NASA/JPL/Space Science Institute; (inset, right) Tiscareno et al., Geophysical Research Letters (2010)

Nothing there. In a rendition (left) of a Cassini image of space near Rhea (which is just off the image to the left), any rings should appear where the black lines are in the identical right-hand frame. None could be detected.

The Moon Rings That Never Were

Dick writes about Earth and planetary science for Science magazine.

They would have been the first rings ever observed around a moon: three narrow bands of icy debris encircling Saturn's second-largest satellite, Rhea. Space physicists announced their existence in March 2008. But a more definitive search finds that they simply aren't there. And that raises the question of what exactly the first team saw.

The original ring detection was based on plasma measurements the Cassini spacecraft made while orbiting Saturn in 2005. In a 2008 paper in Science, space physicist Geraint Jones of University College London and colleagues described how some unseen solid material around Rhea, which is less than half the size of our moon, seemed to be absorbing energetic electrons that are trapped in Saturn's magnetosphere. That absorption appeared to be creating electron shadows, much as a ring creates a light shadow when it passes in front of a star. At Rhea, the stunningly symmetrical electron shadows—three on either side of the moon—implied three narrow ringlets embedded in a broad disk of icy debris in the moon's equatorial plane. That, said Jones at the time, was "the only reasonable explanation we've been able to come up with."

But Rhea's supposed rings bothered ring specialists from the start. Such rings weren't likely to form because just the right sort of grazing impact on Rhea would be required, they said. And the rings would be quickly destroyed by the tug of Saturn's gravity and worn down by eroding small impacts.

Ring specialist Matthew Tiscareno of Cornell University was one of the skeptics. So in late 2008 and early 2009, he and three colleagues took a closer look around Rhea using the visible-light camera on board Cassini. The camera had views of Rhea backlit by the sun—which would light up dust inevitably associated with any rings—as well as other views with the sun behind the camera, when objects up to boulder size would be most easily detected. But nothing showed up. The Cassini search was good enough to detect several orders of magnitude less material than needed to account for the observed electron shadows, the researchers say. "We are now ruling out the possibility that [the electron shadows] are due to rings of solid material," Tiscareno says.

Jones's team concedes. "We can't argue with the conclusions of Tiscareno and colleagues," Jones says. But if not rings, then what? "What we saw is clearly real," he says. "It's probably due to interactions between Rhea and the surrounding magnetosphere." Nothing like the Rhea electron shadows has been seen elsewhere, so Jones doesn't have a clue yet what those interactions might be. Fortunately, Cassini has more Rhea flybys planned during its recently extended mission.

Posted in Space