Swirls are bright, often splashy-looking areas that are
typically associated with strong magentic anomalies. Reiner
Gamma is the most familiar swirl on the Moon. There are only a
few known swirls but the number is now increased by one with the
recognition that the bright rubbly spot (red circle) on the rim
of Descartes crater is the strongest magnetic anomaly on the
nearside of the Moon. Prior to Apollo 16 the bright spot had
been recognized as being unusual in that it wasn’t associated
with a young impact crater, and it was interpreted as ejecta
from a young volcanic eruption. Fast foreward to 1999 when very
low level (18.6 km!) passes over the area by the Lunar
Prospector spacecraft revealed the strong magnetic field,
suggesting to Nicola and her colleagues that the bright area is
a swirl. Because the thorium and iron concentrations over the
swirl are no different than nearby non-bright areas, the source
of the magnetic anomaly is thought to be at depth and not simply
on the surface. The rubbly and furrowed