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[3.01] A Sublimation Model for the Formation of the Martian Polar Swiss-cheese Features

S. Byrne, A.P. Ingersoll (Caltech)

Swiss cheese features were first identified by Thomas et al. [Science, 2000] using Mars Orbiter Camera imagery. They have flat floors and steep sided walls. Their lateral sizes of the order of a few hundred meters. They are quite shallow with shadow measurements indicating a depth of about 8 meters. Although the depressions are quite circular they do display a slight but consistent asymmetry in the form of a small cusp which points poleward indicating that the origin of these features is connected with insolation. As the seasonal frost disappears their walls appear to darken considerably relative to the surrounding terrain. The flat interior of the depression however does not appear to change in this way.

A model to explain the formation of the Swiss-Cheese depressions on the southern residual polar cap of Mars has been developed and tested. The model computes the energy balance of CO₂ frost including all orders of scattering of solar and infrared radiation as well as heat storage in a H₂O substrate. Due to their observed lateral expansion rate of 1-3 m/yr [Malin et al., Science 2001], these depressions must form in a CO₂ medium. However, it was found necessary to include a non-volatile layer within the polar cap to ensure the modeled depressions develop their observed flat floors. We interpret this non-volatile layer to be most likely water ice. Above this non-volatile layer an albedo gradient within the CO₂ ice was inserted to encourage the modeled depressions to develop steep walls. This gradient is consistent with CO₂ ice that contains increasing concentrations of impurities with depth and is consistent with the late summer appearance of the depressions. This modeling suggests that the southern residual cap is exceptionally thin and its contribution to the global reservoir of carbon dioxide is small.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.