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Tiny moon feeds Saturn's big ring

Artist's Rendering of Saturn's Infrared Ring

Saturn's mega-ring is so diffuse that it reflects little sunlight, but its dusty particles shine with infrared light (Source: NASA/JPL-Caltech/Keck)

Astronomers have discovered a new mega-ring around Saturn and believe its genesis is a small, distant moon of the ringed giant.

Phoebe, a Saturnian satellite measuring only 214 kilometres across, probably provides the record-breaking tenuous circle of dusty and icy debris, they report in the journal Nature.

The largest ring identified so far in the Solar System, the ring begins about six million kilometres from Saturn and extends outwardly by another 12 million kilometres, within the orbit of Phoebe.

Led by Dr Anne Verbiscer of the University of Virginia, the astronomers used NASA's orbiting Spitzer telescope in February this year to get a close look at space in Phoebe's neighbourhood.

"This is one supersized ring," says Verbiscer. "If you could see the ring, it would span the width of two full moons' worth of sky, one on either side of Saturn."

Until now, the champion planetary rings in the Solar System were so-called "gossamer rings" surrounding Jupiter and Saturn's E ring.

Phoebe's ring is far fainter than both, and appears to comprise dust from rocks bashed off the little moon by interplanetary debris or other particles.

Iapetus mystery

The team suggest the ring could also explain the mystery of Iapetus, Saturn's bizarre two-tone, black-and-white moon.

Migrating dust from the ring could spiral into Iapetus, coating one side of it with a dark material that, over the life of the Solar System, could be metres thick.

"Astronomers have long suspected that there is a connection between Saturn's outer moon Phoebe and the dark material on Iapetus," says co-author Professor Douglas Hamilton of the University of Maryland.

"This new ring provides convincing evidence of that relationship."

The other side of Iapetus is turning progressively whiter, just as the other half is becoming darker.

There is a so-called thermal segregation theory to explain this.

It suggests that the dark side of Iapetus, by absorbing more sunlight, is able to warm sufficiently to cause local water ice to evaporate.

The vapour then circulates to condense on the nearest cold spot, on the icy, bright side of the moon.

As a result, the dark side loses its surface ice, and thus becomes darker, while the bright side accumulates ice, and gets brighter.
