Empirical Evaluation of S-band Radar Polarimetric Response to Surface Rock Abundance across the Lunar Highlands and Maria

Polarimetric radar measurements are sensitive to viewing geometry, wavelength-scale roughness, and composition. As a result, radar imaging enables characterization of planetary surfaces, but the multiple contributors to backscatter complicate the interpretation of metrics commonly treated as proxies for roughness, such as the circular polarization ratio (CPR). Motivated by renewed interest in lunar surface exploration, here we evaluate how meter-scale rock-related surface roughness influences polarimetric radar backscatter by comparing global monostatic S-band observations from LRO’s Mini-RF with Diviner-derived rock abundance (RA). We identified three scattering regimes across the lunar highlands and maria. Over rock-poor terrains, backscatter is insensitive to increasing RA and is controlled by incidence angle and, in the maria, TiO₂ content. For 0.35 ≲ RA ≲ 1.5%, CPR increases rapidly with RA, loses sensitivity to incidence angle, and is driven by enhanced depolarized backscatter, consistent with an increasing population of wavelength-scale scatterers. Over rock-rich terrains, though, CPR plateaus and weakly decreases. Indeed, in some block-rich, fresh crater ejecta, CPR is negatively correlated with RA. In these terrains, the polarimetric response exhibits suppression of double-bounce and depolarized scattering, resulting in decreasing CPR. In contrast, some rock-rich regions sustain sufficient depolarized scattering to offset double-bounce suppression, resulting in a CPR plateau. In the maria, enhanced attenuation further modulates the balance between scattering mechanisms, leading to lower CPR. As such, we find that CPR is not a monotonic indicator of rock-related surface roughness, but an emergent property of competing scattering mechanisms reflective of target properties. Nevertheless, multiple polarimetric properties alongside CPR can resolve interpretive degeneracies.