There's a new king of rings in the solar system: An enormous
new ring has been discovered around Saturn, made up of debris from the gas
giant's distant moon Phoebe.
Before the discovery of this massive ring — about 12.5 times
the average distance between the Earth and the moon in width and 6 times that distance
in thickness — the largest known planetary rings were Jupiter's
gossamer rings and Saturn's E
ring.
Astronomers have long suspected the presence of this ring,
which orbits Saturn at a radius of about 8 million miles (13 million km) — 200
times the radius of the planet itself.
"There were hints that it could be there," said
Douglas Hamilton of the University of Maryland, one of the astronomers who
found the ring.
One such hint was the unusual coloring of Saturn's
moon Iapetus, which had one dark side and one light side. Some astronomers
suspected that the dark side, which looked suspiciously similar in composition
to another of Saturn's satellite, Phoebe, was actually debris dust from Phoebe
stuck to Iapetus' surface.
But astronomers haven't been able to detect it until now because,
"this thing is just immense," Hamilton told SPACE.com. "If you
look at just a small patch of it, you just see fuzziness."
Hamilton and his colleagues were finally able to see the
behemoth ring with the infrared capability of the Spitzer Space Telescope.
Spitzer was able to detect the sunlight reflected by the tiny dark black
particles. The discovery is detailed in the Oct. 8 issue of the journal Nature.
The particles were likely created when asteroids, meteors or
other bodies collided with Phoebe over the eons. While some of the particles
are small enough to drift out of Saturn's gravitational grasp and into
interplanetary space, others drift inwards toward the planet, where some get
stuck to the leading hemisphere of Iapetus, which trawls through them. Periodic
collisions replace the particles lost in these ways.
Interestingly, Phoebe and its associated dust ring travel in
the opposite direction of Saturn's
other rings and satellites.
The tiny particles are extremely diffuse, with only about 20
in every cubic kilometer of the ring, Hamilton said.
"If you were there, you wouldn't know you were in a
ring," he said.
And because the other gas giants are known to have far-out,
irregular satellites like Phoebe, it is likely that they also have similarly
large, diffuse rings orbiting millions of kilometers out.
"I think this is the tip of the iceberg," Hamilton said.
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