Cassini spied just as many regular, faint clumps in Saturn's narrow F ring (the outermost, thin ring), like those pictured here, as Voyager did. But it saw hardly any of the long, bright clumps that were common in Voyager images.
We often view the solar system as constant and unchanging, at least over human time scales. This, of course, is not entirely accurate and astronomers have detected a surprisingly rapid phenomenon inside one of Saturn's rings: moonlets the size of mountains are created and destroyed over a matter of days or even hours
This discovery centers around the gas giant's F-ring where, over the course of 30 years, has dramatically changed its morphology.
"The F ring is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C," said Robert French of the SETI Institute, at Mountain View, Calif., in a news release "It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps."
French and co-investigator Mark Showalter (also from the SETI Institute) studied photographs of the F-ring taken by NASA's twin Voyager spacecraft when they encountered the ringed planet in the early 1980s.
On comparison with photographs from NASA's Cassini spacecraft that is currently in orbit around Saturn, the F-ring has changed appearance extensively.
Further investigations revealed that bright lumps in the ring come and go over periods of only hours or days - features that the researchers believe are small moons.