Mini-RF Bistatic Radar X-band Observations of the Scattering Properties of Lunar Crater Ejecta

The Mini-RF radar onboard the Lunar Reconnaissance Orbiter (LRO)) is a hybrid dual-polarized synthetic aperture radar (SAR) currently operating in a bistatic configuration in partnership with the 34-m DSS-13 antenna at the Goldstone radar complex. The DSS-13 antenna transmits a signal to the Moon in C-/X-band (wavelength ~ 4.2 cm) and the Mini-RF antenna receives the backscattered signal from the lunar surface as LRO passes overhead. Because the LRO spacecraft changes location with respect to the transmitting antenna, the bistatic angle (the angle between the transmitter and receiver) changes across a given data collect. This provides a unique opportunity to evaluate how the scattering properties of the lunar surface materials change as a function of bistatic angle for spatially resolved data collects. Earlier bistatic observations, in concert with the Arecibo Observatory at S-band (12.6 cm), showed that ejecta of some young craters (diameter = 4-50 km) exhibited a coherent backscatter opposition effect (CBOE) at low bistatic (phase) angles. This opposition effect is consistent with optical studies of lunar soils done in the laboratory, but these observations were the first time this effect was measured on the Moon at radar wavelengths. The style of the observed opposition effect differs between craters, which may indicate differences in ejecta fragment formation or emplacement. The youngest craters observed show a clear opposition effect, while older craters have a fairly flat response in roughness as a function of phase angle. Here, we present new observations in X-band to evaluate the scattering behavior of ejecta at shorter wavelengths. Early analysis suggests an opposition effect is present for some young craters in X-band observations as well. Observing the scattering behavior in multiple wavelengths may provide further information about the rate of breakdown of rocks of varying size to provide increased understanding of how impacts produce regolith on the Moon.