Emily Lakdawalla • Oct 12, 2010
"A very sad story": No rings for Rhea after all
A couple of years ago, a story published in Science made a big splash: Rhea, Saturn's second-largest moon, appeared to have rings. The evidence was from the MIMI instrument on Cassini. During a flyby on November 26, 2005, MIMI detected something blocking the flow of electrons in Rhea's neighborhood, something that made three pairs of dips on each side of the moon.
So he has a limit -- what if the rings are just sparser than the cameras can see? The first response from the MIMI team is that the particles were likely about a millimeter in diameter, which would be enough to make the signatures they saw yet not be detectable by the cameras. But, Tiscareno said, there's a problem. As the particles get larger, not only do they stop scattering light the way tiny particles do (making them harder to see when lit from behind), but they also don't block electron flow the way tiny particles do. The upper limit on the amount of mass in the ring imposed by the camera measurements is four orders of magnitude (that's ten thousand times) smaller than the amount of mass that would actually need to be in the form of millimeter-sized particles in the ring in order to produce the observed MIMI measurements.
So Rhea doesn't have rings. But that leaves the Cassini team with yet another puzzle. Something weird was happening in that MIMI signature. It actually wasn't observed on a subsequent flyby. "There is something really remarkable going on in physics" near Rhea, Tiscareno said. "Unfortunately it's not ring physics. Unfortunately for me anyway, because my funding is to do rings science.
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