Lunar swirls

Lunar swirls are enigmatic features found across the Moon’s surface, which are characterized by having a high albedo, appearing optically immature (i.e. having the optical characteristics of a relatively young regolith), and (often) having a sinuous shape. Their curvilinear shape is often accentuated by low albedo regions that wind between the bright swirls. They appear to overlay the lunar surface, superposed on top of craters and ejecta deposits, but impart no observable topography. Swirls have been identified on the lunar maria and highlands - they are not associated with a specific lithologic composition. Swirls on the maria are characterized by strong albedo contrasts and complex, sinuous morphology, whereas those on highland terrain appear less prominent and exhibit simpler shapes, such as single loops or diffuse bright spots.

The lunar swirls are coincident with regions of relatively high magnetic field strength on a planetary body that does not, and may never have had, an active core dynamo with which to generate its own magnetic field. Every swirl has an associated magnetic anomaly, but not every magnetic anomaly has an identifiable swirl. Orbital magnetic field mapping by the Apollo 15 and 16 sub-satellites, Lunar Prospector, and Kaguya show regions with a local magnetic field. Because the Moon has no currently active global magnetic field, these regional anomalies are regions of remanent magnetism; their origin remains controversial.

There are three leading models for swirl formation. There are two characteristics of lunar swirls that any model of their formation must take into account:

Models for creation of the magnetic anomalies relevant to swirl formation, although independent of the swirls' optical characteristics, point to the observation that several of the magnetic anomalies are antipodal to the younger, large impact basins. Magnetization of these antipodal regions could occur in the presence of an amplified magnetic field, such as would result from a plasma cloud generated by the basin forming impact interacting with a weak magnetic field present at the Moon at the time of basin formation. An unusually thick and/or strongly magnetized deposit of basin ejecta has been invoked to explain swirls that are not antipodal to a basin, such as the Reiner Gamma Formation. An alternative model to a plasma-induced magnetic anomaly, but which also draws upon the basin-antipode correlation is by impact-induced currents and seismic waves ringing the planet.

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