Geological Chronicle
Located in Oceanus Procellarum west of the crater Reiner, Reiner Gamma is one of the most visible lunar swirls from Earth. It was originally thought to be a lunar highland, until observers realized it casts no shadow across the surface. Extending about 70 kilometers in length, the high-albedo feature forms a distinctive swirling, concentric oval, with related loops continuing across the mare to the east and southwest.
Low-orbiting spacecraft have detected a strong magnetic anomaly associated with the feature, measuring approximately 15 nT at an altitude of 28 kilometers, making it one of the strongest localized magnetic anomalies on the Moon. At the surface, this field forms a mini-magnetosphere spanning 360 kilometers, creating a 300-kilometer-thick region of enhanced plasma where solar wind flows around the field. Because solar wind particles are known to darken the lunar surface, this magnetic field may account for the survival of the bright albedo marking.
The origin of the feature remains uncertain, with proposed explanations including cometary impacts and seismic energy from basin-forming impacts. In early lunar maps by Francesco Maria Grimaldi, the marking was incorrectly identified as a crater. His colleague Giovanni Riccioli subsequently named it Galilaeus, a designation later transferred to a crater to the northwest.