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The current Mars exploration strategy calls first for the detection from orbit of materials of interest for astrobiology and landing site selection. This places the productivity of the 1996 Global Surveyor Thermal Emission Spectrometer (TES) and the 2001 Mars Odyssey 9-band radiometer THEMIS in the critical path of this strategy. Most predictions of mineral detection limits for TES and THEMIS are based on laboratory spectra, with the assumption that minerals measured in the field will appear as those measured in the laboratory. Here we examine TES spectra and differences in spectral signatures recorded in the field and laboratory environments, and show what these results indicate for the search for minerals on Mars. We conclude it will be challenging for TES and THEMIS to attain their predicted detection limits unless minerals are present under specific conditions: wellcrystalline, smooth surface, and low downwelling radiance contribution. When these conditions are not met, TES and THEMIS have more difficulty detecting minerals. This should be considered when relying on these data sets for landing site selection. Data: We use the airborne hyperspectral imaging spectrometer SEBASS, field spectrometer, and laboratory spectrometer data to characterize field vs. laboratory spectral signatures [1]. SEBASS is uniquely suited to this task because it measures with the highest combined signal-tonoise ratio (SNR) and spectral resolution of any airborne thermal infrared imaging spectrometer. SEBASS and in situ field spectrometer measurements show that indurated, massive calcite (calcrete) at Mormon Mesa exhibits unexpectedly weak spectral contrast at ~11.2 μm [1]. Laboratory spectra also show the calcrete 6.5, 11.2 and 33 μm bands are weaker than expected. Materials with reduced spectral contrast place higher SNR requirements for detection, which makes them more difficult to detect. Causes of reduced contrast: Spectral contrast varies significantly with properties unrelated to composition, and the two most important effects are the cavity effect and volume scattering. Figure 1 illustrates the increase in effective emissivity (εe) with the number of reflections (rcount) off a cavity wall with true emissivity ε [2]: εe = 1 (1 ε) +1) (1) Pits (e.g. vesicles, pits between pebbles, interstices between grains, and cracks, pits, and grooves in rocks) will cause a cavity effect and reduced spectral contrast according to Equation 1. Surface scattering dominates for coarse calcite particles (Figure 2), so the reststrahlen bands appear as emissivity troughs (spectral minima). In contrast, volume scattering dominates the transmission spectrum, so the reststrahlen bands are offset slightly toward longer wavelength, and appear as emission peaks. Volume scattering results from the presence of rough, angular, or very small particles. The effect of increased volume scattering in the Figure 2 fine calcite spectrum causes reduced spectral contrast as it shifts to appear more as the transmission spectrum.