WATER AND MER: DETERMINING SURFACE HYDRATION FROM MINI-TES

The martian surface has a few percent water by weight in minerals. This has been determined both from orbiting spectral instruments (Mariner 6/7 Infrared Spectrometers and Phobos Imaging Spectrometer for Mars) and the Viking Gas-Chromatograph-Mass-Spectrometer. Spectral observations have relied on the water of hydration feature at 3-μm and past work has shown an increase in band depth (i.e. more hydrated minerals) at the Meridiani (MER-B, Opportunity) landing site. This increase in surface hydration is most likely caused by the massive sulfates found there on the surface by MER. Water also has both vibrational and librational modes that cause absorption from 5 to 8 μm (2000 to 1250 cm^-1). While these features are well known in terrestrial observations where atmospheric water bands limit or interfere with surface measurements, they are not well studied in minerals. Discussion in the literature focuses almost exclusively on the vibrational feature occurring near 6.1-μm (1640 cm^-1). Bracketing the other end of this wavelength range is the occurrence of the Christiansen feature near 8-μm (1250 cm^-1). Preliminary analysis of hydrated, poorly crystalline soils and other hydrated species suggests that spectral features in this range are related to water in the mineral structure. Some complications arise in shifting between reflectance measurements made in many laboratories and surface observations in emission. On Earth it is difficult to get a high quality field emission measurement due to the contribution from sky radiance, and laboratory measurements of emission heat samples which may drive off loosely bound water. Also, laboratory measurements commonly show “roll-off” at the lower wavelengths in particulate samples. The ability to use Mini-TES to explore hydration of surface materials will be described as well as implications for the interpretation of features due to carbonate in this wavelength range.