Researchers have solved what may be the oldest mystery in planetary science: the two-tone surface of Saturn’s moon Iapetus.

The odd feature  the moon’s trailing side is about 10 times brighter than its leading side  has been a mystery since it was first observed by Giovanni Cassini in 1671. In two papers published online by Science, researchers have unraveled the mystery, using images and data from instruments aboard the spacecraft named for Cassini.

The studies confirm an earlier idea that dust, most likely from another of Saturn’s moons, falls on the leading side of Iapetus as it orbits the planet.

Image The Cassini spacecraft took this photo of Saturn’s moon Iapetus in September 2007. Credit... European Pressphoto Agency

“It’s just like a motorcyclist, who only gets the flies on the leading side of the helmet rather than the trailing side,” said Tillmann Denk of the Free University of Berlin, an author with John R. Spencer of the Southwest Research Institute of one of the papers and lead author of the other.