Published online 7 October 2009 | Nature | doi:10.1038/news.2009.979


Huge 'ghost' ring discovered around Saturn

Spitzer Space Telescope reveals a supersized dust belt.

SaturnAstronomers have found an enormous ghostly ring around Saturn.Science Photo Library

Saturn's ring system has just got a lot larger, with the discovery of a faint ring that stretches out millions of kilometres into space.

But this new ring, which follows the orbit of one of Saturn's moons, Phoebe, is unlike any of the other rings that are closer to the planet: as well as being much thicker and wider, it is tilted from the plane of the other rings.

Saturn's rings were first described by astronomer and mathematician Christiaan Huygens in 1655. Since then, astronomers have discovered more details about the number of rings in the system and their composition, aided recently by NASA's Cassini–Huygens mission. It was thought that the farthest ring from Saturn — until now the largest known ring in the Solar System — was the E ring, which stretches from a distance of 3 Rs (where Rs is the radius of Saturn, equal to 60,330 kilometres) to 8 Rs, and is fed by active geysers on Saturn's icy moon Enceladus.

However, this new ring dwarfs all the others, extending from approximately 128 Rs to 207 Rs with a vertical thickness of 40 Rs. "It is an analogue of the 'tiny' rings around Saturn, but on an immense scale," says Michael Skrutskie of the University of Virginia in Charlottesville, and one of the paper's authors1. "It is also a very tenuous ring, with an optical depth of only 2 x 10–8. That is the equivalent of 20 grains of material per cubic kilometre."

Cryogenic clues

The extremely sparse nature of the ring means that it reflects very little light and is practically invisible, which is why it has previously escaped detection. Its existence was proposed based on the discovery of other satellite-associated rings — such as the Janus–Epimetheus Ring around Saturn detected in 2006 — which are formed from material ejected from planetary satellites following an impact.

Phoebe ringPhoebe Ring is much further out than Saturn's other rings.NASA/JPL-Caltech/R.Hurt (SSC) Inset image: NASA/WM.Keck Observatory/JPL-G. Orton

"This was something that was not suspected before the Cassini mission," says Carl Murray, from the astronomy unit at Queen Mary, University of London, and a member of the Cassini imaging team. "We have since come to the conclusion that the origins of many of Saturn's inner rings are its satellites. Of course, these are very optically-thin rings and nothing like the spectacular rings we normally associate with Saturn."

The new 'Phoebe Ring' describes the outer boundaries of Phoebe's orbit, and is at an angle of 27° with respect to the other rings. Phoebe travels in the opposite direction to most of the planet's other moons. The ring was detected by thermal emission using the Spitzer Space Telescope's Multiband Imaging Photometer, which uses a cryogenic system to enable it to see such weak infrared signals. Skrutskie and his team confirmed their observations with images from the Spitzer archive.

And in a bonus discovery, the team think they may also have solved an astronomical mystery. One hemisphere of Iapetus, the next moon in from Phoebe, is much darker in colour than the other half — an observation that astronomers have never fully explained. The team now believes that the Phoebe Ring could be the missing source of the darker material, as well as being responsible for reddish deposits on Hyperion, another Saturnian moon. However, the team has not yet managed to accurately determine the ring's structure or composition to confirm this hypothesis: "Now that Spitzer has exhausted its cryogen, we cannot make any more observations that way," Skrutskie explains.

Further observations of the Phoebe Ring will have to wait until the infrared-optimized James Webb Space Telescope is operational following its launch in 2014. Until then, there will almost inevitably be more surprises from Saturn, Murray predicts. "I doubt this ring could have been seen with Cassini; it shows how important it is to have different instruments working towards the same goal." 

  • References

    1. Verbiscer, A. J., Skrutskie, M. F. & Hamilton, D. P. Nature advance online publication doi:10.1038/nature08515 (2009).
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