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Close study of the swarms of propeller-shaped features the Cassini probe has imaged in the rings of Saturn suggests they may give scientists a better understanding of the way planets coalesce in protoplanetary disks around stars.A new scientific paper published in Astrophysical Journal Letters by members of the Cassini imaging team focuses on a set of particularly large propellers in the outer reaches of Saturn's A ring. First discovered by Cassini in the middle of the A ring - now knows as the "propeller belts" -- the distinctive features are thought to be caused by tiny moonlets that are big enough to disrupt the material in the ring, but not large enough to clear their own orbit like the larger moons Pan and Daphnis.In this image of the sunlit side of the ring, collected on April 11, 2008, one of the large propellers appears to be white. It is indicated with an arrow.NASA/JPL/SSIThis dark image was collected on Aug. 19, 2008, from the unilluminated side of the rings. Although the propellers seem small against the huge expanse of the rings, they are actually several kilometers wide, and can be several thousand kilometers long, according to NASA.NASA/JPLExtrapolating from what it would take to cause such disruptions in the 10-meter-thick rings, members of the Cassini imaging team estimate the moonlets causing the propellers are as big as a half-kilometer in diameter, which is too small for the Cassini cameras to spot directly. But by tracking the propellers over the past four years, they are beginning to understand how objects orbit in debris fields."Observing the motions of these disk-embedded objects provides a rare opportunity to gauge how the planets grew fro, and interacted with, the disk of material surrounding the early Sun," says Carolyn Porco, head of the imaging team. "It allows us a glimpse into how the solar system ended up looking the way it did."
os99, Cassini, Saturn, rings, propellers
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