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Saturn’s Ring Spokes Depend on Sun Angle, Study Says

Cassini Probe Spies Spokes in Saturn's Rings
After months of searching, the Cassini orbiter circling Saturn has finally photographed the spokes in the planet's rings.
(Image: © NASA/JPL/Space Science Institute.)

Theformation of oddspokes in Saturn's rings may depend on the amount of sunlight strikingthe planet's ring plane, scientists said Thursday.

A new studysuggests that the spokes appear more often while Saturn's rings are edge-on tothe Sun, but fade out when the rings are completed tilted out at maximumexposure.

"In fact, theycan be entirely switched off at times," said the University of Colorado's Mih?lyHor?nyi, who led the study, of the spokes in an interview. "This is about the season they should start."

Hor?nyi andhis team used imagesand other data from the Cassini spacecraft currently studying the Saturniansystem to model how spokes rise and fall in the planet's ring system. Theresearch, which is detailed in the March 17 issue of the journal Science, predicts the spokes may return in force by July.

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IMAGE: NASA/VOYAGER

Saturn'srings oscillate on a 15-year cycle as the planet circles the Sun, tilting openat certain time while presenting themselves edge-on at others. According to Hor?nyi'smodel, Saturn's ring spokes appear to be active about eight years at a time,followed by an absence of up to seven years.

Astronomerscaught their first close look of ring spokes during NASA's Voyagermission, which swung past Saturn during the 1980s and returned images ofspokes forming rapidly - within five minutes at times - between subsequentphotographs.

"It'salmost amazing that you're discussing things that occur in seconds and minutes,"Hor?nyi said of the spokes. "Typically, we're always talking about things thathappen over millions of years."

Between1998 and 2005, spoke searches with the Hubble Space Telescope came up empty-handedand researchers believed that unlucky geometry between their Earth vantagepoint, Saturn's ring plane and the Sun was to blame.

But whenCassini entered orbit in 2004, the spokes remainedunseen. The orbiter finallydetected ring spokes in September 2005, which may be an indicator that anew season of spoke activity is underway, researchers added.

"A lot ofus are very curious to see these again," Hor?nyi said.

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