Researchers at the University of Colorado said the spokes, which can be up to 6,000 miles long and 1,500 miles wide, were first spotted 26 years ago by the Voyager spacecraft. When the Cassini mission arrived at the ringed planet in 2004, however, the striking features that cut across Saturn's ring plane had completely disappeared.

Mihaly Horanyi, of the university's Laboratory for Atmospheric and Space Physics, said the event disappointed and puzzled many scientists.

Since the Voyager discovery, the Hubble Space Telescope occasionally observed the ring spokes in the late 1990s, Horanyi said, but the spokes gradually faded, possibly because Saturn's periodic orbital motion and tilted axis of rotation altered the spokes' light-scattering geometry.

"The spokes were switched off by the time Cassini arrived," Horanyi said. "We think it is a seasonal phenomenon, related to the Sun rising and setting over the ring plane that changes the physical environment there, making it either friendly or hostile to their formation."

Writing in the March 17 issue of the journal Science, Horanyi and colleagues at LASP and elsewhere hypothesize that the spokes comprise tiny dust particles less than a micron in width - about 1/50th the width of a human hair – that collect electrostatic charges in the plasma environment of the rings and become subject to electrical and magnetic forces.

The team said the right conditions cause the spoke particles to gain an extra electron, allowing them to leap en masse from the surface of ring debris for brief periods, collectively forming the giant spokes that appear dark against the lit side of the rings and bright against the unlit side of the rings.

Horanyi and colleagues wrote that one of the primary conditions for the spokes to form is a decrease in the angle of the ring plane relative to the Sun. "Because the rings are more open to the Sun now than when Voyager flew by," they explained, "the charging environment above the rings has prevented the formation of the spokes until very recently."

Cassini first imaged a "puny version" of Saturn's spoke rings from a distance of 98,000 miles in early September that were only about 2,200 miles long and about 60 miles wide, Horanyi said.

As the ring-plane angle decreases when Saturn is near its two seasonal equinoxes, conditions appear more suitable for the spokes to form. Although Cassini currently is orbiting too close to the ring plane to make direct observations, the researchers said they expect the spokes to become visible by the time the spacecraft increases its inclination this July.

When the spokes reappear, the team thinks they will remain visible for about eight years, because Saturn takes about 30 Earth-years to complete one orbit around the Sun. The eight-year period should be followed by about six-to-seven years of a spoke hiatus, he said.

The dust grains levitated by plasma during spoke-forming periods probably hover less than 50 miles above the rings, and they scatter light from the Sun differently than the rings themselves, Horanyi said.

"We don't know if they form by rapidly expanding, or if they form all at once," he said, adding that during the Voyager mission, the features were absent during one observation, but fully developed in a follow-up image taken just five minutes later.

"This is a weird phenomena; we don't have the full story on it yet," he said.

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