Eight "moonlets" -- large boulders measuring between 60 and 140 metres (yards) 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 disturbance in the dense particle field of the A ring looks from a distance like tiny propeller shapes.

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, 3,000-kilometres (1,800-mile) section of the ring, some 130,000 kms (81,000 miles) from Saturn.

Combining all the chunks together, they calculate the moonlets came from a parent moon about 20 kms (12 miles) 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.

The problem with this latter theory has been that collisions of such a kind normally create debris in a wide range of sizes, from big lumps a kilometer (half a mile) wide to pebbles a few centimetres (inches) across.

The big pieces are already known, for there are kilometre- (half-mile) moons called Pan, Daphnis and Atlas that jostle their way around the rings, and photographs taken by scout probes have shown countless small pieces.

Until last year, what was missing were the medium-size pieces.

The first evidence of these intermediate-scale rocks was dug out by a team led by Matthew Tiscareno of Cornell University, New York, which spotted the first four "propeller" moonlets, again using pictures from a Cassini flyby.

The eight additional moonlets add strongly to the idea that the rings were created gradually in a "collisional cascade," according to Sremcevic's paper, which is published on Thursday in the journal Nature.

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

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