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The Planetary Society BlogBy Emily LakdawallaUnderstanding Saturn's ring spokesMar. 27, 2006 | 10:55 PST | 18:55 UTC
I'm back from my lovely vacation, more than three days over which I never saw a computer screen nor even turned on the TV; instead we slept in and went for hikes among the giant redwoods of Big Sur, an area of striking natural beauty along California's central coast. Wonderful. Now it's time to dive in to what's been going on off of Earth lately.
And here's what Cassini eventually saw:
As a matter of fact, Mitchell reminds in the article, the spokes were also visible to Hubble from the ring plane crossing of 1995 through 2004. But they seemed to go away as Saturn's season evolved toward southern hemisphere summer, and the rings tilted more away from the Earth-based observers. Scientists thought that their ability to see the spokes was a function of their observing angle on the rings -- easier to see from a flatter point of view, harder to impossible from a more open point of view. As a result, they expected that Cassini would see the spokes any time it had a shallow enough point of view on the rings. But that didn't happen. Instead, Michell says, the best explanation on offer is that the spokes are not a permanent feature of the rings, but a seasonal one instead. How do you get seasonal features in Saturn's rings? Mitchell advances an argument that had been suggested by several other scientists in publications in the early 1980s, just after the Voyagers' flybys, which involves "electrostatic interactions." Mitchell explains: "The interactions involved a transient event to charge [ring] grains to sufficiently large potentials for lift-off from the ring." It's hard to remember that most of the mass of the rings, which form the largest feature in the Saturn system, exists in particles too small to see with the naked eye; particles this small could get electrical charges large enough to repel them from other particles and levitate them above the rings. In support of this idea, Mitchell points out that "Voyager took spoke images that were separated by several minutes and obsered fully-formed spokes that were absent in the image taken just moments before," indicating that some very quick event that generates an electrical potential could cause spokes to appear. Mitchell advances two hypotheses for spoke origin: they could be "triggered by a sheet of hugh-energy auroral electrons connecting the ionosphere to the ring," or they could be "triggered by meteorite impacts," in which case "the transient plasma cloud would have to drift along the full radial extent of the spokes in minutes," the time between sequential Voyager image frames. Cassini should be able to determine which is happening -- the quick auroral one or the slightly less quick meteorite one -- in further studies of spoke formation. Since Cassini is now orbiting in Saturn's ring plane it can't look for spokes right now. But Mitchell concludes, "We expect that spoke activity will have returned by the time [Cassini's] inclination increases again in July of 2006."
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