The nature of bright swirl patterns, some extending for hundreds of kilometers, on the moon’s surface has eluded researchers for years. Now, the mystery seems to have been solved after a computer simulation suggests these were made following comet impacts. The tails of the comet, through made up of light-weight ice particles, likely blew off the upper, dark layer of the moon’s surface, leaving behind a bright trail.

A bright feature known as Reiner Gamma snakes across the lunar surface in this 1967 image from Lunar Orbiter 2. It is one of several features known as lunar swirls. They are thin layers of material that often form swirls or squiggles. Planetary scientists are still trying to understand their origin. [NASA]

Brown planetary scientist Peter Schultz was watching videos of NASA’s Apollo program modules landing on the moon when he noticed how the whole area around landing sites was smooth and bright. He realized what happened: the top lunar soil is darker, but when it was stripped off by the craft’s jet, the underlying layer was exposed, which was much brighter. This is when he first got the idea that maybe those strange light-colored stripes on the moon were caused by comets.

“They simply look as if someone had finger-painted the surface,” says Schultz. “There has been an intense debate about what causes these features.”

He sat on the idea for decades, but only recently when better simulations of dynamic cosmic impacts became available could he actually actually test his hypothesis. Working closely with his former Brown graduate student Megan Bruck-Syal, the two performed a simulation to see what happens when a comet’s icy core and tail hits the moon. Their work, published in the journal Icarus, suggests that comets could have easily displaced gas and dust around the surface creating swirls even thousands of kilometers long, which seems to match current observations.

The Moon’s most intense magnetic field, strong enough in places to hollow-out a mini-magnetosphere in the solar wind, is found on the opposite side of the Moon from Mare Crisium, around the Far side Gerasimovich craters. Accompanying those fields are swirl albedo anomalies, though they are more difficult for the eye to trace out from the brighter highland terrain background. Credit: Lunar Networks

That’s not all. On-site missions from the 1970s came across unusual magnetic anomalies in the vicinity of the bright swirls. If these are indeed made by comet impacts, than the energy of the crash could have melted the iron-rich particles below the surface. Once these cooled, the particles would have recorded the magnetic field of the comet itself, accounting for the anomaly.

“We think this makes a pretty strong case that the swirls represent remnants of cometary collisions,” says Schultz.