U.S. astronomers say they've figured out origins of the mysterious colors of Saturn's moon Iapetus, bright white on one side and coal-black on the other.

The two-faced color pattern has long mystified astronomers but researchers from Cornell University say they have an explanation -- Iapetus' dark "face" is caused by dust thrown off by Phoebe, another of Saturn's moons, which is coating one side of Iapetus.

Since Iapetus doesn't rotate with respect to Saturn, it always presents the same side to the floating dark flakes given off by Phoebe.

The Cornell researchers say Phoebe is a dark and distant moon that circles Saturn in the opposite direction as Iapetus in a retrograde motion that puts it at odds with a number of the ringed planet's outer moons, ScienceNews.org reported Thursday.

"If you're driving on the highway, you'll have more collisions if you're going against traffic," researcher Joseph Burns says.

Collisions between Phoebe and those outer moons produce an enormous, invisible ring of dust lying far beyond Saturn's well-known visible rings.

A large part of that floating space dust eventually ends up on just one side of Iapetus as it swings around Saturn, giving the moon its distinctive black and white yin-yang appearance, the researchers said.