Lunar Crater Interiors with High Circular Polarization Signatures

Abstract:

We analyzed 12.6-cm Earth-based radar images of the Moon to search for older craters (pre-Copernican) that display high values of the circular polarization ratio (CPR) on their interior walls. These craters have highly eroded rims and ejecta, indicating that there must be a source exposed within the crater interior that is continuously creating a rougher surface. Of particular interest are craters between 10-25 km in diameter that occur in smooth plains in the highlands, where competent layers are not expected as they are for the mare. After identifying these high-CPR interiors in pre-Copernican craters, we studied LROC NAC and Kaguya TC images to search for possible albedo and layering on crater interior walls that might signal the presence of anomalous material. Our results indicate that high-CPR craters generally have boulder fields clustered around their upper interior walls. We divide the high-CPR craters into three types: (1) craters on the layered mare lava flows; (2) craters in the highlands that correlate to mapped locations of smooth plains; and (3) craters on the highlands that are not associated with smooth plains. Most of the high-CPR craters in the highlands are associated with Eratosthenian-period craters, and most of these are also on smooth plains, indicating that impact melt sheets are a likely source for the boulders exposed on their interior walls. Statistical analyses will be performed after incorporating multiple lunar datasets into GIS to quantify these preliminary interpretations.

Figure 1. Example of high-CPR crater Zagut A located on smooth plains in the highlands. LROC images showing boulders on (a) northern crater interior wall and (b) southern crater interior wall. (c) Stronlgy enhanced values of CPR are observed for the interior of Zagut A.