Mare Marginis
A section of the outer rim of Descartes is covered by a region that has a higher albedo than the surrounding surface. Low-altitude measurements by the Lunar Prospector showed that this patch is actually a magnetic anomaly—the strongest on the near side of the Moon. This magnetic field may be deflecting particles from the solar wind, and thus preventing the underlying surface from growing darker because of space weathering. This is similar to the process causing Reiner Gamma and other bright swirls such as those of Mare Marginis and Mare Ingenii.