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Last Updated: Friday, 3 August 2007, 07:58 GMT 08:58 UK
Saturn's ring mystery is solved
G ring (Image courtesy of Cassini Imaging Team and NASA/JPL/SSI)
Cassini images of Saturn's G ring captures its single, bright arc
Scientists have made a significant step forward in understanding the dynamics of Saturn's magnificent ring system.

The behaviour of one ring in particular - the G ring - has baffled experts.

The thought its dust particles should have ebbed away because there were no nearby moons to hold them in place or replenish them.

But the Cassini probe has shed new light on the faint, narrow ring; showing that it interacts with a much more distant Saturnian satellite.

The work, published in Science, also unveiled the ring's odd structure.

The G ring is one of Saturn's outermost rings: it is more than 168,000km from the centre of the planet and more than 15,000km from the nearest moon.

"It's a dusty ring," explained Matthew Hedman, a research associate at Cornell University and lead author of the study. "Like the E ring and F ring, it is primarily composed of tiny grains of ice just a few microns across."

However, these minute specks can be easily dispersed or eroded as they whizz around the planet. For the rings to remain in place, they either need something to serve them with a constant supply of new dust and ice, or for a large object such as a moon to confine the particles in the band through its gravitational interactions.

The moon Enceladus directly supplies new material to the nearby E ring. While for the F ring, satellites Prometheus and Pandora may help to keep the particles within this narrow region.

"But the G ring is not near a moon, and that's the thing that makes it odd," explained Dr Hedman.

Arc of debris

Data from the Cassini-Huygens mission to Saturn, a collaboration between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (ASI), has enabled scientists to examine the G ring in more detail than ever before.

Instruments from the Cassini probe revealed that the G ring's structure was unusual.

In addition to the tiny grains of dust spread evenly around the ring, there was also a bright arc across one sixth of the band, that contained larger icy solids. These ranged in size from a few centimetres to a few metres.

Dr Hedman explained: "You would expect this material to shear out, but it was clumped together. So the question was 'how did that work?'"

The team discovered that the ring's orbit was linked to that of the major moon Mimas. For every seven times the arc orbited Saturn, Mimas, which is about 15,000km away, completed six orbits.

"When you get this kind of whole number ratio, there can be some strange things called resonances that occur. These can have interesting influences and can actually confine material within the ring," said Dr Hedman.

The scientists believe the bright arc of material is being held in place through an interaction with Mimas.

Micrometeorites, which litter space, are constantly colliding into the bodies within the arc, generating dust that subsequently spreads out to populate the rest of the G ring.

"The entire G ring could be derived from an arc of debris held in resonance with Mimas," the scientists write in the journal Science.

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