What's a swirl?

Lunar swirls are optically bright, sinuous albedo features observed in both the maria and highlands of the Moon. Though generally bright in appearance, lunar swirls often contain lanes of darker material within their looping patterns. Lunar swirls appear to overlay the lunar surface, superposed on top of craters and ejecta deposits, apparently representing diffuse brightening (or darkening) of unmodified terrains. The type example – the Reiner Gamma Formation – is easily distinguished against the dark mare basalt of Oceanus Procellarum. Lunar swirls have been observed in a number of other locales on the Moon, on both the nearside and the farside, in both mare and highland settings.

Lunar swirls tend to be associated with regions of anomalously high crustal magnetic fields. This relationship has led some to speculate that the swirls result from differential space weathering. In this scenario, the magnetic anomaly protects the surface from solar-wind ion bombardment, suppressing the soil darkening caused by exposure to the space environment, and making the swirls appear brighter than the surrounding, unshielded surface. Another idea postulates that the magnetic anomalies associated with the swirls lead to the formation of electric fields, due to the differential penetration of solar wind electrons and protons into the magnetic field. These electric fields could then selectively attract or repel fine, feldspar-rich electrostatically levitated lunar dust, creating the characteristic bright and dark lanes of the lunar swirls. Still others speculate that the swirls are remnants of collisions with a cometary coma, disrupted comet fragments, or comet-related meteor swarms. In these comet impact-related scenarios, the impact is proposed to expose fresh material from the top < 1 m of the lunar surface, enhancing the albedo of the area without causing a major change in the topography.

The lunar swirl known as Reiner Gamma (60 km width), seen at 750 nm by the Clementine spacecraft.

The lunar swirls in Mare Ingenii (250 km width), seen at 750 nm by the Clementine spacecraft.

The lunar swirls in the crater Gerasimovich, 
seen at 750 nm by the Clementine spacecraft.

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