 CHARACTER OF Mg(ClO4)2 BRINES UNDER MARS REGOLITH CONDITIONS  A. P. Zent1, H. G. Sizemore2,  A. W. Rempel3, 1 MS 245-3 NASA Ames Research Center (Moffett  Field CA, aaron.p.zent@nasa.gov), 2Montani Consulting (HC 64 Box 176 Hillsboro WV,), 3Univeristy of Oregon (Department of Geological Sciences, University of Oregon, Eugene OR   Introduction:  Elsewhere in this volume [1], we report on our investigation of the initiation and growth of ice lenses under Mars like conditions.  In that work, we assume that the soil-water-ice system is gas and solute free.  We conclude that initiation of lens initiation - the unloading of particle-particle contacts by thermomolecular forces at a given soil horizon - may be a common process in the shallow Martian regolith, and that the dominant property controlling the rate of lens growth is the freezing point depression (ΔTf) associated with the interfacial forces of the soil.  Lens growth is thus favored in clay-sized soils over silt soils due to the greater ΔTf, but segregated ice was observed at the Phoenix site, where soils were predominantly siltsized.  Perchlorate salts were also observed at the Phoenix site, and will strongly affect some of the properties associated with potential ice lens growth, over and above increases to ΔTf.  Here, we investigate the nature of Mg(ClO4)2 brines under Mars-like conditions, with particular emphasis on those aspects that might influence the in situ segregation of residual liquids during phase change, potentially leading to the formation of subsurface excess ice.  We also discuss cyclic variations in the water activity (aw) that might affect the habitability of solutions in the shallow regolith. The phase diagram of Mg(ClO4)2 suggests eutectic temperatures on the order of 205 K, meaning that some degree of unfrozen solution would be expected throughout much of the year in a typical shallow martian regolith. In Figures 2 and 3, we compare the unfrozen fraction of H2O due to varying solution concentrations to that expected from interfacial forces in two of our standard soils, Chena Silt and Univik Clay.  While salt addition substantially increases the unfrozen H2O in silt, the large clay surface area is of roughly equal importance in the clays, and substantially different unfrozen amounts are not expected. The density and viscosity of the solutions also increase during freezing.  The viscosity of a binary solution increases by roughly three orders of magnitude, which will severely inhibit potential segregation of subsurface perchlorate brines to form discrete expressions  of excess ice. The precipitation/solvation process for a  (Mg(ClO4)2.6H2O) is very energetic, with roughly a factor of four times more energy released than for freezing pure H2O.  Thus, phase change and precipitation of the hexahydrite salt will be substantially drawn out as the energy associated with phase change is conducted away. Activity:  The nominal limits for metabolic activity in solution is generally thought to be in the vicinity of aw = 0.6.   While eutectic perchlorate brines are thus generally uninhabitable, these activity levels are not reached until the brine cools to ~ 215K.   At higher temperatures, the unfrozen H2O for a saturated pore space solution is well within the habitable range References [1] Sizemore et al., This Volume (2013); [2]Pestrova et al.,(2005); .     Figure 1. Binary phase diagram of Mg(ClO4)2 using the data of Pestrova et a.,  2005, and the path followed by cooling solutions.  Weight percent refers to Mg(ClO4)2 in dry soil.  Figure 2. Unfrozen fraction of H2O during cooling, compared to unfrozen H2O in Chena Silt due to interfacial forces.  Figure 3. Same as above, for Inuvik Clay.  Figure 4. Solution density from freezing onset to the eutectic for 3 initial dry weight concentrations.  Figure 5. Viscosity increase by 3 orders of magnitude in the course of freezing, effectively immobilizing the unfrozen fraction at low T.  Figure 6. The H2O activity coefficient as a function of T during freezing.  
