VIS-IR spectroscopy of magnesium chlorides at cryogenic temperatures

Abstract

Chlorinated compounds have been recently identified or suggested to exist in a number of planetary bodies such as Ceres, Europa, Ganymede, Enceladus and Mars, on the basis of remote sensing and in-situ measurements as well as Earth-based telescopic observations. Sodium and magnesium-bearing chlorinated compounds with different levels of hydration have been suggested to reach the surface of these bodies through ascending flows from the interior; thus, such materials could be geochemically related to the putative presence of subsurface liquid reservoirs and could bring precious information about those layers' salty composition. Laboratory spectroscopic data of chlorinated salts carried out at temperatures representative of real planetary bodies, fundamental to correctly interpret the observational data, are currently missing or incomplete.Here we present laboratory reflectance spectra of two hydrated magnesium chlorides, namely MgCl2•2H2O and MgCl2•6H2O, measured at three different grain sizes and at twelve temperature values in the range 80÷295 K. All spectra have been measured in the visible and infrared spectral range 0.5÷4.7 μm. We examined the absorption features and related spectral parameters as a function of both temperature and grain size. These reflectance spectra, as far as the infrared range beyond 2.5 μm is concerned, are the first measured and published at cryogenic temperatures, therefore these data may prove very useful for the interpretation and modeling of remote sensing spectral data from planetary missions. In particular, this new dataset will be helpful in applying spectral unmixing models to past and future observations of some icy satellites, Mars, Ceres, and possibly other Solar System bodies.