The Lunar Opposition Effect: Dual-mechanism Partitioning and Shallow-regolith Constraints from GaoFen-4 Photometry
Abstract
The lunar opposition effect (OE) probes shallow regolith structure and multiple-scattering behavior, but its physical interpretation is limited by difficulties separating shadow-hiding (SHOE) and coherent-backscatter (CBOE) components and comparing photometry across observational scales. We use geostationary GaoFen-4 (GF-4) observations, which provide a common radiometric frame through single-exposure multispectral full-disk imaging free of terrestrial atmospheric effects, spanning four visible to near-infrared bands (∼491–809 nm) and phase angles of 3 .° 7–113 .° 1. Within a regularized Hapke framework, we invert disk-integrated phase curves and mare and highland pixel populations using Bayesian and nonlinear-optimization schemes, benchmark a Chang’E-3 (CE-3) region against in situ photometry, and obtain zero-phase quantities through model extrapolation followed by finite-solar-disk correction. Physically admissible fits require both SHOE and CBOE: from visible to near-infrared, the zero-phase CBOE fraction rises from ∼52% to ∼64% and the CBOE-dominated interval broadens from ∼2 .° 5 to ∼17 .° 7, indicating substantial coherent backscatter on the low-albedo Moon. The fitted SHOE and CBOE widths map to an effective porosity of 69% ± 4% and coherent-backscatter length-scale proxies of ∼280–460 nm, favoring sub-grain-scale heterogeneity over intergranular spacing. Although highlands are brighter and more strongly backward scattering, mare terrains exhibit a narrower and stronger relative OE, consistent with Fe-rich basaltic materials producing more agglutinitic glass and nanophase iron. At the CE-3 region, the normalized phase-curve shape and relative CBOE fraction (∼79%) agree with less disturbed in situ results, suggesting cross-scale transferability of relative photometric diagnostics. Together, these results link mechanism partitioning, terrain contrast, and scale dependence, providing a lunar reference for opposition photometry of airless bodies.