|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 (phase) angle. Previous work, at optical wavelengths, has demonstrated that the material properties of lunar regolith can be sensitive to variations in phase angle. This sensitivity gives rise to the lunar opposition effect and likely involves contributions from shadow hiding at low phase angles and coherent backscatter near zero phase. Mini-RF bistatic data of lunar materials indicate that such behavior can also be observed for lunar materials at the wavelength scale of an S-band radar (12.6 cm).
Radar observations of planetary surfaces provide important information on the structure (i.e., roughness) and dielectric properties of surface and buried materials. These data can be acquired using a monostatic architecture, where a single antenna serves as the signal transmitter and receiver, or they can be acquired using a bistatic architecture, where a signal is transmitted from one location and received at another. The former provides information on the scattering properties of a target surface at zero phase. The latter provides the same information but over a variety of phase angles. NASA’s Mini-RF instrument on LRO and the Arecibo Observatory in Puerto Rico are currently operating in a bistatic architecture. This architecture maintains the hybrid dual-polarimetric nature of the Mini-RF instrument and, therefore, allows for the calculation of the Stokes parameters that characterize the backscattered signal.
Circular Polarization Ratio (CPR) information is commonly used in analyses of planetary radar data, and is a representation of surface roughness at the wavelength scale of the radar (i.e., surfaces that are smoother at the wavelength scale will have lower CPR values and surfaces that are rougher will have higher CPR values). High CPR values can also serve as an indicator of the presence of water ice.
Bistatic data for the south polar crater Cabeus has been acquired on four occasions and these data cover a phase angle range of 0° to 18°. When viewed at near zero phase, 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 for the floor of Cabeus crater indicates a clear opposition effect at low phase angles that is consistent with the presence of water ice. We suspect 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. This would suggest that the water ice observed would need to be confined to a relatively thin layer, near the surface.