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.
Credit: NASA/JPL/Space Science Institute.

The Cassini spacecraft orbiting Saturn has finally spotted spokes cutting across the planet's rings, a phenomenon astronomers have long hoped their plucky orbiter might find.

While flying past the dark side of Saturn's B ring, Cassini's camera eye photographed the spokes - which appear as radial markings - in a series of three images taken over about 27 minutes. The find is a gem of sorts for mission imaging scientists, who have been hunting for the ring spokes since Cassini arrived at Saturn.

"We've been on the lookout for them since February, 2004," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, CO, of the spokes in an e-mail interview. "Spokes are one of those Saturn-system phenomena that we are keenly interested in understanding."

Saturn's odd ring spokes were photographed during NASA's Voyager mission, which swung passed the planet in the 1980s, and later observed by astronomers using the Hubble Space Telescope.


NASA's Voyager spacecraft photographed spokes in Saturn's rings in the 1980s.

IMAGE: NASA/VOYAGER

But spokes were noticeably absent when Cassini made its final approach toward Saturn in February 2004, and are a prime target for astronomers because of their role and formation within the planet's rings are not fully understood.

"These are among the things we hope to learn," said Porco, who participated in the Voyager mission as well. "[The spokes] are obviously related to a host of processes...and may point to some important effects in understanding the magnetic field and the planet's magnetosphere, and how these systems interact with the rings and atmosphere."

Porco and her imaging team did not initially expect to observe ring spokes until about 2007, when certain models predicted spoke formation and visibility.

"Well, in some sense we should have expected, if the recent models are correct, to see them on the dark side where the photoelectron abundance is low," Porco said of the spokes. "So, I was surprised to see them. But once they showed up, I realized we should have expected them there all along."

While the images were released on Sept. 13, Cassini actually photographed the ring spokes on Sept. 5, 2005, using clear filters and its wide-angle camera from a distance of about 198,000 miles (318,000 kilometers) from Saturn. The spokes themselves are fairly faint, and are about 60 miles (100 kilometers) wide and 2,200 miles (3,500 kilometers) long, researchers said.

Unlike Voyager or Hubble, Cassini is in a unique position to study ring spoke phenomena at Saturn, Porco said.

"Remember, Voyager was just a flyby, Cassini is in orbit," Porco said, adding that Cassini is a vastly superior observation platform when compared to Voyager. "We have the opportunity for monitoring them and their behavior, their comings and goings, how they evolve, when they appear and disappear."

By observing the spokes on Saturn's rings, Cassini rekindled fond memories of Voyager for Porco.

"It felt like the old days, when we first saw the spokes," Porco said. "They are one weird phenomena and it was a joy to see them again...especially since we hadn't seen them yet and were eager to know why."

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