Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p42a0565p&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P42A-0565
Mathematics
Logic
1832 Groundwater Transport, 5400 Planetology: Solid Surface Planets, 5464 Remote Sensing, 6225 Mars
Scientific paper
Gullies were identified in Mars Orbiting Camera (MOC) images, and have been attributed to flowing water (Malin&Edgett, Science 288). These gullies emanate directly from the ground and are visible in the walls of canyons and craters; thus the gullies are likely to be the result of groundwater. As on Earth, the depths of these gullies are likely controlled by the presence of an impermeable layer. Our goal is to examine the morphology and depth of the gullies to determine the nature of this controlling layer. Images from Hale Crater, Nirgal Vallis and Dao Vallis were compared, all of which contain gullies but are situated in different latitudes in the southern hemisphere and different geologic settings. The morphologies of these gullies were examined, especially the relationship between the alcove and the channel of the gullies to the surrounding rock layers exposed. Depths of the features in the images were calculated using Mars Orbiter Laser Altimetry (MOLA) data. Once the MOLA profiles were identified and graphed, the profiles were mechanically manipulated using Photoshop to overlay the path the satellite would have scanned in the MOC image. If the small-scale features of the MOLA profile as well as the large-scale features could be correlated accurately, the match was considered a success. In the north wall of Nirgal Vallis, the majority of the gullies emanate consistently ~260+/-30 meters below the surface, with one image containing a gully with a depth of 640+/-10 meters. Dao Vallis shows a greater variation in gully depth than Nirgal Vallis, from as deep as 1131+/-63 meters below the surface to as shallow as 67+/-5 meters. Many of the Dao images contain multiple gullies with origins at different depths. Images from Hale Crater contain gullies both on the central peak and on the northern wall; the depths of these are variable and originate from near the surface to depths of 348+/-64 meters. In Nirgal Vallis, gullies that emanate from a consistent depth lie within the Noachian plateau subdued crater unit (Npl2) (Scott&Tanaka, 1986); gullies associated with the older Noachian plateau crater unit (Npl1) emanate from a deeper layer. The range of depths of gullies in Dao Vallis correlates with two units, a volcanic layered unit, which has a total variability of ~80-230 meters, and a channeled plains unit with a significantly larger variability of ~60-1131 meters. Gullies in Hale Crater appear only in connection with a relatively dark, competent layer both in the wall and the central peak. The gullies in all three areas appear to be closely associated with a competent rock layer; the alcoves originate from the base of the rock layer, and may appear to have headward erosion, extending the alcoves up into the rock layer above (particularly in Hale). This correlation of gullies to visible rock units, and the variability of gully depths, favors rock as the impermeable layer rather than permafrost. We conclude the gullies emanate from a specific, impermeable rock layer that is related to the geology map-able on the surface.
Gilmore Mark
Phillips Edward
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