Iapetus gets dusted

Saturn moon's yin-yang surface explained

Imagine a powdered sugar doughnut hole plowing through a cloud of dark-chocolate dust. The resulting two-colored treat would resemble one of Saturn’s weirder moons, Iapetus — an icy world with a coal-black face and a bright white backside.

The black-and-white surface of Saturn’s moon Iapetus has puzzled scientists for centuries. Now, astronomers can explain how the bright white Iapetian surface is darkened. Dark, dusty dandruff shed by another Saturnian moon, Phoebe, splatters Iapetus’ forward face. Here, a false-color image from NASA’s Cassini spacecraft illustrates Iapetus’ strange surface patterns. NASA/JPL-Caltech/Space Science Institute

For centuries astronomers have puzzled over the source of this yin-yang color pattern. Now a team led by graduate student Daniel Tamayo of Cornell offers an explanation: Dust flung from another one of Saturn’s moons is coating one side of Iapetus. Because Iapetus doesn’t rotate with respect to Saturn, the same face continually catches the dark moon flakes.

“Iapetus is probably one of the most striking bodies in the solar system, and one of the longest-standing problems in planetary science,” Tamayo says. In a study posted online July 7 in Icarus, Tamayo mathematically describes the movement of dust particles in the outer Saturnian system. He focuses on dust coming from Phoebe, a dark and distant, irregularly shaped moon that circles Saturn in the opposite direction as Iapetus. Phoebe’s retrograde motion puts it at odds with a number of other far-flung moons.

“If you’re driving on the highway, you’ll have more collisions if you’re going against traffic,” explains Tamayo’s coauthor and adviser Joseph Burns.

Collisions between Phoebe and those outer moons produce an enormous, invisible ring of dust lying far beyond Saturn’s well-known photogenic ones. Dust from that Phoebe ring splatters the Iapetian surface like bugs hitting a windshield. Nearly every particle larger than 10 microns across will end up on Iapetus, the team concludes. Smaller particles that miss Iapetus strike the Saturn moons Titan and Hyperion.

An earlier hypothesis pointed to Phoebe as the source of Iapetus’ bizarre black side, but astronomers hadn’t studied the dynamics of the system in detail. “This is a very significant paper,” says astronomer Bonnie Buratti of the Jet Propulsion Laboratory in Pasadena, Calif. “It does the math.”

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