A Radar Scattering Signature of Ballistic Sedimentation Associated with the Lunar Crater Byrgius A

Byrgius A is a 19 km diameter Copernican crater located in the lunar highlands east of the Orientale Basin and west of Mare Humorum. Visible image data of the region obtained by the Lunar Reconnaissance Orbiter Camera Wide Angle Camera (LROC WAC) at a resolution of 100 m/pixel show optically bright ejecta deposits associated with the crater that extend to radial distances of 100s of km, with continuous to near continuous deposits observed to an average radial distance of 70 km. LRO Miniature Radio Frequency (Mini-RF) Circular Polarization Ratio (CPR) information derived from S-band (2380 MHz) data of the region show an increased roughness for Byrgius A and its ejecta deposits relative to the surrounding terrain. This is a commonly observed characteristic of young, fresh craters and indicates that the crater and its ejecta have a higher fraction of cm- to m-scale scatterers at the surface and/or buried to depths of up to ~1 m. A distinct change in the roughness of the ejecta blanket is observed at a radial distance of ~10 km from the crater. However, as observed with visible image data, the generally higher roughness associated with the ejecta of Byrgius A appears nearly continuous to a radial distance of ~70 km. A polarimetric decomposition (m-chi) of the Mini-RF for Byrgius A suggests that the portion of ejecta that extends radially from ~10 to 70 km appears far less continuous than is suggested in both optical data and CPR information. This would indicate that the scattering properties of the top meter of the surface, in that range, are characteristic of a mixture of crater ejecta material and lunar background material. One explanation for this observation is that the thickness of the ejecta at Byrgius A, for this range, is on the order of meters or less. However, this would be inconsistent with estimates of ejecta thickness derived from models of ejecta emplacement (for a given crater diameter) and estimates derived from Earth-based radar observations of craters with radar-dark haloes. Another explanation for the observed radar scattering characteristics is that, at this radial distance, ballistic sedimentation begins to effectively mix the primary crater ejecta of Byrgius A with lunar background regolith.