Mini-RF Bistatic Observations of Cabeus Crater

Abstract:

The Mini-RF instrument aboard NASA’s Lunar Reconnaissance Orbiter (LRO) is currently acquiring bistatic radar data of the lunar surface in an effort to understand the scattering properties of lunar terrains as a function of bistatic angle. As part of the Mini-RF bistatic observation campaign, Circular Polarization Ratio (CPR) information for a variety lunar terrains is being collected. The crater Kepler, and the mare deposits that surround it, offer an example of what this information can tell us about the phase angle response of lunar materials at the transmitted wavelength of the radar (12.6 cm). Data for this particular region has been acquired on multiple occasions and cover a bistatic angle range of 0° to 22°. Isolating the response of Kepler’s ejecta blanket from the surrounding mare deposits, it is clear that the mare deposits have an nearly uniform CPR for bistatic angles < ~17° while materials associated with the ejecta blanket show a steady and significant increase in CPR for bistatic angles < ~5° (i.e., an opposition effect). The difference in the response of these two terrains is likely related the size and distribution of radar scatterers in the ejecta blanket of Kepler crater. Bistatic data for the south polar crater Cabeus has been acquired on four occasions and these data cover a bistatic angle range of 0° to 18°. When viewed at near zero degrees, the floor of Cabeus crater shows an enhancement in CPR with respect to surrounding materials. This is not apparent in data acquired of Cabeus crater when Mini-RF operated in a monostatic mode. Further, when viewed at phase angles of several degrees, the floor of Cabeus crater shows a suppression of CPR with respect to surrounding materials. This scattering behavior clearly indicates an opposition effect but the response is markedly different from that observed for Kepler crater. We propose that the scattering behavior of materials on the floor of Cabeus crater are consistent with the presence of water ice and that the difference in the scattering behavior observed with a monostatic architecture is related to the grazing incidence (~85°) at which the region is viewed by Mini-RF when operating in a bistatic mode (i.e, the water ice observed is confined to a relatively thin layer, near the surface).