Magnetic map of Reiner Gamma

Lunar swirls are strange markings on the Moon that resemble the cream in your coffee—on a much larger scale. They seem to be curly-cues of pale moondust, twisting and turning across the lunar surface for dozens of miles. Each swirl is utterly flat and protected by a magnetic field.

What are they? “We don’t know,” says Bob Lin of UC Berkeley, who has been studying the swirls for almost 40 years. “These things are very strange.”

One of the swirls, Reiner Gamma, can be seen through a backyard telescope. It lies near the western shores of Oceanus Procellarum (the Ocean of Storms) and looks at first sight like a strangely disorganized crater. Indeed, that’s what most astronomers thought it was until 1966 when NASA’s Lunar Orbiter II spacecraft flew overhead and photographed Reiner Gamma from point blank range. Whatever it was in that grainy black and white photo, it was not a crater.

The mystery deepened in 1972 when Lin and colleagues discovered that the swirls were magnetized. “It was an accidental discovery,” he recalls. As often happens in science, “we were trying to learn about something completely different.”

There are a couple of lunar orbiters due in 2008 that should answer the myriad of questions raised — with newer, more extensive data.