T329 Dynamic Planetary Geology Revealed by Long-Term Observations

Wednesday, 22 October 2014
1:00 PM - 5:00 PM
301

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

Paper number: T329-8
Program: Technical Sessions
Time: 3:00 PM – 3:15 PM

Authors

  • Alian Wang Presenting Author
    Washington University in St. Louis, Saint Louis, MO
  • Jie Wei
    Washington University in St. Louis, Saint Louis, MO

Abstract

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.