MIR, VNIR, NIR, and Raman spectra of magnesium chlorides with six hydration degrees: Implication for Mars and Europa

Abstract

AbstractChloride salts have been identified on the surface of Mars and Europa based on the results from the two soil samples analyzed by Phoenix's wet chemistry laboratory for Mars, as well as some interpretations of the data from the remote sensing for Mars and Europa. To help elucidate our understanding of Mars' and Europa's surface mineral composition and relevant environmental conditions, we studied a series of magnesium chlorides with all six hydration degrees that were synthesized using various laboratory conditions. The identifications of synthesized magnesium chloride hydrates were confirmed using X‐ray diffraction. We took a systematic study of the Raman, mid‐infrared (MIR), visible near‐infrared (VNIR), and near‐infrared (NIR) spectroscopies. We found various spectral features that can be linked to the structural characters of H2O molecules in the six crystal structures. On the basis of those knowledge, we found several trends of spectral feature variations that follow the increase of hydration degree of these magnesium chlorides, which can be used to distinguish them during the planetary exploration missions using the Raman, MIR, NIR, and VNIR spectroscopic techniques. Our data will not only fill the gap in the libraries of planetary spectroscopy but also contribute to the interpretation of Mars' and Europa's current and future exploration data. Specifically, identifying the types of Cl salts (hydrous or anhydrous) within geological context at the surface of Mars and Europa will constrain the current and past processes that they experienced, which can help in understanding the evolutions of these planetary bodies.