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.