The spectral variability of kieserite (MgSO4·H2O) with temperature and grain size and its application to the Martian surface

We spectrally characterize (lambda = 0.35-10 mu m) the low-humidity polymorph of kieserite (MgSO4 center dot H2O), which is abundant on Mars and likely present on Europa, at various grain sizes and temperatures (100-300 K) relevant to the surfaces of Mars and Europa. Compositional analysis of these surfaces often relies on remote sensing using imaging spectrometers such as Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), Mars Express Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite, and Galileo Near-Infrared Mapping Spectrometer. To estimate surface abundances from these observations, well-characterized laboratory spectra are required for comparison. Several variables, including temperature and grain size, affect the observed spectra and must be quantified in the laboratory to more confidently evaluate the returned data. Certain spectral features of kieserite exhibit predictable variability with changes in temperature and grain size that may be exploited to better understand the nature of kieserite on the surface of Mars. For instance, trends in our spectral analysis suggest that absorption features centered at lambda < 3.0 mu m were primarily sensitive to temperature changes, while features at lambda > 3.0 mu m were additionally sensitive to grain size changes. We compare our laboratory spectra with selected CRISM data of suspected Martian kieserite and assess the inherent uncertainty that exists in using band center minima to determine surface composition. Incorporation of these temperature and grain size-specific spectra into linear mixture models of planetary surface spectra will improve the compositional interpretation and contribute to our understanding of surface geochemistry and chemical evolution.