Photometry of Lunar Swirls in Comparison to Fresh Crater Ejecta

Abstract Swirls are sinuous high-reflectance lunar surface features collocated with crustal magnetic anomalies. These bright features and their geographical relationship to magnetic anomalies have been studied for decades, although their cause remains unclear. Several mechanisms for swirl formation have been proposed, including altered space weathering due to solar wind shielding, scouring of the surface by cometary impacts, and sorting of regolith materials. We use an empirical phase curve analysis and Hapke radiative transfer modeling of single-scattering albedo, bidirectional reflectance, and the angular width of the shadow hiding opposition effect to investigate regolith and crater ejecta properties at three prominent swirl sites: Reiner Gamma, Firsov, and Mare Ingenii. We assess the photometric differences between swirls, nearby background regolith, and fresh impact crater ejecta, and discuss implications for swirl formation. Our results suggest that both swirl and background regolith are consistently less backscattering than fresh crater ejecta, although swirl regolith and crater ejecta within swirls maintain higher single-scattering albedo values than the corresponding background materials. Physical processing via micrometeoroid impacts may explain the similar backscattering characteristics of swirl and background regolith, while additional space weathering of background regolith by solar wind causes differences in single-scattering albedo. In contrast, fresh crater ejecta is more backscattering than both swirl and background regolith, likely due to the impact process that exposes blocky material and alters the nearby surface. As a result, an anomalous space weathering environment caused by solar wind shielding remains a compelling hypothesis for the high-reflectance nature of lunar swirls.