Missing Holes in the Hills? Studying the crater record of the Columbia Hills

Details Missing Holes in the Hills? Studying the crater record of the Columbia Hills Missing Holes in the Hills? Studying the crater record of the Columbia Hills

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p21a0132h&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P21A-0132

Statistics

5415 Erosion And Weathering, 5420 Impact Phenomena, Cratering (6022, 8136), 5464 Remote Sensing, 5470 Surface Materials And Properties, 6225 Mars

Scientific paper

Now that Spirit has obtained panoramic views from the Husband Hill summit area in the Columbia Hills (CH), we are afforded the opportunity to reexamine the role of cratering in their geomorphic evolution. The orbital images of the CH taken with the Mars Orbiter Camera (MOC) on Mars Global Surveyor, at first glance, show a lower crater density in the CH as compared to the basaltic volcanic plains that Spirit crossed on the floor of Gusev Crater. This apparent paucity of craters is at odds with most of the proposed formation mechanisms for the hills that hypothesize an older age than for the plains basalts. This problem must be reconciled if we are to come to grips with the variety of rocks in the CH and their relationships to one another. We suggest that three factors conspire to mask the CH crater record, likely to varying degrees. First, slopes will change the distribution of ejecta from craters. While this effect has been assumed for previous planetary studies, it appears not to have been extensively researched. Geomorphic surfaces on the Earth collect debris (produced by various processes) in hollows and on flat surfaces, and we hypothesize that slopes in general, and in the CH on Mars in particular will respond to ejecta emplacement in a similar manner. Second, the target properties of the CH, potentially quite different than for the plains, i.e. softer rocks, may result in a more poorly expressed cratering record that cannot be clearly detected in orbital imagery. Third, the CH are topographically more exposed to eolian effects on average than are the plains, which may serve to soften the topography of craters, or simply mask them, more quickly. Prior to Spirit's arrival in the CH, the MOC images already showed areas of sediment accumulation, and areas where stripping may dominate. Spirit's views from the eolian-bedform-covered crest of Husband Hill into the Inner Basin show drifts, dunes and hollows. We test these proposed factors with a "ground-truth" mapping of smaller craters near the rover to check against the plains crater statistics. We estimate the size and position of impact craters within the Columbia Hills from both ground observation and MOC image data, and compare the two. We also evaluate the relative abundance of eolian bedforms as compare with the plains to assess their importance in masking the crater record. Surficial processes appear to have degraded impact craters smaller than a few meters whereas those ten meters or greater may be detectable. We feel that it may also be valuable, since the CH are thought to be old compared to the plains, to compare the crater counts we do observe with counts in the nearby regional southern cratered highlands.

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