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G. B. Hansen (HIGP/SOEST, Univ. Hawaii)
We have examined data from two orbits in the assessment period of the Mars Global Surveyor (MGS), focusing on the known properties of pure CO2 ice. For CO2 snow surfaces with grains less than a few centimeters in size, there should be a depression in the thermal infrared brightness temperature with a minimum near 25 \mum. These grain-size properties of solid CO2 may explain both the continuous and transient low-brightness-temperature regions discovered by the Viking orbiter infrared radiometers (IRTM). Using the Thermal Emission Spectrometer (TES) data from Orbit 21, we show how the spectra from an annulus near the edge of the retreating south polar cap are consistent with smaller CO2 grain sizes. We suspect that this behavior results from the fracturing of nearly solid sheets of frozen CO2 (formed in this way, or reaching this state by a process of grain-coarsening), as they are heated (mostly from below) by the rising sun. The unfractured layers near the pole behave more like a blackbody, and probably reflect visible light much like the surfaces underneath them, while the cracked surfaces should appear bright. We also investigated a low-altitude pass over the autumn north polar region in orbit 35, using data from both TES and the Mars Orbiter Laser Altimeter (MOLA). We found an area with properties similar to the Viking transient low-brightness-temperature regions, located precisely on the rim of a 22-km crater (from the MOLA profile) at the outlet of the Chasma Boreale. The MOLA reflectivity and the 30-\mum emissivity are anticorrelated across this feature (bright at 1 \mum, low emissivity at 30 \mum), implying that it is caused primarily by the CO2 particle size. We propose that this region is characterized by a local change in particle size, and that it is correlated with topography, a fact hinted at but not proven by Viking IRTM studies. A likely cause of such a feature is the flow over the obstacle by a possibly dusty atmosphere near saturation, causing a local precipitation of CO2 snow nucleated on the dust grains. The spectra of both south and north polar deposits indicate varying amounts of dust contamination, with the north spot having much more dust than the south.