Temperature-dependent, VIS-NIR reflectance spectroscopy of sodium sulfates

Hydrated sodium sulfates have been suggested to be present in variable amounts in Solar System objects such as Mars and Europa, among the possible others. The presence of these hydrated species is related to current/past aqueous environments, thus has an importance regarding the potential habitability of planetary objects. In this study, we analyzed anhydrous sodium sulfate (thénardite) and the hydrated sodium sulfate (mirabilite) by means of visible-infrared reflectance spectroscopy in the 0.4–5 μm spectral range, at different low temperatures between 80 and 298 K. Each mineral has been analyzed in three different grain sizes, between 36 and 150 μm. The anhydrous compound, thénardite, is characterized by a nearly flat spectrum in the visible and near IR up to 2.6 μm, while in the 3–4 μm region, the spectrum shows a few weak features due to H2O and SO42− overtones/combinations. The first strong SO42− overtone is visible at 4.6 μm. Spectra of mirabilite are substantially characterized by H2O absorption features in the 1–3 μm region, and by sulfate overtone/combination bands occurring at 3.8 and 4.7 μm. A weak feature appearing at 2.18 μm is also putatively attributed to the sulfate ion. The bands show changes as a function of temperature. The hydration absorption features in mirabilite show the strongest dependence with temperature, both in terms of shift in position and change of spectral shape. Bands at 3.1–3.24 μm in thénardite, as well as absorption features located at 1.78 and 2.47 μm in mirabilite, could be used as diagnostic proxies for the detection of these two minerals on planetary bodies.