Emissivity and reflectance spectra at different temperatures of hydrated and anhydrous sulphates: A contribution to investigate the composition and dynamic of icy planetary bodies

Kurzfassung

Accurate analyses of existing spacecraft data and telescopic observations are of fundamental importance to describe in detail the surface composition of icy planetary bodies, such as the icy galilean moons. However, the spectral library data to compare the remote data with planetary observations are usually restricted to small spectral ranges and collected only at room temperature. In this study, selected hydrated Mg-sulphates were studied. Emissivity and reflectance spectra were collected in the 3-20 μm and 0.25-16 μm range, respectively, with emissivity collected in the temperature range 300-673 K, and reflectance from 300 to 193 K. All samples were recovered after the heating and cooling cycles and were characterized by means X-ray powder diffraction. Rietveld refinements of the collected data were performed to evaluate the mineralogical composition of the samples before and after the thermal treatment. Both reflectance and emissivity measurements gave us information about the vibrational modes and overtones of SO4 and H2O. Moreover, the careful analysis of the collected data allowed us to study the influence of the cation substitution in the sulphate's crystal structures on the wavelength position of the SO4 vibrational modes. In particular, in simple salts [kieserite MgSO4·(H2O); hexahydrite MgSO4·6(H2O); gypsum CaSO4·2(H2O); thenardite Na2SO4; arcanite K2SO4; anhydrite CaSO4; barite BaSO4], the increase of the cation's radius gives a shift of the ν3 overtone towards higher wavenumbers, varying in the range 1880-2300 cm-1. On the other hand, it was observed that, in hydrated sulphates, that the increase of the strength of the hydrogen bond gives a shift of the ν3(SO4) overtones towards lower wavenumbers. At high temperature, the depth of several absorption bands in the emissivity spectra increases; however, when total or partial de-hydration occurs, a discontinuity in the deepening is observed due to the endothermic character of the dehydration phenomena. Among the investigated samples, several hydrated and anhydrous sulphates undergo de-hydration and/or phase transition at specific temperature conditions. This leads to the stabilization of new crystal structures with higher density compared to the low temperature hydrated ones. The likely occurrence of minerals dehydration will strongly affect the availability of free water in planetary depths and, as a consequence, the thickness of the icy crust. Likewise, the density changes between the different polymorphs will affect the buoyancy. This means that the structural behavior of the "non-icy" components of the icy crust have a significant impact on the structure and dynamics of the planetary bodies and have to be considered in planetological models.