Diffusion barriers at Mars surface conditions: Salt crusts, particle size mixtures, and dust

摘要

The diffusion coefficient of water vapor through porous media at Mars-like surface conditions is measured for a variety of complex particle size distributions and soil compositions. Micron-sized dust simulants, mixtures of sand- and dust-sized particles, and salt-encrusted sand are examined. We find that while the value of the diffusion coefficient, D, can be reduced by up to a factor of 10 for heavily salt-encrusted soils (minimum observed D = 0.4 ± 0.04 cm2 s?1), moderate amounts of salt only produce minor reductions in D. Mechanical packing of pure dust can lower D by a similar amount, while mixtures of dust with sand-sized particles produce at most a factor of ～4 reduction. We conclude that present-day processes of aeolian redistribution, moderate levels of salt encrustation, and volatile loss from dirty ice would be inefficient at producing soil deposits and lags on Mars that pose significant barriers to diffusion. Therefore, subsurface ice deposits that are thermally unstable would not be protected against sublimative loss by such materials.