Mathematics – Logic
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p31a10s&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #P31A-10
Mathematics
Logic
1823 Frozen Ground, 1824 Geomorphology (1625), 1829 Groundwater Hydrology, 5470 Surface Materials And Properties, 6225 Mars
Scientific paper
High-resolution (<10m/pixel) images from the Mars Orbiter Camera (MOC) have revealed kilometer-scale channels which have a general alcove, channel, apron morphology and emanate from exposed subsurface layers; these have been interpreted as recent groundwater seepage features by Malin and Edgett (Science 288). Here we report on the examination of these gullies along a transect exposed by the Dao Vallis in the southern hemisphere of Mars. This allows us to examine gullies that have presumably experienced similar climatic conditions. We examine the morphlogy of the gullies, the geology of the units with which they are associated and the depth of the gullies below the surface. Dao Vallis is a 600 km outflow channel which empties into the Hellas Basin. Eleven MOC images were examined that include gullies along ~450 km of the Vallis, from its head SW towards Hellas. The gullies incise into two geologic units (Crown et al., Icarus 100) from NE to SW: Hadriaca Patera volcanic unit (AHh) and Channeled Plains Rim unit (Ah5) downstream. Despite the differences in geology, all channels appear to have the following morphological characteristics. All gullies emerge at some distance (variable) below the surface where exposed walls contain apparent rock units. We observe the gullies to emanate either directly from individual rock layers or from an unstratified smooth material with positive relief. When associated with rock layers, the gullies often spring from a particular continuous layer within a layered sequence. Gullies also emerge directly from the bottom of a layered unit that caps the valley wall. In other images, smooth material lies within the alcove at the tops of gullies. The gullies originate within the smooth materials. We interpret the smooth material to be rich in ground ice, thereby supporting the theory that permafrost is related to and may feed seepage gullies. In some areas, the smooth material exists without gullies. Once seepage occurs, the channels follow a semi-braided or branching network pattern. If the seepage source is concentrated to one area, sharply incised single channels are produced. Braided morphologies arise under more dispersed seepage conditions. The gullies produce depositional structures analogous to terrestrial alluvial fans. In the northern reaches of Dao Vallis, the recent depositional structures are more well-developed than their downvalley counterparts and the channels often continue out along the valley floor. Downvalley, fewer well-developed depositional structures are observed, and the channels are often attenuated once they reach the valley floor. These characteristics may reflect variations in sediment load or temporal relationship with Dao floor materials. These observations suggest a direct relationship between gullies, rock units and ground ice. We hypothesize that the rock layer is a necessary catalyst for groundwater as an impermeable layer and/or due to the rock's thermal properties with respect to surrounding units. Ground ice may indicate groundwater flow over longer timescales than that recorded by the gullies. We intend this characterization of the morphology of features in the Dao Vallis to provide a model of the aggregate conditions necessary to produce modern groundwater emissions on Mars.
Gilmore Martha S.
Stopper A. R.
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