31st Annual Meeting of the DPS, October 1999
Session 39. Mars Surface: Evidence of Change
Contributed Oral Parallel Session, Thursday, October 14, 1999, 8:30-9:50am, Sala Plenaria

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[39.04] MOC observations of cliff tops and wall rock layering in the Valles Marineris, Mars

R.A. Beyer, A.S. McEwen (LPL, University of Arizona)

New Mars Orbital Camera (MOC) images taken since March of this year with resolutions of up to 1.4 meters per pixel have been obtained of the cliff tops and walls of the Valles Marineris canyons. In many cases, at the cliff edges we observe diverse morphologies not previously seen. We also observe the ubiquitous layering in the canyon walls at a greater resolution than before.

In some areas we see a plateau surface that simply appears to have been eroded away to form the canyon wall. The edge appears either sharp or slightly weathered. In others, there is a distinct terrace that steps down from the plateau. This terrace could represent a less resistant solid layer over the more competent bedrock or could be evidence of a thick eolian layer that has weathered back from the cliff edge. Alternatively, this material may be volcanic tephra. There is also a morphology that may either be very fine layering or some kind of etching at the cliff's edge. If this is layering, it is significantly different from the layering we observe further down the walls. These features curiously occur only in a few places near a set of valleys and channels cut into the south wall of Ius chasma, often cited as sapping channels.

The wall rock layering that we observe appears rough and irregular at the highest resolutions, this may be consistent with a volcanic origin, but water-lain sediments greatly disrupted by impacts or tectonism cannot be ruled out. Large boulders can be seen downslope in some cases. In some places bright layers can be seen along with the more common dark ones. This could be an actual compositional layer, bright talus collecting on a ledge, or simply sun-facing facets that are returning a bright reflection.

We will continue to observe the Valles Marineris and perhaps a more complete coverage of the cliff tops and wall rock will be reached in the future.

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