Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p72c..09g&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P72C-09
Mathematics
Logic
1829 Groundwater Hydrology, 1832 Groundwater Transport, 5415 Erosion And Weathering, 5470 Surface Materials And Properties, 6225 Mars
Scientific paper
We continue to study the apparently recent Martian gullies in Nirgal Vallis, Dao Vallis, Hale Crater and Gorgonum Chaos using Mars Orbiting Camera (MOC) images. All gullies analyzed here are found on poleward-facing slopes. Every observed gully emanates from within a single competent subsurface rock layer or from a specific competent layer among multilayered strata. The gullies emanate from the layer, even when the layer is faulted, suggesting a causal relationship. We have measured the elevation of gully heads using Mars Orbiter Laser Altimeter (MOLA) data and found the depth of the gully heads measured ranges from approx. 10 to 800 m (average: 280 m; n=70) below the local surface and 2 km above to 5 km below the MOLA datum. To test the hypothesis that the subsurface depth is related to subsurface geology, we calculated average depths below the local surface of the gully heads as a function of mapped surficial geologic unit. Our data show that the average depth of gully heads is more variable in strata beneath geologic units predicted to have greater primary or secondary permeability. We posit that gully heads with a more regular range of depths within a geologic unit indicate a regionally continuous impermeable layer (e.g., ash or clays) approximately parallel to the local surface. Gully heads that show a greater variability in depths within a geologic unit may indicate the occurrence of faulted and/or semipermeable layers (strata with abundant unconformities or disrupted units at craters and chaos). The average elevations of gully heads show no correlation with latitude, suggesting the layer associated with the gully heads is not permafrost. We advocate a model in which gullies are produced in areas where 1) ground ice is present and 2) is melted in the near-surface within a climate that can sustain liquid water which 3) percolates through any permeable materials down the hydraulic gradient until encountering an impermeable layer (aquiclude). If the impermeable layer dips toward an exposed wall, this groundwater may flow along the layer and discharge at the surface, forming a gully. Implicit in this model is that regions without impermeable layers would lack gullies. At Dao Vallis (approx. 33S to 38S), exiting groundwater contributes to both gullies and deposits of smooth, positive-relief material we interpret to be ice-rich regolith, likely indicative of cooler local surface temperatures than the other gullies examined. The average gully alcove depth of 280 m below the Martian surface probably reflects the relatively high permeability of the upper hundreds of meters due to various weathering processes. The presence of gullies may mark the distribution of subsurface impermeable layers globally. As some of these geologic units are mapped as Noachian, the aquicludes marked by gullies may have controlled the flow of groundwater throughout much of Martian history.
Gilmore Martha S.
Phillips Eleyne L.
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