Thermal infrared observations of the condensing Martian polar caps: CO2 ice temperatures and radiative budget

The physical processes that control the formation of the Martian seasonal polar caps are not completely understood. On the one hand, climate models reproducing the annual variations in atmospheric pressure caused by the condensation of the polar caps have shown that the amount of CO2 actually trapped in the polar regions in winter is lower than expected from simple energy balance considerations. On the other hand, the available spacecraft observations of the condensing polar caps are complex and puzzling. They are characterized by highly variable low-emission zones exhibiting anomalously cold brightness temperatures. To better understand these results we have carefully reanalyzed the Viking infrared thermal mapper (IRTM) measurements obtained during the polar night in both hemispheres. First, by removing the signature of the low-emission zones in the data, we have retrieved the actual surface temperatures of the polar caps. We find that they were lower than the frost point of CO2 for the topography of the polar regions usually used in models, especially in the south polar region, However, our analysis reveals that the low-emission zones were more frequent and more intense in the northern hemisphere. They strongly altered the polar radiative budget which is computed and analyzed here, and thus the CO2 condensation rate. We conclude that the models' tendency to overestimate the amount of CO2 ice condensing in the polar caps is explained by different causes in each hemisphere. In the north, the models did not simulate the low-emission zones and underestimated the heat advected to the polar cap region during the dust storms, especially by the upper atmosphere polar warming. In the south, they overestimated the polar cap surface temperatures and also did not simulate the low-emission zones.