Artist's impression of rings around Rhea
Credit: NASA/JPL

The Cassini spacecraft has detected what may be evidence for rings around Rhea, Saturn's second largest moon. This is the first time that rings have been found around a planet's moon. The discovery is the result of work by a team led by Cassini scientist Geraint Jones, of University College London, and is reported in the 7 March issue of the journal Science.

Rhea is roughly 1500 km in diameter and appears to be surrounded by at least one ring and a broad dusty disc extending up to 5900 km from the moon. The evidence for rings came from measurements made by Cassini's Magnetospheric Imaging Instrument (MIMI). MIMI measured distinctive drops in the flow of electrons around Rhea. Interestingly, the drops occur symmetrically on either side of Rhea, suggesting the presence of rings circling the moon. The material making up the rings probably ranges in size from small pebbles up to boulders, and could be the remnant of a collision with an asteroid or comet in Rhea's distant past.

“Seeing almost the same signatures on either side of Rhea was the clincher. After ruling out many other possibilities, we said these are most likely rings. No one was expecting rings around a moon”
Geraint Jones, University College London