• Dick writes about Earth and planetary science for Science magazine.

Mystery Solved: The Dark Side of a Moon

6 October 2009 (All day)

Iapetus has a dirty face, and it's getting dirtier every day. That's the conclusion of astronomers studying Saturn's oddest moon, a sort of yin-yang symbol in space that's almost pitch black on one side and icy bright on the other. Iapetus's bizarre coloration has been a mystery since Giovanni Cassini discovered it in 1671, but now scientists have fingered the source: a newly discovered gigantic dust ring encircling Saturn--the largest ring in the solar system. Fed by dust from embedded moons, the ring steadily deposits dirt on Iapetus's once-clean façade. "It's nice to finally see a smoking gun that tells us exactly what happened," says ring specialist Joseph Burns of Cornell University.

Planetary scientists announced their discovery today at the Division for Planetary Sciences annual meeting in Fajardo, Puerto Rico. Anne Verbiscer and Michael Skrutskie of the University of Virginia, Charlottesville, and Douglas Hamilton of the University of Maryland, College Park, reported that images taken by the infrared Spitzer satellite orbiting Earth revealed the giant dust ring.

The ring is vanishingly faint but broad, with the band extending 17 million kilometers beyond Phoebe, one of Saturn's moon. That dwarfs the solar system's previous record holder, Saturn's dusty E ring, which is supplied by the icy geysers of the moon Enceladus. But the micrometer-size dust doesn't stay in the ring forever, the group points out; it drifts inward, coating the leading face of the first sizable body it encounters, which is Iapetus.

Hamilton and his colleagues also tracked down the source of the dust. At least three dozen or so "irregular" satellites, including Phoebe, whiz every which way within the giant ring. As wandering asteroids and comets hit these objects, they kick off debris, which in turn collide with other debris and strike yet more satellites. So the grinding down of irregular satellites produces the dust ring that rains onto the leading face of Iapetus. "It all fits together neatly," says planetary dynamicist Jack Lissauer of NASA's Ames Research Center in Moffett Field, California.

The grind may not just be happening at Saturn. All four of the outer planets have swarms of dust-generating irregular satellites. Planetary scientist Bonnie Buratti and colleagues at the Jet Propulsion Laboratory in Pasadena, California, reported at the meeting that their ground-based telescopic observations show that the leading faces of the two outermost large uranian satellites--Titania and Oberon--are somewhat darker than their trailing faces. Voyager mission scientists had seen similar contrast on Jupiter's outermost major satellite Callisto. So, Buratti and colleagues suggest, Iapetus is not the only moon having dust kicked in its face by neighbors.

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