By Ken Croswell
Saturn's rings are such a spectacle that you can see them through even a modest telescope. Made mostly of water ice, the rings contain countless particles, large and small, that orbit the planet in a thin plane. For decades scientists have known that gravitational tugs from Saturn's many moons imprint patterns on the rings. Now they have discovered a new ring sculptor: oscillations of the planet itself, which promise insight into the interior of the solar system’s second-largest planet.
The discovery came about because of a close inspection of Saturn’s rings. From outermost to innermost, the three main rings are named A, B and C. In 1980, when the Voyager 1 spacecraft flew past, it found grooves in each ring that resemble those on a vinyl record. The gravitational pulls of Saturn's moons make waves, mostly in the A ring, because that's the one closest to the moons.
In 1991, however, Paul Rosen, then at Stanford University, and his colleagues used Voyager data to discover waves in the C ring, the one nearest the planet. Although the moons accounted for some of these waves, no one knew what caused the others.
Then, in 2004, the Cassini spacecraft began orbiting Saturn, and Matthew Hedman and Philip Nicholson at Cornell University used Cassini to observe background stars through the C ring. By measuring how much starlight different parts of the ring absorb, the scientists mapped six of the mystery waves in detail. "We found that Saturn probably is affecting the rings just as the moons do," Hedman says. As he and Nicholson will report in a future issue of The Astronomical Journal, the pattern speeds of the six waves around Saturn match those that should result as Saturn's interior sloshes back and forth and alters its gravitational field.
The discovery has stirred other planetary scientists. "It's incredibly important," says Jonathan Fortney at the University of California, Santa Cruz. "It opens up the possibility of using seismic data to understand the structure of a giant planet, which has been phenomenally successful for the sun and for Earth."
"The rings are a seismograph," says Mark Marley of NASA Ames Research Center, who in 1990 earned his doctorate for predicting that Saturn's oscillations would affect its C ring. Then, after the discovery of the mystery waves, he and planetary scientist Carolyn Porco, then at the University of Arizona, proposed that the planet's oscillations were producing those waves, whose characteristics match the properties of the six waves Cassini has examined. The recent discovery is "a brand-new window into the interior structure of the planet," Marley says.
"I've been skeptical for a long time that you could do this kind of seismology at all," says David Stevenson of Caltech, who nonetheless thinks the discovery is genuine and notes that French astronomers have recently reported detections of oscillations on Jupiter. "Maybe we are on the verge of having a new way of understanding giant planet structure."
The waves' properties indicate that current models of Saturn's interior are approximately correct. The waves also present a puzzle, however. Marley and Porco's calculations had suggested that each ripple in the rings should arise from a different type of oscillation inside Saturn—but instead Hedman and Nicholson found multiple waves resulting from the same type of oscillation. "That was not predicted," Hedman says. "That implies there is something missing in the models."
Now that Hedman and Nicholson have succeeded in mapping six Saturn-induced ring waves, other planetary scientists will refine their models of the planet's interior. They already know Saturn consists mostly of hydrogen and helium, which surround a core of ice, rock and iron, but the core's exact weight—thought to be roughly 10 to 20 Earth masses—is unknown.
Models of Saturn carry implications beyond the solar system. "Many of the extrasolar planets we see are gas giants," Hedman says. "Jupiter and Saturn provide close-by versions of what gas-giant planets are, so if we can understand the interior structure of these guys pretty well, we should be able to better understand other giant planets too."
No one knows whether giant planets elsewhere sport beautiful rings. Astronomers can't yet discern rings around extrasolar planets, and the sun's other giant worlds—Jupiter, Uranus and Neptune—possess only faint rings. This observation raises the intriguing possibility that rings as spectacular as Saturn's could be rare in the Milky Way, which would make our solar system one of the few to boast such a stunning sight.