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Search for Buried High-grade Water Ice at Low-to-mid Latitudes of Mars Using MONS Neutron Data

Wednesday, February 10, 2010

Global maps of hydrogen near the surface of Mars, interpreted as mass fraction of water-equivalent hydrogen, WEH, have been generated from neutron leakage fluxes from Mars. Although these data provide an unambiguous indicator of WEH, quantitative details of its magnitude and burial depth depend on the regolith model used to interpret the data. Presently, this model assumes a spatially uniform surface cover layer having one-to-two mass percent of WEH and thickness, D, covering a semi-infinite 'permafrost' deposit containing water mass fraction Wdn. Although general characteristics of these maps compare favorably with other experimental observations and theoretical estimates at high latitudes, no comparisons have been possible at low to mid latitudes. Recent HiRISE observations of white deposits uncovered temporarily by five recent small craters between 43 deg and 55 deg N latitude, 150 deg to 190 deg E longitude [Byrne et al., 2009], prove unambiguous disagreement with the neutron model results. However, the spatial scales of the MONS and HiRISE data are very different. In this talk we develop a new model that gives consistent results with the new HiRISE observations using MONS data. Use of this model shows that large areas at low-to-mid latitudes contain high-grade water ice deposits that are buried less than one-to-two meters below the surface. These deposits could have been emplaced by the freeze-out of plumes of brine driven by enhanced geothermal temperature gradients or the residue of past high obliquity-driven water-ice precipitation.

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