Descartes is a heavily worn lunar impact crater that is located in the rugged south-central highlands of the Moon.

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.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Descartes.