Photometric Properties of Lunar Swirls Relative to Fresh Crater Ejecta

Swirls are sinuous high-reflectance surface features collocated with crustal magnetic anomalies on the lunar surface. These bright features and their geographical relationship to magnetic anomalies have been studied for decades, though what causes them is still unclear. Several mechanisms for swirl formation have been proposed, including decreased or altered space weathering due to solar wind shielding, scouring of the surface by cometary impacts to expose fresh material and/or compact the regolith, and sorting of electrostatically levitated dust. The Lunar Vertex mission is set to launch in 2024 and will conduct an in-situ investigation of Reiner Gamma, a prominent swirl located in the mare. Understanding the photometric properties of swirls may help constrain plausible formation mechanisms and contribute to the science return of the Lunar Vertex mission. This work is an overview of the results of radiative transfer modeling to assess the differences between swirl and non-swirl regolith and fresh crater ejecta, and the corresponding implications for the hypotheses for swirl formation.

Phase curve analysis and radiative transfer modeling results suggest that swirls have scattering properties similar to non-swirl regolith with the difference being only albedo. In contrast, fresh impact crater ejecta which is also high in reflectance, is more backscattering than both swirl and non-swirl regolith and is therefore darker than swirl material at high phase angles. This is likely due to the impact process that exposes blocky material and alters the nearby surface. This observation suggests swirl formation does not require a recent cometary impact event or another process that would alter the regolith structure and is consistent with thermophysical observations of swirl material. As a result, an anomalous space weathering environment due to solar wind shielding remains a compelling hypothesis for the cause of the high reflectance of lunar swirls.