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

After months of searching, the Cassini orbiter circling Saturn has finally photographed the spokes in the planet's rings. CREDIT: NASA/JPL/Space Science Institute.

The formation of odd spokes in Saturn's rings may depend on the amount of sunlight striking the planet's ring plane, scientists said Thursday.

A new study suggests that the spokes appear more often while Saturn's rings are edge-on to the Sun, but fade out when the rings are completed tilted out at maximum exposure.

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

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

NASA's Voyager spacecraft photographed spokes in Saturn's rings in the 1980s. IMAGE: NASA/VOYAGER

The spokes in Saturn's rings intrigue astronomers, who believe the phenomenon may aid investigations into the planet's magnetic field. They can reach 6,000 miles (9,656 kilometers) in length and span 1,500 miles (2,414 kilometers) in width, but their role and formation within the ring plane are not completely understood.

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

Astronomers caught their first close look of ring spokes during NASA's Voyager mission, which swung past Saturn during the 1980s and returned images of spokes forming rapidly - within five minutes at times - between subsequent photographs.

"It's almost 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 that happen over millions of years."

Between 1998 and 2005, spoke searches with the Hubble Space Telescope came up empty-handed and researchers believed that unlucky geometry between their Earth vantage point, Saturn's ring plane and the Sun was to blame.

But when Cassini entered orbit in 2004, the spokes remained unseen. The orbiter finally detected ring spokes in September 2005, which may be an indicator that a new season of spoke activity is underway, researchers added.

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

• Cassini Probe Spies Spokes in Saturn's Rings
• On Final Approach, Cassini Photographs Saturn
• Special Report: Cassini's Mission at Saturn