The Distribution and Extent of Lunar Swirls

Abstract:

Patterns of 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 swirls are much more extensive than previously known within the South Pole–Aitken Basin, and constitute a single grouping larger than the previously mapped swirls that extend from Mare Marginis to King crater. We also identify a host of smaller features including swirls near craters Abel, Crozier, Dewar, and Dufay X. All mapped 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 all magnetic anomalies. Swirls are found in regions with iron abundances shifted to higher-than-background values; their formation may be 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 higher optical maturity (OMAT) parameter values, stronger 1-µm bands, and shallower normalized continuum slopes than their surroundings, consistent with materials that have experienced less space weathering. However, some swirls cannot be discerned in OMAT or band-depth images, and have only moderate reflectance. Areas with low 321/415 nm ratios but non-distinct visible–near-infrared properties could be related to the presence of a glassy component with limited embedded submicroscopic iron. Swirl color properties vary with distance from Copernican and some Eratosthenian craters; they have lower UV ratios, higher reflectance, and higher OMAT values close to these craters. The association with Eratosthenian craters may suggest swirls preserve fresh material exposed by impact craters longer than in non-swirl regions; the difference between swirls and Copernican craters implies that the majority of swirls have reached some degree of maturity or have some other property that results in spectral characteristics that are distinct from those of fresh craters.