Fourteen months after entering orbit around Saturn, Cassini has spotted evidence of dusty spokes in the planet's rings. First seen 25 years ago by instruments on the Voyager spacecraft, the spokes are most likely charged dust particles floating above the rings. The new pictures should help astronomers piece together a better understanding of the Saturnian environment.
A sequence of images captured on September 5 reveals faint, narrow spokes in the outer B ring stretching 3,500 kilometers long and just 100 kilometers wide. "We had convinced ourselves that conditions wouldn't be right for seeing spokes on the lit side of the rings until about 2007," says Cassini imaging team leader Carolyn Porco of the Space Science Institute in Boulder, Colo. "But this finding seems to be telling us that conditions on the dark side of the rings are almost as good right now for seeing spokes." The visibility of the spokes depends in part on the angle between the sun and the rings, with more sunlight increasing the chances of spotting them. Scientists had expected the orientation of the sun with respect to the ringed planet to become optimal for viewing the spokes in 2007.
The spokes are thought to be composed of small icy particles about the size of the wavelength of visible light. Analysis of the Voyager images indicated that the spokes strengthened and weakened in sync with powerful radiation called Saturn kilometric radiation (SKR), which emanates from the planet's magnetic field. By monitoring the appearance of the spokes over a number of years with Cassini, the researchers hope to better understand how they wax and wane over time. "Cassini has found that the SKR period has changed since Voyager, which though hard to believe, may mean that the rotation of Saturn's interior has changed," Porco explains. "That would be a finding of enormous consequence, so we'll be looking very closely to see if the frequency of spoke activity has changed, too."