Saturn's outermost ring stuns astronomers

© NASA/JPL/Space Science Institute

The biggest planetary ring in the solar system is much bigger than previously thought, say scientists.

A new study, reported today in , has found that Saturn's outermost ring is nearly 300 times the size of the planet it orbits.

That's more than 30 per cent larger than scientists had thought.

"Nobody expected [planetary] rings to ever be this large," says lead author, Professor Douglas Hamilton of the University of Maryland.

"The textbooks all say that rings are small, located close to their planet."

Saturn's largest was originally discovered by Hamilton and colleagues in 2009, and is named after the Saturnian moon Phoebe, which is the source of the particles that make up the ring.

In the previous study the ring was detected between distances of 128 and 207 Saturn radii, but new measurements using NASA's WISE spacecraft have given the researchers a better picture and increased its size by 30 percent.

"Before we knew [the Phoebe ring] was big, but we didn't know exactly how big, and now we have that answer. It's nearly 300 times the size of the planet Saturn," says Hamilton.

Particle size

The new study studied the size of the particles in Saturn's outermost ring, providing new insights into how the ring formed.

Millimetre to centimetre-sized chunks of ice in the tails of passing comets continually bombard Phoebe, producing a constant stream of debris that forms the giant ring.

The observations in 2009 couldn't determine the size of the particles in the ring, but the new more detailed picture shows it is composed of mostly very small dust particles.

Hamilton and colleagues worked out the size of ring particles by examining how the brightness of the ring changed as the spacecraft moved across it.

Larger rocks, between fist and soccer-ball-sized, only make up around 10 per cent or less of the ring's total composition, the researchers say.

"We didn't really expect the smallest ones to be dominant," says Hamilton.

He says larger particles collide to form smaller particles, and these smaller particles are eventually lost by interacting with sunlight.

However, because the ring is so large, individual particles don't collide very often so this means the ring could be very old, says Hamilton.

"Most of these particles can last for millions to billions of years because there's so much space and so few particles, and they're moving so slowly that the collision rate is so slow."

Two faced moon

When small particles are lost from the Phoebe ring, they move inwards towards Saturn until they cross the orbit of the neighbouring Saturnian moon Iapetus.

"Iapetus is a moon that's unlike anything else in the solar system, it's black on one side and white on the other due to this Phoebe ring," says Hamilton.

Just like the Earth's Moon where the same face is always locked towards Earth, Iapetus orbits Saturn with the same face always leading in its orbit. That forward looking face is turned black by the black dust grains from the Phoebe ring striking it during its orbit.

"That's really what led us to go looking for this outer ring in the first place," says Hamilton.

Saturn's many rings

Saturn's famous main rings are composed of chunks of ice that are between the size of a house all the way down to millimetre-sized particles. The particles in these rings are very densely packed and constantly colliding with each other on time scales of minutes to hours.

Beyond Saturn's main rings is the E ring which is produced by geysers erupting from fissures at the south pole of the moon Enceladus.

These geysers are spraying water into space which quickly freezes into very fine ice grains, which remain in orbit for around a hundred years until they're swept up again by Enceladus.

"Finally we go out the Phoebe ring which is a totally different regime, where most of the particles are fine dust, although not as small as particles in the E ring," says Hamilton.