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Baby moons spark theory that explains the rings of Saturn

A clutch of baby moons detected in Saturn's outermost ring has bolstered a theory that the giant planet's magnificent circles were created from icy satellites that smashed up over tens of millions of years.

Eight "moonlets" -- large boulders measuring between 60m and 140m across -- were spotted by the US-Italian probe Cassini as they swung through Saturn's A ring.

As they move forward, the rocks scatter aside smaller debris in front and behind, rather as a ship's bow pushes aside water and its stern leaves a wake.

The astronomers, led by Miodrag Sremcevic of the Laboratory for Atmospheric and Space Physics at the University of Colorado, found the "propellers" were concentrated in a narrow, 3,000km section of the ring, some 130,000km from Saturn.

Combining all the chunks together, they calculate the moonlets came from a parent moon about 20km across that shattered some 30 million years ago, perhaps by an impact with a comet or asteroid.

Over the years, further collisions by meteoroids and other rubble smashed the chunks to their present size.

Two hypotheses prevail as to how Saturn acquired its seven rings.

One is that the rings were born at the same time as the planet itself -- they were left-over debris that became enslaved to the gas giant, doomed to orbit it for eternity.

The other is that the rings were the remains of large icy moons that broke into smaller pieces over time.

They calculate that it takes roughly 100 million years for the moon's constituent parts to whacked, bumped and ground to less than 100m in size.
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