Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13a1257t&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13A-1257
Other
[0702] Cryosphere / Permafrost, [1034] Geochemistry / Hydrothermal Systems, [5422] Planetary Sciences: Solid Surface Planets / Ices, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Recent findings (e.g., Byrne et al, 2009) indicate that water ice lies very close to the surface at mid-latitudes on Mars. Re-interpretation of neutron and gamma-ray data is consistent with water ice buried less than a meter or two below the surface. Hydrothermal convection of brines provides a mechanism for delivering water to the near-surface. Previous numerical and experimental studies with pure water have indicated that hydrothermal circulation of pore water should be possible, given reasonable estimates of geothermal heat flux and regolith permeability. For pure water convection, the upper limit of the liquid zone would lie at some depth, but in the case of salt solutions, the boundary between liquid and frozen pore water could reach virtually to the surface. The principal drivers for hydrothermal circulation are regolith permeability, geothermal heat flux, surface temperature and salt composition. Both the Clifford and the Hanna-Phillips models of Martian regolith permeability predict sufficiently high permeabilities to sustain hydrothermal convection. Salts in solution will concentrate in upwelling plumes as the cold surface is approached. As water ice is excluded upon freezing, the remaining solution becomes a more concentrated brine, reaching its eutectic concentration before freezing. Numerical simulations considering several salts (NaCl, CaCl2, MgSO4), and a range of heat fluxes (20 - 100 mW/m2) covering the range of estimated present day heat flux (20 to 40 mW/m2) to moderately elevated conditions (60 to 100 mW/m2) such as might exist in the vicinity of volcanoes and craters, all indicate the same qualitative behavior. A completely liquid, convective regime occurs at depth, overlain by a partially frozen "mushy" layer (but still convecting despite the increased viscosity), overlain by a thin frozen layer at the surface. The thicknesses of these layers depend on the heat flux, surface temperature and the salt. As heat flux increases, the mushy region lies closer and closer to the surface, and the frozen layer thins. At the higher heat fluxes (> 60 mW/m2), upwelling plumes can deliver liquid brine to within a few meters of the surface, even breaching it for the salts with very low eutectic points.
Feldman William C.
Maurice Sylestre
Travis Bryan J.
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