Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p41a0892a&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P41A-0892
Other
5416 Glaciation, 5420 Impact Phenomena (Includes Cratering)
Scientific paper
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.
Allen Christine
Bell Mary Sue
Kanner Lisa C.
McBride S. A.
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