HR: 17:00h
AN: C12C-05 INVITED     [PDF]
TI: Permafrost Landscapes on Earth: Lessons for Mars and Europa
AU: * Hallet, B
EM: hallet@u.washington.edu
AF: Quaternary Research Center, Box 351360 University of Washington, Seattle, WA 98195 United States
AU: Sletten, R S
EM: sletten@u.washington.edu
AF: Quaternary Research Center, Box 351360 University of Washington, Seattle, WA 98195 United States
AU: Fletcher, R C
EM: rfletche@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences 335 Deike Building, University Park, PA 16802 United States
AB: The cold and hyper-arid Dry Valleys of Antarctica have long been recognized as having environmental conditions on Earth closest to those on Mars. They contain valuable clues about the presence and character of ice in the permafrost, and about surficial processes on Mars. Extensive regions in the Dry Valleys are commonly mantled by poorly sorted glacial or rock fall material ranging in size at the surface from boulders to small pebbles. The abundance of coarse material at the surface reflects vertical sorting due to re-current thermally induced volumetric changes and to vertical cracks forming (and closing) in the upper few meters of the permafrost, as well as removal of fines from the surface by eolian transport. In Beacon Valley, an area underlain by massive ice, the low relief (1 to 10 m) ground surface with scattered 50-100m-wide depressions is veneered with $\sim$0.5m of sublimation till. The existence of this till, which formed over 1-10 million years as debris initially contained in the ice accumulates at the surface, demonstrates the long-term persistence of ice under surface conditions seemingly unsuitable for ice to exist. Polygonal patterned ground, with a typical polygon size of $\sim$20 m, is characteristic of extensive sediment-covered areas and some exposures of bedrock in the Dry Valleys. The thin snow cover, which is due to the hyper-arid conditions and high winds, is conducive to extreme ground cooling and frequent permafrost cracking in the winter. The cracks tend to be vertical, and as they grow and interact, they form polygons. Due to the paucity of surface water, the cracks do not tend to fill with water during the warm season; instead, they fill with sand. Although water is scarce, polygonal patterned ground reflects sufficient ice in the upper potion of permafrost ($\sim$2-3 percent by volume) to permit thermal stresses to develop on scales vastly larger than the grain scale. Recurrent cracking along pre-existing cracks and addition of sand from the surface progressively forms sand-wedges that reach 5 m in width. Wedge growth involves surface divergence in the center and compression at the sides, which results in distinct troughs bordered symmetrically by ridges that delineate the polygons. The ratio of the polygon radius to the divergence rate (0.1-1 mm a$^{-1}$) yields a characteristic time scale of 10$^{4}$-10$^{5}$ years for wedge growth to pervasively disrupt or renew the surface, but this depends on the frequency of permafrost cracking and the amount of material available for infilling open cracks. The striking micro-relief associated with growing sand-wedges, the symmetrical ridges along a linear trough that are most clearly developed in frozen sediments, closely resemble troughs with paired ridges on Europa, which implies recurrent cracking and near-surface introduction of material into the cracks with potential implications about ice thickness. Inclined areas with significant subsurface ice can show diverse indications of gravitationally-induced flow, including rock glaciers with prominent surface ridges and lobes. They reflect differential down-valley motion that can be detected through remote sensing on both the Earth and Mars. Transverse ridges are also common on rock glaciers, and ongoing theoretical analyses suggest that they merit special attention because they may contain rich information about subsurface ice. This includes the thickness and spatial extent of massive ice layers, and the rheology of these layers and the surrounding permafrost.
DE: 1824 Geomorphology (1625)
DE: 5462 Polar regions
DE: 5470 Surface materials and properties
DE: 6218 Jovian satellites
DE: 6225 Mars
SC: Cryosphere [C]
MN: 2003 Fall Meeting