Saturn's gauzy G-ring is being swept into its orbit, grain by grain, from a region of icy chunks on its inner edge. So say researchers working on data from NASA's Cassini spacecraft in the 2 August issue of the journal Science .

A single bright arc on the inside of Saturn's G-Ring. Credit: Ciclops

Most of Saturn's dusty rings are associated with a moon: for example, it is thought that the moon Enceladus' leaky bottom creates the E-ring; and the F-ring can trace its origins to the moons Prometheus and Pandora. But the G-ring doesn't have a moon, and its origin has been a mystery.

But now researchers say they have uncovered a mechanism that explains how it is formed.

In the ring's inner edge there is a region of relatively large particles that extends roughly one sixth of the way around the circumference of the ring. It is about 250 kilometres wide, much narrower than the full 6,000-kilometre width of the G-ring.

The large particles are broken up by constant impacts from micro-meteoroids, and as plasma in Saturn's magnetic field moves through the region, the finer, dust-sized particles are swept up to form the G-ring.

"Distant pictures from the cameras tell us where the arc is and how it moves, while plasma and dust measurements taken near the G-ring tell us how much material is there," said Matthew Hedman, a Cassini imaging team associate at Cornell University, and lead author on the Science paper.

The arc is thought to be a relatively long-lived feature, confined by the gravitational influence of the moon Mimas. Part of Hedman's research involved constructing a model that demonstrated the mechanics of this confinement.

Further, the research has revealed that there must be plenty of unseen material in the arc - chunks ranging from pea-sized to small boulders. In total, the arc's mass is equivalent to that of an ice-rich moon around 100 metres across.

The team plans to take a closer look for the missing material when Cassini passes through the region again in 18 months. ®