HR: 1340h
AN: P13A-0973    [Abstracts]
TI: Pitting within the Martian South Polar Swiss Cheese Terrain
AU: * Pathare, A
EM: avp@gps.caltech.edu
AF: Caltech, MC 150-21 California Institute of Technology, Pasadena, CA 91125 United States
AU: Ingersoll, A
EM: api@gps.caltech.edu
AF: Caltech, MC 150-21 California Institute of Technology, Pasadena, CA 91125 United States
AU: Cushing, G
EM: gcushing@usgs.gov
AF: USGS - Flagstaff, 2255 North Gemini Drive U.S. Geological Survey, Flagstaff, AZ 86001 United States
AU: Titus, T
EM: ttitus@usgs.gov
AF: USGS - Flagstaff, 2255 North Gemini Drive U.S. Geological Survey, Flagstaff, AZ 86001 United States
AB: The morphology of the Martian South Permanent Residual Cap is dominated by enigmatic quasi-circular landforms commonly referred to as "Swiss cheese" terrain. These large Swiss cheese depressions, which typically have widths of more than 100 m and extend down to the base of the layer, are expanding at rates of a few meters per Martian year due to CO2 sublimation. We present high-resolution Mars Orbiter Camera (MOC) images detailing extensive "pits," by which we mean small cavities generally less than 10 m in diameter that do not penetrate completely through the Swiss cheese terrain. This pitting is only observed upon the thickest (~10 m) Swiss cheese mesas ("Unit A" as classified by Thomas et al. 2004), and moreover only occurs within 50 meters of the edges of these deposits. We argue that the pits are collapse features caused by the release of CO2 gas from a pressurized layer several meters below the mesa top. As the walls of the mesa retreat due to radiation imbalance, the pressurized layer is exposed, and CO2 vents out laterally, weakening the layer and causing the collapse. We can think of no other process that communicates laterally over distances of 50 meters in one Martian year, which is the time scale over which the pits form. For a layer 6 meters thick, the hydrostatic head is ~200 mbar, which provides an upper bound to the gas pressure in the sealed lower layer. However, for that maximum pressure to be attained, the CO2 in the lower layer must be approximately 30 K warmer than CO2 on the surface. Such a temperature differential is difficult to maintain, though, given that 6 meters is also the thermal skin depth for CO2 over 1 Martian year. We are exploring a number of mechanisms that might continually or cyclically warm this layer and enable rapid venting when the seal is broken. The persistence of polygonal cracks on the mesa tops could be further evidence of subsurface thermal variations.
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting