The Dark Ring in Southwestern Lunar Orientale Basin: Origin as a Single Pyroclastic Eruption

Prior to Clementine, the low albedo 'dark ring' circular feature in the lunar Orientale basin was interpreted as a ring of localized dark mantle deposits formed at vents located at the position of an ancient pre-Orientale-basin impact crater scar. We have analyzed the deposit and its surrounding region using Clementine data and find information that supports a different interpretation. First, we have located and mapped an elongate 7.5 km by 16 km depression lying in the center of the dark ring. This feature was obscured in previous images but is clearly shown in the Clementine data. It is unusual in terms of its morphology in that it is relatively rimless, is not associated with any other craters that might suggest that it was part of a secondary cluster and it is not specifically morphologically or stratigraphically associated with any deposits of the Orientale Basin. On the basis of our analysis, we interpret it to be a collapse depression and investigate the possibility that it is the location of a source vent for the dark ring deposit.