Control of the radiative behavior of the Martian polar caps by surface CO2ice: Evidence from Mars Global Surveyor measurements

The seasonal polar caps of Mars are composed primarily of solid CO2, and th eir growth and decay play a large part in the planet's atmospheric CO2 cycl e. The thermodynamic temperature of the caps is similar to 145 K, in equili brium with the similar to 6 mbar atmosphere. The optical properties of CO2 ice require that pure snow with a sufficiently small particle size has a hi gh visible/near-infrared albedo and a low infrared emissivity in parts of t he thermal infrared, particularly in the region between 20 and 50 mu m. Dus t mixed into or on top of the CO2 ice will lower the visible albedo and bri ng the thermal infrared emissivity closer to 1. Water ice mixed with the CO 2 will have little effect in the visible and near-infrared to 1.4 mu m but can raise the 20-50 mu m emissivity if the particle size is >50 mu m. Obser vations of both seasonal polar caps during two of the assessment orbits of the Mars Global Surveyor spacecraft are used to show that there is a strong correlation between visible brightness and low 20-50 mu m emissivity. The infrared spectra from the Thermal Emission Spectrometer from regions with l ow 20-50 mu m brightness temperature are consistent with surface deposits o f CO2 with millimeter-sized grains and containing varying small amounts of dust, and they are not consistent with the expected signature of water ice or clouds. Large regions of low emissivity in the spring seasonal caps have not been observed previously. They are correlated with visible bright regi ons which are known to become brighter as the spring progresses. The visibl e brightness of the south polar deposits examined is inferred from historic al observations, while the brightness of the north polar region is determin ed from Mars Orbiter Laser Altimeter measurements at 1.06 mu m. The model o f seasonal CO2 caps with bright, low-emissivity regions agrees with previou s visible observations of bright crater rims, streaks, and other bright are as within the polar caps, some of which may evolve in time from dark, high- emissivity sheet ice to brighter, fractured, lower-emissivity ice layers. I t is also in agreement with models of the CO2 cycle, which require average polar cap emissivities <0.9.