Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmep53f..04p&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #EP53F-04
Physics
[5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes
Scientific paper
Using a stereo pair of HiRISE images of a pole-facing crater slope at 38°S, 218°E, we measure relative elevation changes between manually-selected points to generate topographic profiles along 9 gullies. Based on the topography measurements, the gully alcove accounts for a volume of 6 x105 m3 of eroded material and typical slopes of the interior channel region (above the depositional apron) is ~20°. Using these observations, we focus on the hypothesis that sediment transport on gully slopes occurs via fluvial transport processes in order to determine if it is consistent with the expected timescales and water volumes available for gully formation. To test this hypothesis, we develop a numerical sediment transport model based on steep slope flume experiments performed by Smart [1984]. These "low flow," steep slope conditions make Smart's sediment transport predictor the most relevant to Martian gullies. The finite difference numerical model we employ calculates sediment discharge rate along a gully channel of specified width, depth, and sediment grainsize. We crudely account for three dimensional effects of sediment infilling from valley sides and fan-shaped deposition. Our results suggest that fluvial sediment transport on the ~20° gully slopes is rapid. One of our simulation results is shown in the figure below. Based on our simulations, channels 1 m deep by 8 m wide and 0.1 m deep by 3 m wide transport a sediment volume of 6 x105 m3 in 10 h and 40 d, respectively, under constant flow conditions and require a total water volume of 106 m3 and 0.5x106 m3, respectively. These results suggest a sediment:water volume ratio of roughly 35 - 50% by volume. If these volumes of water are discharged as groundwater, the required aquifer thicknesses and aquifer drawdown lengths would be unrealistically large for a single discharge event. Alternatively, the water volume required by the fluvial transport model could be discharged in 15-30 episodes for an aquifer 10 m thick. Snowpack melting at a rate of 2 mm per Martian yr cannot produce the water discharge rates necessary for fluvial sediment transport unless the water is somehow stored below an ice-rich mantle and then released periodically as suggested by Christensen [2003]. As an alternative to fluvial processes, slope failure and/or wet debris flows may be the dominant processes eroding gully alcoves and depositing sediment on distal fans. a) Perspective view of a sediment transport simulation lasting 14 h under constant flow conditions for a channel 1 m deep by 8 m wide with a sediment grainsize of 10 cm. 6.2x105 m3 of sediment was transported over the course of this simulation. Topographic, slope, and change in elevation profiles are shown every 5000 s (one tenth of total simulation time) in parts b, c, and d, respectively. A visual comparison between the simulation and an observed gully at the same scale is shown in the inset.
Nimmo Francis
Parsons Robert
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