The northern and southern seasonal polar caps of Mars are formed in the polar night, during their respective autumn and winter seasons, by the condensation of atmospheric CO2 directly to the solid phase as ice, snow, and possibly frost. During spring and summer, the seasonal ice sublimes, returning CO2 to the atmosphere. The caps advance and recede in response to seasonal variations in solar insolation, extending as far as 40¿ from the poles, and have been noted by telescopic observers since the 17th century. Roughly 25% of the atmosphere, which is 95% CO2 by volume, is cycled into and out of the seasonal caps. The CO2 cycle dominates atmospheric circulation on Mars, and must be thoroughly understood in order to answer fundamental questions about climate history and the global distribution of near-surface water. The general aspects of atmospheric circulation are well known; however, many details remain hidden in the polar night. General circulation models (GCMs) have been developed that model the seasonal condensation and sublimation of CO2, accounting for the dynamics of the atmosphere, surface thermal properties, and the effect of atmospheric dust on the rate of frost deposition. The GCMs are tuned to match pressure variations measured by the Viking landers by adjusting the albedo and emissivity of the seasonal caps. While this approach gives an accurate estimate of the total amount of CO2 cycled through the caps, the cap properties used by the models differ significantly from those observed by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), which revealed a heterogeneous cap with time varying emissivity and albedo.