Recognition of Volumetric Scattering of Lunar Regolith Media in the PSR via Fully Polarimetric SAR Data

Data of fully polarimetric synthetic aperture radar onboard Chandrayaan-2 make it feasible to analyze the geometric and physical properties of the lunar soil layer. A specific mission of Chandrayaan-2 is to investigate if there is water–ice in the regolith of the permanently shadowed region (PSR). Volatile substances are likely to accumulate in the ancient regolith media. Volumetric scattering of the regolith media might indicate specific characteristics of the regolith. Scattering from surface, rocks should be excluded. Such flat regions are difficult to identify in the digital elevation model. In this article, Chandrayaan-2 full-pol data at L-band are used for this purpose. The co-pol and cross-pol scattering coefficients and decomposition parameters, such as H, $\alpha $, degree of polarization, and circular polarization ratio of lunar surface, are studied. High-order scattering from rough surface is numerically simulated and analyzed by the bidirectional analytical ray-tracing code. It is found that rocky rough surface causes high co-pol scatterings. High-order scatterings of the rough surface and volumetric scatterers can enhance cross-pol scattering. Criteria are proposed to select volume scatterings at flat regions at Mare Serenitatis and a PSR crater. Regions with weak co-pol backscattering in a PSR crater at 85.9°N, 111.7°E are classified as flat surface. It is found that a flat region with weak co-pol and high cross-pol terms appears different from surrounding regions. The enhanced cross-pol is likely to be caused by volumetric inhomogeneity in the regolith. This unusual region shows its inhomogeneous soil layer and deserves further study.