Rebecca Brunt

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 Bchains of depressions that occur in high latitude lowlandsB 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 or randomly along troughs. To investigate hypotheses for the origin and patterning of depressions, I 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 24 Mars patterns, each characterized by hundreds to thousands of individual depressions, are tested against three synthetic patterns in which depressions were distributed randomly and randomly along 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 occurring along the SW margin of Utopia (280^oW 40^oN, 280^oW 55^oN, 260^oW 40^oN and 260^oW 55^oN) 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, < 1deg, with broad SE 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 down sloping troughs, consistent with a surface flow or subterranean pipeflow connection between beads.

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Pages: 154
Supervisor: Lawrence Plug