Modeling Radar Albedos of Laboratory-Characterized Particles: Application to the Lunar Surface

In a recent publication in the Journal of Geophysical Research: Planets, AO planetary lead Dr. Anne Virkki and Dr. Sriram S. Bhiravarasu of the Lunar and Planetary Institute analyzed radar images of the moon obtained by the Mini-RF instrument on-board the Lunar Reconnaissance Orbiter (LRO) to better understand anomalous radar scattering observed in some lunar craters. After comparing the images with numerical simulations of radar scattering properties, the work showed that the shapes of the surface particles affect the radar albedo and circular-polarization ratios more than the composition. The circular polarization ratio is the ratio of the echo power of each pixel in the same-sense polarization as the transmitted way to that in the opposite circular polarization. It has been traditionally used as a zeroth-order gauge of surface roughness, although there is existing literature of the role that various physical parameters of particles and volumes play in interpreting the measured circular polarization ratio values. Also the role of the size distribution on the radar signatures of test surfaces was presented: a power-law size distribution with a bigger weight on larger particles than smaller particles caused stronger radar echoes. This is aligned with existing knowledge that younger craters, which are more reflective at radar wavelengths than older craters, host often larger particles than older craters.

The study found significant differences in the radar albedos within the lunar craters compared to the surrounding surface, which the authors attribute to the near-surface bulk volume densities of the surfaces. The analysis did not show major variation between polar and non-polar craters, but it demonstrated a systematic trend difference between the internal and external parts of craters.

The numerical modeling results indicate that radar reflectivity enhancements are primarily determined by the relative fraction of particles with sizes on the order of the wavelength or larger. They also showed that particle shape has a greater effect on the radar reflectivity than the composition does, and criticized the interpretation of circular-polarization ratio as a commonly used measure of surface roughness, which has important implications to the way radar signals are typically interpreted. The radar albedo can be a more linear measure of the particle abundance as the polarization ratio, if the incidence angle and surface bulk density are constrained. Elevated radar circular-polarization ratios have also been used to identify ice deposits in permanently shadowed craters on Mercury and possibly on the Moon, but this study suggests that the elevated circular-polarization ratios can also be due to irregular particle shapes rather than the material composition. Nevertheless, the authors note that the material of the radar target can be a significant contributing factor of the shapes that the particles take. Volume scattering and coherent backscattering effect were not considered in this study.

About Arecibo
The Arecibo Planetary Radar Program is funded by NASA’s Near-Earth Object Observations Program. The Arecibo Observatory is operated by the University of Central Florida (UCF) in partnership with Universidad Ana G. Mendez - Universidad Metropolitana and Yang Enterprises Inc., under a cooperative agreement with the National Science Foundation (NSF).