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Brine Aquifers, Salt Deposits and Ice Lenses on Mars

Wednesday, January 21, 2009
Travis (Los Alamos National Lab)

A recent study, using THEMIS data, has found evidence of chloride salt deposits associated with a number of features on the surface of Mars, primarily in the southern hemisphere. Based on their interpretation of the data, precipitation from briny aquifers and surface ponds is a likely source of those salts. Other studies indicate high hydrogen content (and by inference, water content) at the surface of Mars in certain locales, even at the equator. Such ice lenses could be dusty remnants of exchange of H2O between soil and atmosphere, but they might also represent remnants of frozen brine aquifers.

Could brine aquifers in fact readily transport water and salt to the cold surface? The dominant Cl-containing salts in Martian subsurface aquifers are likely to be CaCl2 and NaCl. Salt depresses a solution s freezing point; the NaCl eutectic point is about -22 oC, while for CaCl2, the eutectic point is -52 oC. Geothermal gradients on Mars have been estimated to be in the range of 20 to 40 mW/m2, with considerably higher values earlier in Mars history, and possibly even now in certain areas such as the Tharsis region.

This talk discusses results of a numerical study of possible brine aquifer dynamics in the shallow subsurface subject to geothermal gradients. A previous simulation study of aquifer dynamics in the Martian subsurface considered only pure water aquifers. It found that hydrothermal convection develops for a range of geothermal gradients, for reasonable soil properties. A subsequent, combined experimental and numerical study considered both pure water and salty aquifer dynamics in response to a geothermal gradient. Salt not only depresses the freezing point, but can add an unsteady characteristic to hydrothermal convection. Those studies and this one use the MAGHNUM code. MAGHNUM solves the time-dependent governing equations for water and vapor flow, and heat and salt transport in porous, permeable media in 2-D or 3-D geometries. It allows changes between liquid, vapor and ice phases, depending on local thermodynamics. It has been used for a number of astro-hydrological studies, ranging from hydrothermal flows in asteroids, to water moons of Jupiter and Saturn, to Martian impact crater systems. This study suggests that significant surface salt deposition and ice lens formation could be associated with briny aquifer dynamics.

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