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A.P. Rossi, G. Komatsu (IRSPS, Universita' d'Annunzio, Pescara, Italy), J.S. Kargel (Astrogeology Team, USGS, Flagstaff, Arizona, U.S.A.)
Recent high resolution MOC images have revealed the presence of deformed impact craters on flow-like features characterized by narrow bands of alternating light and dark material on the walls of Valles Marineris. The maximum crater elongations are consistent with the flow directions. Moreover the directions of these flows follow the topography downslope. In some cases, the flows emanate from cirque-like depressions, and the flows are divided by sharp ridges similar to arête. These landforms have resemblance to (1) alpine-type glacier morphology, including cirques, arêtes, and glaciers containing medial moraines; and (2) Grand Canyon-type sapping and mass wasting features. Certain aspects of the features in Valles Marineris seem more consistent with the first hypothesis involving a viscous rheology of the flows driven by ice-assisted creep processes. This hypothesis includes direct analogies to glaciers and rock glaciers. In the case of rock glaciers, flow is produced by freeze-thaw and by internal deformation of ice cores or lenses, whereas in the case of glaciers, movement occurs by internal deformation plus basal sliding in some cases where the glacier is melted at its bed. The amounts and roles of ice in the genesis of the Martian glacier-type landforms in Valles Marineris are not clear at this point. The population density of undeformed fresh impact craters on these flows appears to be low compared with the surrounding plateau areas. This may indicate relatively recent ages of the flow processes. Despite the limited coverage of the MOC images, the occurrence of the flow-like features associated by the deformed impact craters is common in most areas of Valles Marineris that have been imaged. We currently are working on the systematic survey of these flow-like features. We are also in the process of analyzing MOLA data in order to constrain the geometry of these landforms.