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Saturn's largest ring is bigger than we thought

Morgan Rehnberg, Correspondent
Jun 12, 2015, 17:31 UTC

Sen—Sometimes the big just get bigger. Ever since its initial discovery in 2009, Saturn’s Phoebe ring has reigned supreme as the planet’s largest ring. It was already known to be more than ten times bigger than the next-largest ring. Now new research unveiled this week has revealed that  Saturn’s outermost ring is nearly twice as large as previously thought. Stretching out to more than 15 million kilometers from the planet, it is now believed to be the largest in the Solar System.

How did such a large structure evade our detection for so long? Unlike most other rings, this one is not predominately comprised of water. Instead, it is made of tiny chips of rocks blasted off of one of Saturn’s darkest moons by micrometeoroid impacts. The surface of its namesake moon Phoebe reflects just eight per cent of incoming light, a far cry from the shiny surfaces of most other large Saturnian moons.

The ring is so distant and dark that it is actually more easily observed from Earth than by the Cassini spacecraft in orbit about Saturn. This latest work made use of NASA’s Wide-field Infrared Survey Explorer, an Earth-orbiting space telescope.

Although we did not spot the Phoebe ring until 2009, hints of its existence had long been present. Orbiting four times closer to Saturn than Phoebe, the moon Iapetus sports a dramatic color dichotomy. While one half of Iapetus is as bright as many of the planet’s other moons, the other half appears as dark as Phoebe. Scientists had long speculated that material from Phoebe must be falling in onto Iapetus, a hypothesis confirmed by the discovery of the ring.

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Phoebe, the source of the ring that bears its name, is seen in this image captured by NASA's Cassini spacecraft. Image credit: NASA/JPL/Space Science Institute