The thin, outermost ring shown in this picture from the Cassini probe is Saturn’s F ring, and the bright dot near it is the moon Prometheus. The thicker white band is the A ring, and the smudge near what looks like the tip of that ring was caused by Peggy (Image: NASA/JPL/Space Science Institute)

A rebellious moon might have just popped out of Saturn’s rings. Images from a NASA spacecraft show a disturbance along the rings’ edge that is probably being caused by an unseen object stirring up the icy bands. The region has since quietened down, suggesting that we may have witnessed the birth of a small moon.

Models had previously shown that Saturn’s rings might double as moon factories. Material on the outer edge of a ring could clump up and grow into an object large enough to hold together under its own gravity. The newly made moon could then migrate away to become an independent satellite. This process could be how Saturn made the family of small moons that orbit close to – and sometimes inside – its famous rings, but so far no one has seen the factory in action.

Carl Murray of Queen Mary University of London and colleagues were looking at pictures of the small moon Prometheus taken by NASA’s Cassini orbiter. In an image from 15 April, they noticed an unexpected distortion in the A ring, the outermost of the planet’s thick, bright rings.


“I’d never seen anything quite like this at the edge of the A ring,” Murray said today during a talk at a meeting of the American Geophysical Union in San Francisco.

Vanishing act

Searching through earlier images of the A ring, Murray found 107 images of the distortion taken since June 2012. Dozens of similar patterns, shaped like aircraft propellers, have been spotted deeper in Saturn’s A ring, further from the edges. Astronomers think they are the wakes left by moons that are too small to be seen directly that are ploughing through the rings. Murray nicknamed the unusual object causing the A-ring distortions Peggy, after his mother-in-law, because he analysed the first sighting on her 80th birthday. Peggy could be a moon in the making tugging at the edge of the A ring, he says.

“We can’t see the object, but it’s pushing ring material around, which we can see,” says Matthew Hedman at the University of Idaho in Moscow, a member of the Cassini imaging team who was not involved in Peggy’s discovery.

Unfortunately we may never know the object’s true fate. Murray hasn’t found the bright spot in later images of the A ring, and it is possible the nascent clump was destroyed in a collision. But it is equally likely that it became a moon that has since moved too far from the ring to be noticeable. Peggy would be less than a kilometre wide, so if it has escaped from the A ring it is probably too tiny for Cassini’s cameras to spot.

Even if we never see it again, though, Peggy has already left its mark. The Cassini team plans to search for similar objects along the edges of Saturn’s rings in the coming months, in the hope of spying more potential moons being born. “It’s not clear if Peggy is unique, but it is special,” Hedman says.