Lunar swirls

Lunar swirls are high-contrast surface patterns, ranging from broad, complex formations to smaller bright markings. Reiner Gamma, a conspicuous feature in Oceanus Procellarum on the Moon's near side, is a well-known example. Orbital camera observations have mapped swirls in both dark basaltic maria and brighter highland regions. Mapping has found all known swirls associated with crustal magnetic anomalies, although many magnetic anomalies have no corresponding visible swirl.

The Moon has no global magnetic field today, but parts of its crust retain localized magnetization. One leading explanation proposes that these local fields deflect some charged particles in the solar wind. Because solar-wind exposure contributes to space weathering and gradual darkening of lunar soil, regions receiving less of this bombardment could remain comparatively bright. Modeling of the Reiner Gamma magnetic environment has reproduced important aspects of its observed bright and dark pattern, lending support to this explanation. It is not a complete account of every swirl or every detail of its appearance.

Swirls are defined partly by their brightness contrast, but brightness can reflect several properties of the regolith, including composition, grain size and structure. Measurements and analyses of Reiner Gamma report that bright and dark areas differ in soil properties and that topography may also be associated with the pattern. A 2024 study of this region found evidence that compositional differences dominate the brightness variation in its study area, while grain size may contribute; it did not find compaction differences there. Researchers have also considered impact-related excavation and electrostatic movement of fine dust. Current evidence is consistent with interactions among processes rather than a single explanation applying identically everywhere.

Researchers combine orbital images, spectral and thermal measurements, topographic data, and measurements of crustal magnetism. These observations can test whether a proposed mechanism matches both the shapes and physical properties of the swirls. A 2015 study using data from NASA's Lunar Reconnaissance Orbiter Diviner instrument found thermal-infrared evidence consistent with reduced space weathering and argued for solar-wind deflection. Later modeling work reproduced aspects of Reiner Gamma's albedo pattern. Important uncertainties remain because fine-scale measurements of the magnetic field, near-surface plasma and soil are limited. Future surface investigations and higher-resolution measurements could help distinguish the relative contributions of competing mechanisms.