HR: 14:25h
AN: P23C-04 INVITED     [Abstracts]
TI: Infrared and Visible Observations of South Polar Spots and Fans
AU: * Christensen, P R
EM: phil.christensen@asu.edu
AF: Arizona State Univeristy, Dept. of Geological Sciences, Tempe, AZ 85287-6305 United States
AU: Kieffer, H H
EM: phil.christensen@asu.edu
AF: Celestial Reasonings, 2256 Christmas Tree Lane, Carson City, NV 89703 United States
AU: Titus, T N
EM: titus@usgs.gov
AF: U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ 86001 United States
AB: A variety of relatively low-albedo features that form in regions of the south polar cap during early spring have been previously reported. Four types of these dark features have been recognized: spots, fans, blotches, and halos. The spots are typically <15-50 m in diameter with typical spacings of several 100 m. Fans are 10's to 100's of m in length, 10-30 deg. in angular size, typically originate from a preexisting dark spot, and all point in a similar direction within a given area. Blotches are larger than spots, 100's of m to 10's of km in size, with less distinct boundaries, and, unlike most spots, have albedo patterns that match from winter to summer. Halos are roughly circularly symmetric annuli 10's to 100's of m in diameter surrounding spots. All of these features are only slightly darker than the surrounding ice, with TES-derived albedos of 0.22 versus 0.23 for the ice. We have observed these features with THEMIS in 100-m per pixel infrared and 18-m per pixel visible imaging, focusing intensely on a specific region (99 E, 86.2 S) where spot and fan formation was observed the previous Mars year. A few dark spots form before sunrise, with significant spot formation occurring immediately following sunrise. A large number of spots form fans within 10-20 days. All of these features are within 5 C of CO2 ice temperatures, indicating that they must be must be a very thin layer (<1 mm) that is on top of the CO2 ice, and do not represent surface defrosting. H. Kieffer initially developed a conceptual model to account for the observed observations of the spots, fans, and blotches. In this model relatively dark, granular material lies at the surface during summer. One to two meters of CO2 ice forms the residual cap during winter. This ice anneals to form a translucent slab, allowing sunlight to penetrate to the subsurface, and forming an impermeable layer. After sunrise, insolation reaches and heats the substrate, leading to sublimation of the CO2 ice from the base. Pressure builds at the base of the slab, resulting in the formation of vents that jet CO2 gas, carrying dark granular material to the surface to form spots. Some of this material is subsequently blown downwind to form fans. The small differences in albedo between the dark material on the surface and the dark substrate results in nearly equal solar energy input and nearly equal sublimation. However, in the spots the sublimation occurs at the surface, whereas over the rest of the cap the sublimation occurs at the base. As a result of this near equality, the dark spots do not burrow down to the substrate, but instead the entire upper surface slowly lowers. Eventually the ice is completely removed and the dark granular material is back on the surface. Operating each year, this process will winnow out the finest grained materials that can be carried away by the wind in suspension, resulting in a layer of material that has been well sorted by a unique aeolian process that operates vertically.
DE: 5422 Ices
DE: 5462 Polar regions
SC: Planetary Sciences [P]
MN: Fall Meeting 2005