By Jonathan Amos
Science reporter, BBC News
A colossal new ring has been identified around Saturn.
The dusty hoop lies some 13 million km (eight million miles) from the planet, about 50 times more distant than the other rings and in a different plane.
Scientists tell the journal Nature that the tenuous ring is probably made up of debris kicked off Saturn's moon Phoebe by small impacts.
They think this dust then migrates towards the planet where it is picked up by another Saturnian moon, Iapetus.
“
The particles smack Iapetus like bugs on a windshield ”
Dr Anne Verbiscer, University of Virginia
The discovery would appear to resolve a longstanding mystery in planetary science: why the walnut-shaped Iapetus has a two-tone complexion, with one side of the moon significantly darker than the other.
"It has essentially a head-on collision. The particles smack Iapetus like bugs on a windshield," said Anne Verbiscer from the University of Virginia, US.
Observations of the material coating the dark face of Iapetus indicate it has a similar composition to the surface material on Phoebe.
The scale of the new ring feature is astonishing. Nothing like it has been seen elsewhere in the Solar System.
The more easily visible outlier in Saturn's famous bands of ice and dust is its E-ring, which encompasses the orbit of the moon Enceladus. This circles the planet at a distance of just 240,000km.
The newly identified torus is not only much broader and further out, it is also tilted at an angle of 27 degrees to the plane on which the more traditional rings sit.
This in itself strongly links the ring's origin to Phoebe, which also takes a highly inclined path around Saturn.
Scientists suspected the ring might be present and had the perfect tool in the Spitzer space telescope to confirm it.
The US space agency observatory is well suited to picking up the infrared signal expected from cold grains of dust about 10 microns (millionths of a metre) in size.
The ring would probably have a range of particle sizes - some bigger than this, and some smaller.
Modelling indicates the pressure of sunlight would push the smallest of these grains towards the orbit of Iapetus, which is circling Saturn at a distance of 3.5 million km.
"The particles are very, very tiny, so the ring is very, very tenuous; and actually if you were standing in the ring itself, you wouldn't even know it," Dr Verbiscer told BBC News.
"In a cubic km of space, there are all of 10-20 particles."
Indeed, so feeble is the ring that scientists have calculated that if all the material were gathered up, it would fill a crater on Phoebe no more than a kilometre across.
The moon is certainly a credible source for the dust. It is heavily pockmarked. It is clear that throughout its history, Phoebe has been hit many, many times by space rocks and clumps of ice.
"We've got a 'smoking gun'," said Professor Carl Murray, a scientist working on the US-European Cassini probe, which is currently touring the Saturnian system.
"We know now that this is where this coating at Iapetus comes from. Phoebe is the source. Something has hit Phoebe, produced lots of material that moves around the orbit of Phoebe and then gradually spirals in. We've solved several mysteries with this observation," the UK researcher told BBC News.
Jonathan.Amos-INTERNET@bbc.co.uk
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