October 13, 2013
A Rim Mound
image by Damian Peach, Selsey, UK
Piccolomini is a standard complex crater with terraced walls, flat floor and central peaks. Although it only has a few subsequent craters on its interior, It disappears at full Moon, showing that it is old enough to have lost its rays and rim brightness. Because it cuts through the main basin rim of Nectaris it is prominently placed but not especially unique. However, Damian's image shows something I never noticed before. The southern (bottom) inner rim seems to be covered by a mass of material continuous with the surrounding terrain. My first thought was that it was ejecta from the formation of the Nectaris Basin, but a millisecond later I realized that that was imposible because Piccolomini formed on top of the basin, after it formed.The higher Sun, higher resolution LRO image shows that the upper rim exists there but it is lower height than nearby. The LRO image reveals that the area that looks odd on Damian's image is odd on the satellite view - it is more highly textured, full of small bumps; it is a different surface than nearby locations on the rim. Topographic profiles using LRO QuickMap altimetry data show that a rim to floor transect over the anomalous area has a flat to bulbous profile, while a few kilometers away the normal rim has a concave profile. Material has been deposited on this part of Piccolomini's rim and there is no likely nearby source. The color topo map of QuickMap shows that this is a wider high area than anywhere else inside the rim. The only thing I can think of as an explanation is that there has been a collapse of the rim that has tumbled material that was previously surrounding Piccolomini inside the crater.
Sept. 24, 2013.
21st Century Atlas chart 6.
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Yesterday's LPOD: Age Dating a Moon Photo
Tomorrow's LPOD: A Flat Rim?