Lunar Impact Craters: New Perspectives From Full-Polarimetric Analysis of Chandrayaan-2 Dual-Frequency SAR Data

With the availability of the full-polarimetry data from Chandrayaan-2 Dual-frequency Synthetic Aperture Radar (DFSAR) for the first time, the lunar impact craters are studied for comprehensive characterization, which was earlier limited due to hybrid-polarimetry based observations. In conjunction with the conventional Circular Polarization Ratio (CPR), we explore the unique full-polarimetric parameters such as Single-bounce Eigenvalue Relative Difference and T-Ratio which are sensitive to surface roughness and dielectric constant, respectively. We studied the polarimetric behavior of a total of 115 nos. of impact craters within a specific size range (similar to 1-25 km diameter) belonging to different categories viz. fresh, degraded, polar and non-polar anomalous craters. Our results confirm that both kinds of non-polar and polar anomalous craters overall hold centimeter-decimeter scale surface roughness similar to the exterior of the fresh craters, which mainly leads to the anomalous nature of the craters. Our analysis indicates possible control of the crater degradation process on the change in the radar scattering behavior of the studied lunar craters. The crater degradation causes a reduction in the surface roughness and dielectric property (density) of the scatterers present in various craters leading to a change in the radar scattering behavior. Alongside, water ice, if at all present admixed into the regolith in the polar anomalous craters, is not in the form of dominant scatterers that can influence the radar signature. However, the occurrence of relatively higher CPR values in smooth low dielectric crater interiors supports the possibility of the presence of water ice in some of the studied polar anomalous craters belonging to Pre-Imbrian to Imbrian lunar geological periods. Radar studies of impact craters on the Moon have so far been limited to Earth-based radio telescopes and Moon-orbiting sensors operating with hybrid polarimetry configuration. However, to comprehensively characterize the lunar geological features, an enhanced set of measurements as provided by a fully polarimetric radar is required. Dual-Frequency Synthetic Aperture Radar (DFSAR) on board Chandrayaan-2 orbiter is the first SAR with full-polarimetric capability in the lunar orbit, and therefore enables new techniques to uniquely characterize the target surface. Consequently, in this study, we investigate the full-polarimetric radar parameters for holistic characterization of the physical behavior of the surface materials present in various categories of lunar impact craters. Our findings reveal that the crater degradation process controls the change in the physical nature of the targets present in various impact craters leading to a deviation in the radar scattering behavior. We report the presence of centimeter-to-decimeter scale surface roughness in both polar and non-polar anomalous craters, which is the major reason for the anomalous nature of the craters. In addition, our results suggest the possible presence of water ice in some of the polar anomalous craters situated in the Peary crater of the north pole and in Shoemaker, Faustini, Haworth and Cabeus craters near the south pole. We explore unique full-polarimetric radar parameters to characterize the physical properties of the lunar surfaceInteriors of both non-polar and polar anomalous craters have centimeter-to-decimeter scale surface roughness, leading to an anomalous natureWe explore the possible presence of water ice deposits within polar anomalous craters Peary, Shoemaker, Faustini, Haworth and Cabeus