A moon orbiting the spectacular ringed planet Saturn has at least one ring of its own, space scientists have discovered. They are encircling the giant planet’s second largest satellite, Rhea.
The rings, which are too faint to have been spotted before, are the first rings ever detected around a moon in the solar system.
The find was made by an international team led by UK astronomer Geraint Jones, of the Mullard Space Science Laboratory in Surrey.
He used instruments aboard Nasa’s unmanned Cassini probe to detect a disk of debris and at least one ring around 950-mile wide Rhea.
The disk measures several thousand miles from one end to the other. In addition, there may be a dust cloud extending up to 3,000 miles from Rhea’s centre.
Saturns’s own rings, which can be seen shining brightly in even small telescopes, are formed from a disk of countless particles of rock, grains of dust and small boulders.
The new discovery is reported in the latest issue of the journal Science. Cassini has been orbiting Saturn since 2004. It is teaching us lots about Saturn and its vast family of moons, including Titan, with its vast oil reserves, and Enceladus, whose pull is making it hard to measure the length of the planet’s day.
Dr Jones began his investigations after being first alerted to dust around Rhea when Cassini made a close flyby in November 2005.
Similar patterns of dust and electrons on either side of Rhea showed that rings and dust must exist. “Seeing almost the same signatures on either side of Rhea was the clincher,” said Dr Jones.
“After ruling out many other possibilities, we said these are most likely rings. No one was expecting rings around a moon.”
Space scientists say the rings could be the remnants of a collision with an asteroid or comet in Rhea’s distant past. Other moons of Saturn, such as Mimas, show evidence of a catastrophic collision that almost tore that moon apart.
Felllow Cassini scientist Professor Michele Dougherty, of Imperial College London, said: “Cassini has made great strides in identifying new features in Saturn’s system, from rings around the planet to new moons.
“This is the first evidence for a ring around a moon and provides a new opportunity for exploring how rings form, evolve and disperse.”
Picture: A Nasa artist’s impression of the rings around Rhea.