Model for Formation of Martian Residual Cap Depressions (Swiss Cheese)

Details Model for Formation of Martian Residual Cap Depressions (Swiss Cheese) Model for Formation of Martian Residual Cap Depressions (Swiss Cheese)

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p22a0536b&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #P22A-0536

Physics

5400 Planetology: Solid Surface Planets, 5462 Polar Regions, 5470 Surface Materials And Properties, 6225 Mars

Scientific paper

In an effort for explain the formation of the `Swiss-cheese' terrain visible on the southern residual cap of Mars (Thomas et al., Nature, 404,2000); we have developed a radiative model to follow the growth/decay of an initial depression due to sublimation/condensation of carbon dioxide. The pits making up this terrain have many distinctive features; they are shallow (~10m deep), with steep walls and flat floors and contain an interior moat that runs along the bottom of the walls. Their diameters range from a few 10's of meters to a kilometer. The model accounts for incident sunlight, emitted thermal radiation, and scattered short and long wave radiation. We have included the effects of a layer of water ice placed under the carbon dioxide at adjustable depth. The water ice layer is free to store heat during the summer (when the carbon dioxide has been removed) through subsurface diffusion of heat. Release of this heat at the end of the summer can inhibit frost formation. We have investigated many cases involving pure dry ice with constant albedo, albedo as a function of insolation, and differing albedo for fresh and residual frost (the latter has lower albedo). In most cases the initial depressions heal themselves and disappear into the surrounding terrain. Cases involving the layer of water ice provide a much closer approximation to the shape of the observed features (especially the flat bottoms). A problem arises of how much exposed water ice we can have during the summer season and still have temperatures averaged over the footprint of the Thermal Emission Spectrometer be close to the carbon dioxide sublimation temperatures. The depth to the water ice layer is a strong controlling factor of the evolution of depression shape and depth in our model. Matching this shape with observations yields important information regarding the depth to any putative water ice layer within the residual cap itself. It is known from laboratory measurements that carbon dioxide is too weak to support the edifice of the southern cap (Nye et al., Icarus, 144, 2000). A likely scenario, based on analogy with the northern cap is that the main structure is water ice with a covering of frozen carbon dioxide. If this covering is thin enough then it may be that the southern cap could loose its permanent carbon dioxide covering during times of higher obliquity. >http://www.gps.caltech.edu/~shane/agu2001/index.html

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