Propelling evidence: Cassini finds clues to source of Saturn's rings.

Four propeller-shaped gaps in one of Saturn's main rings are the latest evidence that a shattered moon produced the planet's dazzling hoops. The discovery supports the theory that a comet or asteroid struck a large, icy Saturn moon about 100 million years ago and that the distributed debris formed rings. They cover a region broader than the distance between the Earth and its moon.

The 5-kilometer-long gaps turned up in images taken by the Cassini spacecraft on July 1, 2004, as it slipped through the rings before settling into orbit around Saturn. By performing a thorough analysis of faint features in the images, planetary scientists led by Joseph A. Burns and Matthew Tiscareno of Cornell University found the gaps in the bright, mostly homogeneous middle section of Saturn's A ring.

They suggest that the gaps were cleared out by moon fragments about 100 meters across, which still exist but are too small for even Cassini to see.

The moonlets represent an intermediate-size population of ring objects whose presence had been predicted by computer models but never discerned. Previous A-ring observations by spacecraft only found evidence of water-ice particles up to 20 meters across and two much larger, icy moonlets: 30-km-wide Pan and 7-km-wide Daphnis (SN: 11/19/05, p. 328).

Pan and Daphnis are massive enough to each clear a circular gap extending all the way around the ring. In contrast, a smaller moonlet would clear out two short arcs, one on either side of its location.

The Cassini images of the short gaps thus provide the first evidence for intermediate-size moonlets. Such ice chunks would be common if Saturn's rings arose from the breakup of a moon into a variety of large and small pieces, says Burns. He and his colleagues describe the findings in the March 30 Nature.

The new data don't support an alternative to the breakup model. In the alternative scenario, most of the material in Saturn's rings comes from small, primordial leftovers from the planet-forming disk of gas, dust, and ice that surrounded the young sun, notes Burns.

However, the evidence for the intermediate-size mooulets "connects the small particles to Pan and Daphnis" indicating that they're all fragments of a large moon that once orbited the planet, Burns adds.

Theorist Frank Spahn of the University of Potsdam in Germany agrees. He says that if the rings' particles were indeed relics from the birth of the solar system, Saturn's gravity and collisions among the particles would have prevented them from growing much larger than 10 m across.

Cassini photographed the four propeller-shaped gaps within a 2,500-square-kilometer patch of the A ring. The Cornell team's extrapolation suggests that the A ring alone houses some 10 million of the 100-m-diameter moonlets.

The gaps and moonlets could offer researchers insights about planet formation in the solar system and around other stars, Spahn says. In the process of accumulating matter, fledgling planets may create propeller-shaped gaps in the primor-dial disk from which they arise. The features now found in Saturn's A ring may be small-scale versions of such structures, Spahn suggests.

SATURN'S WINGS A propeller-shaped gap (white streaks in inset) in Saturn's A ring (arrow) supports the theory that the planet's rings were created when a comet or an asteroid shattered a large Saturnian moon. A moonlet about 100 meters wide would have created the gap.

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