Unique regolith characteristics of the lunar swirl Reiner Gamma as revealed by imaging polarimetry at large phase angles

Context. Lunar swirls are high-albedo irregular markings that are generally associated with prominent magnetic anomalies. The formation of swirls is still unknown. Near-infrared spacecraft-based imaging suggests reduced space weathering at the locations of swirls. However, the reduced space weathering alone cannot explain the observed spectral properties. Aims. We provide detailed physical characteristics of the regolith at the Reiner Gamma swirl. For the first time, systematic telescopic observations in a range of phase angles are used to derive the surface roughness, opposition effect strength, and grain size distribution at a spatial resolution of 1 km. Methods. Imaging polarimetric observations of Reiner Gamma were obtained at the Mount Abu IR Observatory between January and March, 2021. These observations were collected with the two narrow-band continuum filters, GC (green) and RC (red), in a range of phase angles. The georeferenced polarimetric images were used to derive the single-scattering albedo, photometric roughness, and amplitude of the opposition effect by adopting the Hapke reflectance model. We further computed median regolith grain size maps of Reiner Gamma using the derived photometric roughness, albedo, and degree of polarization. Results. A comparison of the polarization properties of Reiner Gamma swirl with the craters Kepler and Aristarchus suggests grain size variations within the swirl structure. The Hapke modeling of the Reiner Gamma swirl suggests significant changes in the opposition effect strength at the central oval, but only marginal differences in surface roughness from its surroundings. Within the swirl, the median grain size varies significantly in comparison to the background mare grain size of similar to 45 mu m. Conclusions. Our results confirm the occurrence of surface alteration processes that might have disrupted the regolith microstructure in the Reiner Gamma swirl. These findings are consistent with an external mechanism of swirl formation, by considering interaction between the regolith and cometary gas. Subsequent to its formation, the swirl structure was preserved due to shielding by crustal magnetic field.