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Rings of Saturn Formation 几乎不同理论都和文章中一样 Saturn's rings may be very old, dating to the formation of Saturn itself. There are two main theories regarding the origin of Saturn's rings. One theory, originally proposed by Édouard Roche in the 19th century, is that the rings were once a moon of Saturn whose orbit decayed until it came close enough to be ripped apart by tidal forces (see Roche limit). A variation of this theory is that the moon disintegrated after being struck by a large comet or asteroid. The second theory is that the rings were never part of a moon, but are instead left over from the original nebular material from which Saturn formed. It seems likely however that they are composed of debris from the disruption of a moon about 300 km in diameter, bigger than Mimas. The last time there were collisions large enough to be likely to disrupt a moon that large was during the Late Heavy Bombardment some four billion years ago.[15] The brightness and purity of the water ice in Saturn's rings has been cited as evidence that the rings are much younger than Saturn, perhaps 100 million years old, as the infall of meteoric dust would have led to darkening of the rings. However, new research indicates that the B Ring may be massive enough to have diluted infalling material and thus avoided substantial darkening over the age of the Solar system. Ring material may be recycled as clumps form within the rings and are then disrupted by impacts. This would explain the apparent youth of some of the material within the rings.[16] The Cassini UVIS team, led by Larry Esposito, used stellar occultation to discover 13 objects, ranging from 27 meters to 10 km across, within the F ring. They are translucent, suggesting they are temporary aggregates of ice boulders a few meters across. Esposito believes this to be the basic structure of the Saturnian rings, particles clumping together, then being blasted apart.[17] |