SYDNEY: Astronomers have identified the source of a faint and mysterious Saturn ring that was discovered by the Voyager space probe and lies beyond the main retinue of rings.

The G ring is more than 15,000 kilometres from the nearest moon and 168,000 kilometres from the centre of the Saturn. Since it was first captured in images by Voyager 1 in 1980, the existence of the ring has been a mystery, as it is not flanked by moons that might either shepherd it and carve it out or infuse that region with vapour.

The E and F rings, for example are closely associated with moons (Enceladus for E ring and Prometheus and Pandora for the F ring) that directly supply them with material and sculpt and perturb their particles. The location of the G ring continued to defy explanation, until now.

Complex interactions

But experts report today in the U.S. journal Science that the ring is likely produced by relatively large, icy particles that reside within a bright arc on the ring's inner edge – the finding is evidence of the complex interaction between Saturn's moons, rings and magnetosphere.

See a NASA movie sequence of the G ring over a full orbital revolution here.

Observations form the Cassini spacecraft between 2004 and 2005 confirm that the icy particles are confined within the ring by gravitational effects from Saturn's moon Mimas. Furthermore, micrometeoroids collide with the particles, releasing smaller, dust-sized particles that brighten the arc.

The plasma in the giant planet's magnetic field also sweeps through this arc continually, dragging out the fine particles, which create the G ring.

"Distant pictures from the cameras tell us where the arc is and how it moves, while plasma and dust measurements taken near the G ring tell us how much material is there," said Matthew Hedman, a Cassini team scientist with Cornell University in Ithaca, New York, and lead author on the Science paper.

Saturn's ring system is an enormous, complex structure, and its origin is puzzling. The rings are labelled in the order they were discovered. From the planet outward, they are D, C, B, A, F, G and E. The main rings – A, B and C – from edge-to-edge, would fit neatly in the distance between Earth and the moon.

Cassini images show that the bright arc within the G ring extends one-sixth of the way around Saturn and is about 250 km wide, much narrower than the full 6,000-kilometre width of the G ring. The arc has been observed several times since Cassini's 2004 arrival at the ringed planet and appears to be a long-lived feature.

Charged particles

As part of their study, Hedman and colleagues conducted computer simulations that showed the gravitational disturbance of Mimas could indeed produce such a structure in Saturn's G ring. The only other places in the solar system where such disturbances are known to exist are in the ring arcs of Neptune.

Cassini's magnetospheric imaging instrument detected depletions in charged particles near the arc in 2005. According to the scientists, unseen mass in the arc must be absorbing the particles.

"The small dust grains that the Cassini camera sees are not enough to absorb energetic electrons," said Elias Roussos of the Max Planck Institute for Solar System Research, Germany, and member of the research team. "This tells us that a lot more mass is distributed within the arc."

The researchers concluded that there is a population of larger, as yet-unseen bodies hiding in the arc, ranging in size from that of peas to small boulders. The total mass of all these bodies is equivalent to that of a small 100-metre wide, ice-rich moon.

The experts will have a much better opportunity to spot the G ring's source bodies when Cassini flies about 1,000 km from the arc late next year.