THE NATURE OF LUNAR SWIRLS (Invited Presentation)

Lunar swirls are high-reflectance surface features co-located with crustal magnetic anomalies. Models for their formation include reduced 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 compositional or magnetic sorting of the soil. Discriminating between these formation models will provide new information about the nature of lunar space weathering, magnetic anomalies, and lunar surface processes. Paststudies of lunar swirls have suggested that swirls are more forward scattering (reflect relatively more light at high phase angles) compared to fresh crater ejecta. These photometric differences were interpreted as indicative of distinct physical properties for the swirl regolith such as variations in the mm-scale regolith structure or removal of fines. Such small-scale regolith textural differences are not thought to be preserved over long periods of time, and thus this evidence has been used to suggest that swirls formed recently in lunar history. However, thermal infrared observations from the Diviner Lunar Radiometer show that swirls do not show anomalous thermophysical behavior expected for such regolith properties. Here we examine new photometric observations from the Lunar Reconnaissance Orbiter Camera (LROC) of the Reiner Gamma, Mare Ingenii and Firsov swirls, in order to document the photometric properties of swirls over a broad range of phase angles and to interpret their nature.