The gravitational pull of Saturn's largest moon, Titan, causes giant, circling "tsunamis" of icy particles in one of the planet's rings, new data suggest. The discovery may solve the 30-year-old mystery of a gap in Saturn's faint, inner C ring.
NASA's Voyager 1 probe—observing Saturn's rings from a single, shallow angle—first recorded a rippling region within the C ring during a November 1980 flyby.
The otherwise regular ripple was interrupted by a gap that seemed to be almost 9.3 miles (15 kilometers) wide, based on radio data. Complicating the matter, later pictures of the C ring showed no large gap.
Now scientists working with NASA's Cassini orbiter have confirmed the gap exists.
Because the ripples and gaps are generally too subtle to discern in Cassini images, the researchers observed the gap largely indirectly, via light shining through the opening as stars passed behind the ring.
"A reasonable analogy is a person walking behind a picket fence and shining a flashlight through," astronomer Phil Nicholson of Cornell University said Monday. "Light from bright stars drifting behind the rings gets blocked by denser regions of particles but passes through the gaps."
The recent Cassini observations taken from several different angles show the opening to be much narrower than previously thought—a mere third of a mile (half a kilometer) wide. Voyager's slantwise perspective, it turns out, had resulted in the gap seeming much wider than it is, Nicholson said during a briefing at a meeting of the American Astronomical Society's Division of Planetary Sciences in Pasadena, California.
What's more, in about half of Cassini's readings the gap becomes a peak, as if something were blocking the view—"like a gap in the fence had been turned into a fence post," Nicholson said.
The new starlight data suggest that, at some angles, spiraling walls of icy particles on either side of the gap line up so that they block the gap from view, he said.
Based on the Cassini data, he added, each peak seems to be "like a tsunami propagating away from an earthquake fault," he said. "If you were standing next to this tsunami, it'd be pretty big"—with each peak just under a mile (1.6 kilometers tall)—"but it's moving pretty slowly, about 250 meters [820 feet] a day."
(See Saturn pictures.)
Several known gaps elsewhere in Saturn's rings can be explained by tiny moons that orbit inside the rings, carving paths through the particles. But some gaps, like this one in the C ring, are associated with no known moons, making the gaps' origins a mystery. (Related: "'Pinball' Collisions Seen in Saturn Ring.")
As it turns out, the tsunamis' rotations matched the orbital rate of Titan, the researchers realized—circling Saturn once every 16 days. Because of that correlation, the scientists suspect the peaks are results of the C ring's gravitational relationship with the moon.
As Titan orbits Saturn, its gravity likely yanks the section of ring particles that are in resonance with the moon—or moving at the same speed as Titan's gravitational field, explained Larry Esposito, an astronomer at the University of Colorado at Boulder who was not involved in the new research.
"For decades people have said these resonances can [also] open gaps" in Saturn's rings, Esposito said. "This data confirms those predictions."
Saturn Tsunami Shines
Although the tsunami feature is normally too small to see in Cassini's pictures, the craft saw it directly for the first time during Saturn's equinox, when the sun's rays hit the rings edge-on, Cornell's Nicholson added. The low angle of sunlight caused the tilted wall to shine like a bright arc in the otherwise dark ring.
"Think of the sun illuminating gently undulating topography on Earth right after sunrise or just before sunset," he said. "On average there's not much illumination ... but a slope [facing the sun] can look quite bright."
In general, Nicholson said, the new find adds to evidence that Saturn's disk of rings is actually less two-dimensional than once believed.
According to the University of Colorado's Esposito, the finding may not have any major implications for studies of Saturn's rings, but there's no denying "it's an intriguing case."