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Saturn ring created by remains of long-dead moon

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Raven
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« on: August 04, 2007, 10:45:20 pm »

Saturn ring created by remains of long-dead moon

19:00 02 August 2007
NewScientist.com news service
Anna Davison


Dust cast off the shattered remains of a long-dead moon of Saturn is the source of a mysterious ring around the giant planet, suggest observations by the Cassini spacecraft. The moon's broken body appears to be kept in line by the gravitational influence of another moon called Mimas.

Saturn's faint G ring lies beyond the planet's main set of rings and has puzzled scientists since it was first detected by NASA's Pioneer spacecraft in 1979. They wondered how it could continue to survive with no moon nearby to feed it with debris or stop its dust from dispersing the nearest moon, 400-kilometre-wide Mimas, lies more than 15,000 km away.

Now, images captured by the Cassini spacecraft are providing an important clue. They reveal a bright arc at its inner edge that is thought to consist of bigger chunks of debris, some a metre in diameter. As they are bombarded by micrometeorites, dust flies off these large chunks "and spreads out to form this ring", says team leader Matthew Hedman of Cornell University in Ithaca, New York, US.

If the chunks were unified into a single object, they would form a 100-metre-wide icy moonlet a body that astronomers believe actually existed at one time.

"There used to be a moon there," says Jeff Cuzzi of NASA's Ames Research Center in Moffett Field, California, US, who was not involved in the study. "It got destroyed at some time, leaving a bunch of rubble out there. It's slowly being ground down."

Remote control
The movement of this rubble appears to be controlled by the gravity of Mimas, which orbits Saturn farther beyond the G ring.

The evidence for this comes from Cassini's observations that the rubble-filled arc orbits Saturn seven times for every six that Mimas does, a gravitational relationship known as a resonance.

"That gave us the clue that Mimas was the reason that that chunk of material was there," Hedman told New Scientist. Watch a movie of the G ring orbiting Saturn over a period of about 20 hours (4.5 MB MPEG).

The big question, astronomers say, is how long the G ring will last. That depends on how the arc of rubble fares as it is bombarded by debris. Come back in 100 or 1000 years, Cuzzi says, and the G ring will probably look a little different.

"We think of these outer planets as pristine and unchanging, but they're really not," he told New Scientist. "There's a lot of action going on out there."

Cassini will get a good look at the arc in 18 months, when it flies within about 1000 kilometres of it.

Cassini: Mission to Saturn - Learn more in our continually updated special report.

Journal reference: Science (vol 317, p 653)

http://space.newscientist.com/article/dn12406-saturn-ring-created-by-remains-of-longdead-moon.html
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Raven
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« Reply #1 on: August 04, 2007, 10:49:47 pm »


A bright arc moves from right to left in the G ring in this sequence of images taken every half hour or so (starting at the top) on 19 September 2006. The arc is about 250 km wide and reaches about a sixth of the way around the G ring (Image: Science)
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