High circular polarization ratios in radar scattering from geologic targets

Abstract

We examine and model the occurrence of circular polarization ratio (CPR or μc) values greater than unity in terrestrial and planetary radar observations as a guide to the range of associated surface morphology. Lunar crater deposits exhibit maximum μc values at 12.6 and 70‐cm wavelength of 2 to 3. CPR values for Maxwell Montes on Venus range up to about 1.5 at 12.6‐cm wavelength. Echoes from SP Flow in Arizona exhibit μc up to 2 at 24‐cm wavelength. Scattering from rock edges and cracks (dipole‐like) produces μc of unity for single scattering and up to about 2 for multiple reflections. Scattering from natural corner reflectors (dihedrals) formed by pairs of rock facets can yield an average μc of 3–4, but likely requires non‐random or scale‐limited surface roughness properties in order to dominate the observed echo. The dihedral mechanism is required to satisfy the highest lunar μc observations, while echoes from SP Flow and Maxwell Montes could be explained by just the dipole mechanism. The dihedral scenario requires rocky facets on scales many times the radar wavelength, which the blocks at SP Flow (and the unknown surface texture of Maxwell) may not provide. The random dipole model is less demanding in terms of structures and scales, and likely increases the CPR of lunar or asteroid regoliths through scattering from and between rocks.