Peppered around lava flats and mountaintops all over the moon are strange sinuous shapes known as lunar swirls. Their winding, dusty curves are brighter than the surrounding area and, so far, their formation remains a mystery to scientists.

“They look like someone took a white paintbrush and painted across the moon,” says Catherine Neish, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. She helped organize the 2011 Lunar Swirls Workshop Without Walls, an online conference dedicated to these puzzling features. At the meeting Sept. 7, scientists presented new data that may help finally determine the cause of the swirls.

Known about since the Renaissance, lunar swirls came under increased scrutiny after orbiting satellites in the 1960s noticed that they tended to be associated with magnetic fields. Unlike the Earth’s large global magnetic field, these lunar magnetic fields are small local phenomena that are strewn more or less randomly on its surface. Wherever researchers find lunar swirls, they find these magnetic fields.

Three theories are currently proposed to explain the swirly enigmas. The most prominent hypothesis suggests the magnetic fields are shielding the surface from weathering by the solar wind, a stream of charged particles emanating from the sun that darkens the lunar soil over time. Another theory is that the fields are transporting charged lunar dust around into twisting formations. In both of these ideas, the lunar swirls are essentially the shadow of an invisible magnetic field.

But some researchers suggest this thinking is backwards, and it is actually the swirls that are causing the magnetic fields. Tiny impacts from a shattered comet or meteorite could have uncovered bright fresh soil beneath the darker lunar surface, generating heat that turned the surrounding area magnetic. If this model is correct, researchers could expect to see a population of very small impact craters in ultra high-resolution images within the lunar swirls.

Most presentations during the conference gave evidence favoring the solar wind as the culprit in creating lunar swirls. Neish presented radar data suggesting the lunar-swirl areas are very flat and the features themselves aren’t more than four inches deep, which doesn’t support an impact hypothesis. But she admits there still wasn’t enough information to forge a final agreement among the attendees.

To say anything more conclusive, researchers will need data from a new suite of probes and satellites, such as NASA’s Lunar Atmosphere and Dust Environment Explorer, an orbiter that will make measurements of the moon’s dust, scheduled to launch in 2013.

Solving the lunar swirl mystery could help scientists understand other planets whose surfaces are exposed to the vacuum of space. For instance, Mercury is similarly blasted by the solar wind, yet no swirly patterns have been spotted there. “In addition to being beautiful, these lunar swirls are important to understanding many solar system space-weathering processes,” says Neish.

Top, middle images: NASA/GFSC/Arizona State University.
Bottom image: JAXA/NHK.