Mysterious Lunar Swirls

A new round of lunar exploration could help explain why there are swirls on the surface of the Moon.

Courtesy of Science@NASA The Reiner Gamma swirl, photographed by the ESA's SMART-1 lunar orbiter.

Picture this: A cup of coffee, steaming and black. Add a dollop of milk and gently stir. Eddies of cream go swirling around the cup. Magnify that image a million times and you've got a Lunar Swirl.

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.

Before long, two more swirls were found on the Moon's farside. They lie directly opposite the nearside impact basins Mare Imbrium (the Sea of Rains) and Mare Orientale (the Eastern Sea). Impacts on one side of the Moon, it seemed, made swirls on the other side. No one could explain how.

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. "Their target was Earth's magnetic tail, a ropey pasta of magnetic force fields extending from Earth more than a million miles into deep space. The solar wind blowing against Earth's magnetic field makes the tail, and in the days of Apollo not much was known about it.