Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p72c..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P72C-01
Other
1823 Frozen Ground, 6200 Planetology: Solar System Objects (New Field), 6225 Mars
Scientific paper
In its present climate Mars is globally covered by permafrost, a region of the subsurface that regardless of ice content is always below 273 K. The upper boundary of this zone ranges from the surface to a depth of only a few centimeters. The lower boundary is as much as several kilometers deep, where geothermal energy warms the deeper regolith, but can be less than a kilometer near the equator. Water may be present in the permafrost as an interstitial vapor, as a solid (ground ice), or as a thin adsorbed layer on the surface of soil grains. Liquid water can persist only as a concentrated brine. Subsurface ice (ground ice) within the permafrost can persist if stability conditions allow. In the absence of a liquid phase, the transport of water within the permafrost occurs primarily by vapor diffusion. As a result ground ice can be dynamic, exchanging with atmospheric, polar, and other ground-ice reservoirs over time. Based on results from numerical dynamic-equilibrium models, ground ice is stable against sublimation in geographic locations and at depths where the annual-mean subsurface saturation density of water vapor is below the water-vapor density of the atmosphere. These are also geographic locations where the annual-mean temperatures are below the frost point of the atmosphere (the temperature at which atmospheric water vapor saturates and begins to condense as frost). In a porous regolith in diffusive contact with the atmosphere, water vapor will condense in the pore space forming ground ice where these stability conditions exist. Numerous theoretical investigations over the past few decades have suggested that near-surface ground ice should be found in the mid and high latitudes of Mars at depths less than a few tens of centimeters from the surface. In contrast, the equatorial region should be absent of ice unless it is isolated from the atmosphere. Recent observations of abundant near-surface hydrogen have confirmed the presence of ice approximately in these higher-latitude regions and provide constraints for refinement of our models. The theory of ground ice stability and its distribution, dynamics, and nature will be reviewed. Comparisons will be drawn between martian and terrestrial ice-rich permafrost and placed into the context of various observations of Mars.
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