HR: 0800h
AN: P41A-0892    [Abstracts]
TI: Cratering Evidence For The Age And Thickness Of An Extensive Ice-Rich Mantle In Western Utopia Planitia, Mars
AU: * Allen, C C
EM: carlton.c.allen@nasa.gov
AF: NASA Johnson Space Center, Astromaterials Acquisition and Curation Office -- ST, Houston, TX 77058 United States
AU: Bell, M S
EM: mary.sue.bell1@jsc.nasa.gov
AF: Lockheed Martin Space Operations, Basic and Applied Research Department, Houston, TX 77058 United States
AU: Kanner, L C
EM: lisa\_kanner@brown.edu
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AU: McBride, S A
EM: sam97@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY 14853
AB: Polygonal features with characteristic dimensions of 100 +/- 30 m, bounded by cracks, are commonly observed on the martian northern plains. These features have previously been attributed to thermal cracking, in direct analogy to ice-wedge polygons in terrestrial polar regions. Polygons were mapped in the northern mid latitudes (30 to 65 N) using all Mars Orbiter Camera (MOC) narrow-angle images (5 m per pixel) from September 1997 through September 2003. Three fourths of MOC images showing polygons are centered in western Utopia Planitia (40 to 50 N; 258 to 288 W). This region, notable for its dense concentration of polygonal terrain, is otherwise non-unique in its mapped geology, topography, gravity, or albedo. Previous authors have suggested that this concentration of polygons indicates the presence of a generally continuous ice-rich mantle. Ice stability models, neutron spectroscopy data, and the common occurrence of thermokarst indicate that the ice is concentrated below 1 m depth and is currently subliming. The MOC images show 687 craters, with diameters between 100 m and 4 km, on polygonal terrain. The size-frequency distribution of these craters larger than 1 km is concordant with the Barlow distribution for craters larger than 8 km in western Utopia, indicating preservation of a late Hesperian crater population. Approximately 20 of the 687 craters on polygonal terrain postdate the adjacent polygonal cracks, indicating Amazonian-age deposition or activation of the ice-rich layer. The size-frequency distribution of craters on polygonal terrain shows a marked deficiency of craters smaller than 1 km, suggestive of mantling. Some such craters with diameters between 460 m and 1.1 km are buried to their rims by polygonal terrain; below this range all rims are buried, and above all rims are exposed. Based on the MOLA-derived relationship between rim height and crater diameter, this range indicates that the ice-rich layer is locally 30 to 40 m thick. These findings are in accord with recent models of obliquity-driven deposition and sublimation of ice-rich mantles in the northern mid latitudes of Mars.
DE: 5416 Glaciation
DE: 5420 Impact phenomena (includes cratering)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting