On Chlorine Salts: Their Detection, Stability and Implications for Water on Mars and Europa

Chlorine salts (e.g. chlorides, chlorates and perchlorates) are an important factor in the stability of water on the surfaces of planetary bodies. Here we have shown that perchlorate and chlorate salts will lower the freezing point of water, allowing it to be liquid down to ~204 K. These salts will also slow down the evaporation rate, extending the lifetime of the liquid water solution. Chlorine salts have been detected on Mars, which has significant implications for the stability of water and hence its habitability. To study their effects on the stability of water on planetary surfaces, we need to first locate where these chlorine salts exist; this is typically done by remote sensing. To date, only anhydrous chlorides have been remotely detected, mostly due to the lack of hydrated chlorine salts in the spectral libraries used to identify features. To address this deficit, we measured reflectance spectra for numerous chlorine salts. Hydration bands were most common in near-infrared spectra, with band depth and width increasing with increasing hydration state. In the mid-infrared, oxychlorine salts exhibit spectral features due to Cl-O vibrations. We also investigated the spectral features of these salts at low temperature (80 K) to compare with remote sensing data of the outer satellites, specifically Europa. At low temperature, water bands become narrower and shallower than their room temperature counterparts. We show that chlorine salts do possess distinct spectral features that should allow for their detection by remote sensing, though care must be taken to acquire laboratory spectra of all hydrated phases at the relevant conditions (e.g. temperature, pressure) for the planetary body being studied