Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p41a1354f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P41A-1354
Mathematics
Logic
0720 Glaciers, 0722 Rock Glaciers, 0776 Glaciology (1621, 1827, 1863), 0798 Modeling
Scientific paper
An integrated system with glacial features exists at 34E, 41N in the Deuteronilus-Protonilus Mensae region. This 30,000 km2 valley system is typical of dozens of fretted valleys in this region along the dichotomy boundary. We compare features described in current geological observations with results from the University of Maine Ice Sheet Model (UMISM) that we feel support the glacial interpretation of these features and also allow speculation as to the timing and processes responsible for the formation of these features. Geological observations identify evidence for a number of features that are felt to be indicative of glacial flow. These include: 1) localized alcoves from which emanate narrow, lobate concentric-ridged flows interpreted to be remnants of debris-covered glaciers; 2) alcove depressions perhaps indicating sublimation of material from relict ice-rich accumulation zones; 3) plateau-ridge remnants between alcoves typical of glacially eroded aretes; 4) horseshoe-shaped ridges upstream of topographic obstacles; 5) convergence and merging of LVF fabric in the down-valley direction; 6) deformation, distortion and folding of LVF in the vicinity of convergence; 7) LVF with pits and elongated troughs in distorted areas; 8) distinctive lobe-shaped termini with associated pitting where the LVF emerges into the northern lowlands. This evidence defines a coherent, unified flow regime extending from the upper valley reaches down to the northern lowlands. Additional support for the glacial hypothesis comes from a GCM for a dusty-atmosphere Mars with obliquity set to 35o and a water source in the Tharsis region. The GCM generates a pattern of ice accumulation in good agreement with these geological observations. This climate is what one might expect to follow a high- obliquity excursion of the sort that built ice sheets on the Tharsis volcanoes. UMISM as used here is an adaptation for the Martian environment of a thermo-mechanically coupled shallow- ice approximation terrestrial ice sheet model used for time-dependent reconstructions of Antarctic, Greenland, and paleo-icesheet evolution on Earth. Starting with no ice, the model is run for 2 million years. While this is longer that is expected for any steady climate to hold on Mars, it delivers a flow pattern that can be compared to the geological interpretations. We present ice thicknesses, surface elevations, and velocity maps at four times during the growth of this ice sheet. At 300Ka the flow from the sides has not yet merged in the centers of the valleys, a configuration that would not produce the turning flow observed. By 500Ka the beginning of a coherent downvalley flow is observed with ice from each side of the valley merging in the center. By 1000Ka there is a well-established valley glacier extending to the mouths of the valleys. Velocities are as high as 250 mm/year. By 1500Ka, the glacier extends out of the valleys onto the northern lowlands. Either 1000Ka or 1500Ka would produce the observed landforms. We also present comparisons of flow features in high-resolution THEMIS images with modeled flow at 1000Ka.
Fastook James L.
Head James W.
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