“It’s a little bit like a tsunami propagating away from an earthquake fault,” said planetary scientist Phillip Nicholson of Cornell University in a press briefing October 4. Nicholson presented a new model explaining the ring gap at the American Astronomical Society’s Division for Planetary Sciences meeting in Pasadena, California.

Saturn’s rings are riddled with gaps, many of which are held open by small moons. But in the last five years, since Cassini discovered the moon Daphnis, no new moons have shown up in the other known fissures.

“It’s become an increasing problem, as to what determines where these gaps are in the rings and what keeps the gaps open,” Nicholson said.

One such gap has been a mystery since the Voyager 1 spacecraft flew by Saturn in the 1980s. Using radio observations, Voyager detected what looked like a 9-mile-wide gap in the middle of Saturn’s C ring. Just outside the gap, astronomers saw a wave-like structure circling the ring, which they interpreted as an extra-clumpy region pushing through the ring’s flat disk.

But Cassini found the gap to be much narrower, only about a mile and a half wide. Even weirder, the gap seemed to disappear about half the time.

Both puzzles can be resolved by thinking of the ring in three dimensions, Nicholson says. Last year, the angle of sunlight during Saturn’s spring equinox revealed that many of Saturn’s rings have mountains.

“Mostly the rings are very flat. It’s the most two-dimensional structure we know in the universe,” Nicholson said. “But there are exceptions to every rule, and there are exceptions to the rule that Saturn’s rings are flat everywhere.”

The new model suggests the actual gap in the ring is only about a third of a mile wide, but part of the ring rises 2 miles in the air. The crack looked wider to Voyager than to Cassini because of the angles each spacecraft was observing from.

“In hindsight, what looked like a 15-kilometer-wide gap actually was this gap with a vertical displacement of about 3 kilometers (1.8 miles), projected and seen almost edge on,” Nicholson said. “If we assume this was vertical and not horizontal and do the projection, it fits perfectly with this model, better than you have any right to expect.”

The ring’s corrugation comes from a gravitational relationship with Titan, whose orbit around Saturn falls at a slight angle to the ring plane. At a certain point in its orbit, Titan yanks the ring particles upward, starting a wave that travels around the ring.

“The whole pattern rotates around at the same rate as the satellite Titan orbits Saturn, once every 16 days,” Nicholson said. The wave rolling along under Cassini occasionally blocked the spacecraft’s view. “That accounts for the fact that the gap seems to come and go,” he added.

This sort of wave could explain some of the other gaps in Saturn’s rings that are not held open by moons, although it could also be unique to Titan and the C ring, Nicholson said.

“This and some other work suggests there might not be one explanation for gaps, there may be 3 or 4 or even more different dynamical circumstances that can give rise to these gaps.”

The insights gleaned from Saturn’s rings can be applied to disks all over the galaxy, including disks around stars that will eventually coalesce into planets, added Linda Spilker, Cassini deputy project scientist at the Jet Propulsion Laboratory.

Images: 1) Cassini’s view of Saturn shortly after the spring equinox in August, 2009. NASA/JPL/SSI 2) NASA/JPL/Cornell

See Also:

• Video: Moon Builds Snowballs in Saturn Ring
• Cassini Spacecraft Spots New Object in Saturn’s Rings
• Saturn Equinox Reveals Mountains in Rings
• This Summer’s Sexiest Images From Saturn

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Authors: Lisa Grossman

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