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

Abstract

We spectrally characterize (λ = 0.35-10 μm) the low-humidity polymorph of kieserite (MgSO 4 ·H 2 O), 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 Minéralogie, l'Eau, les Glaces et l'Activité, 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 λ < 3.0 μm were primarily sensitive to temperature changes, while features at λ > 3.0 μm 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.

Europa's surface composition from near‐infrared observations: A comparison of results from linear mixture modeling and radiative transfer modeling

“…In order to move forward with such efforts, greater investment in laboratory investigations of the optical constants of candidate materials is needed. While some progress has been made here [ Dalton and Pitman , ; Hanley et al ., ; Jamieson et al ., ], much still remains to be done. As noted above, RT methods may be optimal for analyzing spectra from certain key locations on Europa where significant quantities of optically dark nonice materials may exist.…”

Crystallography relevant to Mars and Galilean icy moons: crystal behavior of kieserite-type monohydrate sulfates at extraterrestrial conditions down to 15 K

“…A related debate concerns the existence and nature of a postulated second structural polytype of kieserite with a broader stability range but similar spectroscopic properties, the so-called low-humidity (LH) kieserite, presumed by several authors to prevail on Mars (Wang et al, 2009;Jamieson et al, 2014). While elucidation of the actual structure and properties of this enigmatic 'LH phase' is still pending to date, it is one of the goals of the present study to provide reliablehitherto missing -low-temperature reference data of 'classical' MSO 4 ÁH 2 O compounds, to serve as a benchmark for comparison and as a basis for spectroscopic and thermodynamic modeling purposes.…”