That's one plausible interpretation, at least, of a rhythmic wobble Mimas displays as it orbits Saturn once every 23 hours or so, says study lead author Radwan Tajeddine, a planetary scientist at Cornell University. The other possibility, says Tajeddine, is that Mimas might be solid throughout but that its rocky core might not be spherical even though its icy outer layers clearly are. "Instead," he says, "the core might be elongated—shaped like a rugby ball."

Whatever the reason, the wobble Tajeddine and several co-authors discovered by carefully examining images from the Cassini space probe was unexpected. The scientists weren't surprised at the wobble itself, since many moons, including our own, oscillate slightly as they orbit. Mimas's shudder, however, is