Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.p33d..12b&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #P33D-12
Physics
1824 Geomorphology (1625), 5400 Planetology: Solid Surface Planets, 5470 Surface Materials And Properties
Scientific paper
Circular to elliptical depressions connected by troughs, visible in Mars Orbiter Camera images, occur in association with small-scale polygons in Utopia Planitia. Possible terrestrial analogues --chains of depressions that occur in high latitude lowlands-- are the result of thaw-derived subsidence over ice-wedges, especially at wedge intersections, linked by straight or angular water courses that follow ice wedge troughs. The similar plan-view shape and apparent connectivity between depressions support the hypothesis that Utopia patterns also may form by interactions between patterned ground, ice-rich sediment and flowing surface water. Alternatively, depressions might form by localized subsidence from sublimation of water or CO2 ice that initiates randomly, randomly along troughs, or preferentially along wide troughs. To investigate hypotheses for the origin and patterning of depressions, we compare their distribution in Utopia to remote sensing data on topography, thermal inertia and hydrogen content of the shallow subsurface. In addition, the spacing and connectivity along troughs of 25 Mars patterns, each characterized by hundreds to thousands of individual depressions, are tested against three synthetic patterns in which depressions were distributed randomly, randomly along troughs, and randomly along broadest troughs. Mars depressions have diameter 4-100 m and broadly variably separation distances of 5-300 m, compared to ranges of 1-50 m and 5-50 m for beaded depression patterns in northern Alaska. Principally occuring along the southwest margin of Utopia (40° -47° N, 257° -276° W) on surfaces of Hesperian age, Mars patterns occur in regolith classified as fine grained loose material with dispersed rocks and/or bedrock outcrops. Water ice content of the upper metre of regolith is low, < 5% by weight. In comparison, fine grained perennially frozen soils in terrestrial settings typically contain 30-70% water ice, but this may be topped by a drier seasonally frozen layer. Mars features principally occur on shallow slopes, < 1° , with broad southeast exposure. The crisp morphology of meter-scale depressions suggests recent or ongoing formation. Comparisons with synthetic terrains indicate that depressions are not uniformly distributed across polygonal patterns, but instead preferentially occur along downsloping troughs, consistent with a surface flow or subterranean pipeflow connection between beads.
Brunt R. X.
Plug L. J.
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