Mapping and model simulations of Lunar Prospector magnetometer measurements show that the source of the strongest known magnetic anomaly on the lunar near side (42 nanoTeslas at 18.6 km altitude) coincides approximately with a high-albedo region of the Descartes mountains centered 60 km south-southeast of the Apollo 16 landing site. The Descartes mountains represent primary ejecta from one or more basin-forming events (Imbrium and/or Nectaris), supporting the hypothesis that basin ejecta materials emplaced >3.8 Gyr ago are the main sources of lunar magnetic anomalies. The higher albedo of the surface at this location is consistent with a significant role for solar wind ions in the optical maturation (or space weathering) of the lunar surface.