The distribution and extent of lunar swirls

The mysterious high-reflectance loops and ribbons known as swirls are not uncommon on the Moon, but are apparently unique to this body. We mapped their distribution and extent using ultraviolet-visible images from the Lunar Reconnaissance Orbiter Camera. We find two main geographic groupings of swirls (South Pole-Aitken Basin and Marginis-King) and a host of smaller features including swirls near craters Abel, Crozier, Dewar, and Dufay X. All mapped swirls are associated with magnetic anomalies and swirls have magnetic field strengths shifted to higher values than their background, though there is not a 1:1 correspondence between the locations of swirls and magnetic anomalies. Swirls are also found in regions with iron abundances shifted to higher-than-background values, which could indicate that their formation is inhibited by low iron content. The most distinguishing characteristic of swirls is a low 321/415 nm ratio coupled with moderate to high reflectance, and swirls generally have high optical maturity (OMAT) parameter values, stronger 1-µm bands, and shallower normalized continuum slopes than their surroundings, consistent with a surface that has experienced less space weathering. However, some swirls cannot be discerned in OMAT or band-depth images. Areas with low 321/415 nm ratios but non-distinct visible-near-infrared properties could be related to the presence of fresh silicates or a glassy component that does not have a substantial abundance of embedded large submicroscopic iron grains (i.e., a difference in the agglutinate fraction of the soil). Swirl color properties vary with distance from Copernican and some Eratosthenian craters; their association with Eratostheninan craters suggests fresh material may be preserved longer in swirls than in non-swirl regions.