Iapetus gets dusted

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.

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, 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.

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