UCL in the News: Saturn satellite reveals first moon rings

Rings are not just for planets anymore - astronomers have found them around Saturn's moon Rhea, the first ever observed around a moon.

"It is a huge surprise - we didn't have any suspicions at all" that such rings existed, says Geraint Jones [UCL Mullard Space Science Laboratory]. …

The rings revealed their presence by the way they block the flow of energetic electrons that zip around Saturn, trapped by the planet's magnetic field. The effect was observed by the Cassini spacecraft as it passed about 500 kilometres away from Rhea in November 2005.

As Cassini approached the moon, the flow of electrons had a mostly gradual decline that included three sharp downward spikes before the moon itself completely blocked electron flow to the spacecraft. When the spacecraft emerged on the other side of the moon, the flow had an overall gradual rise, which included another three sharp downward spikes.

Jones says the three sharp spikes reveal narrow rings of higher particle density within the disc extending around Rhea. …

Cassini found no trace of rings around Saturn's similar icy moons Dione and Tethys, which Jones speculates cannot hold onto rings because they are much closer to Saturn than Rhea and therefore subject to more interference from the planet's gravity. …

Normally, narrow rings require a small moon to shepherd the particles and keep them confined, but no satellites of Rhea have been spotted so far.

"Whatever is going on there is a real puzzle, so ultimately there may be another explanation, but from what we have at the moment a debris disc and embedded rings are the best explanation," Jones told New Scientist.