiometer THEMIS. On the other hand, SEBASS data demonstrate that these deposits can be detected by spectra recorded with sufficient signal-to-noise ratio (SNR). The observed reduction in band contrast is significant, and we conclude it is cause by surface roughness effects [1]. The nature of carbonate and other formations on Mars is uncertain, as well as the amount and kind of subsequent weathering, but a rough surface is certainly a possibility that must be taken into account. These results should be considered in planning for future instruments, and when utilizing thermal infrared spectra for landing site selection. This effect was found by drawing on expertise and unique technology most commonly used for the Department of Defense (DoD). The significance of the lessons learned illustrate the importance both of extending spectral studies to the field, and of drawing on data sets and expertise from non-traditional groups, in order to best define what is needed to detect and identify interesting materials and desirable landing sites on Mars using infrared spectroscopy. Background. If Mars had a denser CO2 atmosphere in its past and surface water, then large deposits of carbonate likely formed [2]. Critical to the validation of this model is the determination of whether carbonates are present. In addition, carbonate deposits would provide important input for landing site selection [3]. Currently, there is no strong spectral evidence for carbonates on Mars, which has long been a puzzle in spectral studies. Three spacecraft spectrometers have returned thermal infrared spectra from Mars, and these spectral data sets are available to the planetary community. Table 1 gives instrument details. In the thermal infrared, calcite has clear bands centered near 6.5, 11.2, and 35 μm. However, weathering and surface roughness dramatically reduce the band contrast of most materials. Field site. 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 [4] (Figs. 1 and 2); and localized regions contain limestone in a conglomerate [1,4].