Here's a neat animation captured last month by Cassini and assembled by Mike Malaska: spokes in Saturn's B ring. The spokes are especially prominent just now, so close to equinox.
NASA / JPL / SSI / animation by Mike Malaska
Spinning spokes in Saturn's rings
Cassini captured this 22-frame animation of the "spokes" in Saturn's rings on September 7, 2009. Scientists think that the spokes are made of fine dust electrostatically levitated above the ring surface, made more visible by the low-angle lighting that occurs near equinox. (It may also be possible that the spoke phenomenon only occurs at this season; scientists don't yet know for sure). The images were aligned and their contrast adjusted to make this animation smooth. A higher-resolution version can be downloaded from Mike Malaska's flickr page.
Spokes were first observed by the Voyagers nearly 30 years ago, when Saturn happened to be in the same season it is now:
NASA / JPL
Spokes in the B ring movie
This movie is made from many separate Voyager 1 images of the B ring. Spokes are apparently a seasonal phenomenon; they were readily visible to the Voyagers, when Saturn was near its equinox, but were not visible to Cassini when it arrived during Saturn's southern summer.
Hubble continued to observe them until 1998. They consist of tiny particles levitated above the ring plane by electrostatic forces; it's not clear if the angle of incoming sunlight has a role to play in the production of spokes, or if it merely takes low-angle spring and autumn sunlight to make the spokes visible. Recently it was suggested that they are connected to thunderstorms in Saturn's atmosphere. Cassini first spied spokes in September 2005, and over its extended (and hopefully extended-extended) mission it'll continue to watch them (probably) fade back into invisibility.
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