Investigating the hysteretic behavior of Mars-relevant chlorides

Abstract

Liquid solution stability has been a highly studied topic in the Martian community since the detection of perchlorate (ClO4−) at the Phoenix landing site and the global detection of chloride (Cl−) by THEMIS (Thermal Emission Imagining System, onboard Mars Odyssey). Understanding how brines form and react to changing environmental conditions helps identify potentially habitable environments on Mars, both at present and in the past. Here we measure the extent of metastability of magnesium chloride (MgCl2) and sodium chloride (NaCl) brines when freezing. We find that the metastable eutectic temperature of MgCl2 depends on the maximum temperature (Tmax) reached before freezing. If Tmax < −15 °C, the metastable eutectic temperature (mTeu) is only 3 °C below the stable eutectic temperature, and if Tmax > −15 °C, mTeu is 15 °C below the stable eutectic temperature (Teu). We speculate that this metastable behavior follows the phase diagram for the transition into the 8 hydrate for MgCl2, thus, yielding a different freezing temperature, the peritectic for MgCl2·8H2O. However, mTeu for NaCl is independent of Tmax and was constantly at 3 °C below Teu with no peritectic (consistent with the phase diagram). We also found that MgCl2 brine can exist for at least 60 h at 5 °C below its Teu. Applying our findings, we determined the potential time evolution of brines at Palikir crater, using a time-series of modeled temperature profiles. Surficial layers melt more frequently, but layers at 2–3 cm depth are able to warm above Tmax > −15 °C and maintain brine for longer than surficial layers. The evaporation rate of brine buried by 2–3 cm of regolith is greatly reduced due to the generally cold temperatures, solute concentration, and by the regolith overburden. We also found that at Palikir crater, only the deep subsurface (~9.5 cm depth) has water activities (~0.75) high enough to support life. Overall, the metastable properties of brines can drastically affect their formation and longevity on Mars, and should be considered in future models.