June 11, 2010
A Hole with No Sides?
image by Juergen Oberst & NASA/GSFC/Arizona State University/DLR]
Old basins are hinted at by roughly circular patterns of mare that occur in shallow depressions. The problem is in defining their rims. Mare Nubium has long been on lists as a suspected ancient basin but there is very little rim topography to confirm it or even its exact diameter. Using topographic maps made from LRO data Juergen has made a new attempt to identify possible rim segments to fix the diameter. His measurement of 675 km (black circle) is essentially the same as Don Wilhelm's 1987 value of 690 km (white circle). The difficulty in deciding the diameter is apparent as you look closely at the shaded relief image (right), where you will see that many of the putative basin rim segments are also rims of craters. One thing that is clear is the depression within the floor of the basin - Juergen finds that the average floor elevation is 1.8 km lower than the average rim. Surprisingly the center of the depression is offset significantly to the west compared to the likely rim. Perhaps the diameter is larger than these circles suggest, extending further to the west. But that would make the northern rim further away from the the deepest parts of the mare than the south side is, leading to another offset. It is fortunate that even with the best modern data some questions cannot be confidently answered, otherwise explorers who finally return to the Moon would have nothing to do.
Rükl plate 54