CHLORIDE HYDRATES -- SOURCE MATERIALS FOR RECURRING SLOPE LINEAE (RSL) ON MARS

RSL is an important phenomenon that reveals present-day surface-near surface H₂O-relevant processes on Mars. We propose and tested a hypothesis that suggests subsurface chloride hydrates to be their source materials. The hypothesis is initially built on the basis (1) chloride-rich brines or dry-out products do not have Vis-NIR bands, that agrees with the orbital observations; (2) the deliquescence of a chloride hydrate can generate 90-120% brine in volume ratio, that agrees with a ground-truth field investigation on Slope Streak in Antarctica, i.e., a few % of additional water in subsurface soils can cause obvious brightness change observable by satellite images.

We conducted two sets of experiments to test this hypothesis: (1) deliquescence of six chloride hydrates at three temperatures (T) and ten relative humidity (RH) levels; (2) formation of chloride hydrates at low T (-78°C) through vapor-solid reaction. We found that at 5°C< T <50°C, deliquescence-solid boundaries of many chloride hydrates occur near 33%RH. For comparison, the similar boundary of Mg-, Fe²⁺-, Fe³⁺-sulfates occur at 85-95%RH, 95-100% RH, 75-80%RH. It means the deliquescence of chloride hydrates is much easier to start at the raising of T, than the deliquescence of sulfates, and the mid-high RH maintained in a subsurface salty layer can provide suitable RH environment for it at present-day Mars. Furthermore, we found the rate of deliquescence of chloride hydrates has strong temperature dependence. At T range relevant to RSL sites during Mars summer, the deliquescence of CaCl₂.2H₂O and MgCl₂.6H₂O can accomplish within hours or days. Finally, we have successfully demonstrated the formations of several MgCl₂.xH₂O at -78°C through vapor-solid reactions.

These experimental results support a model that the deliquescence of subsurface chloride hydrates would generate RSL on equator-facing slope during Mars summer, then recharging could happen during cold seasons by cold-trapping atmospheric H₂O vapor.