Infrared remote sensing of Mars and the Mars astrobiology exploration strategy

The Mars exploration strategy calls first for the detection from orbit of minerals indicative of environments conducive to the support of life or the preservation of biomarkers. That information would then be used for astrobiology landing site selection. The near-term search will be conducted by the 1996 Global Surveyor Thermal Emission Spectrometer (TES) and the 2001 Mars Odyssey 9-band radiometer Thermal Emission Imaging System (THEMIS). This places the productivity of TES and THEMIS in the critical path of the Mars astrobiology strategy. Most predictions of mineral detection limits for TES and THEMIS are based on laboratory spectra of fresh mineral surfaces. However, standard laboratory measurements of fresh mineral surfaces generally do not reproduce all the spectral effects of weathering and surface roughness that are very apparent in field spectra, and these differences can critically affect interpretations of TES and THEMIS data. Here we examine causes of variations in spectral contrast, and differences in spectral signatures recorded in the field and in typical laboratory measurements, and show what the results indicate for the search for minerals and landing sites using TES and THEMIS. We conclude that for TES and THEMIS to attain their predicted mineral detection limits, minerals must be present under specific conditions: well-crystalline, smooth-surfaced at several scales, and low atmospheric downwelling radiance contribution. As a result, TES and THEMIS should not necessarily be used to exclude landing sites that are of interest for other reasons (e.g. geomorphology), but that exhibit no clear detections of minerals of interest to astrobiologists.