Something is disturbing the famed, majestic rings of Saturn as well as the lesser-known rings around Jupiter. The ring systems, which appear at first glance to be planar, wafer-thin bands of ice and dust, have on closer examination been found to be rippled, like a corrugated tin roof.
The culprit in both cases appears to be cometary debris strikes that tilted the rings, a tilt that over the years became twisted up into a spiral pattern of ripples within the rings. That is the conclusion of two studies published online March 31 in Science—one study on Saturn, one about Jupiter—which go on to pinpoint specific dates that debris plowed into the rings to generate ripples: late 1983 for Saturn; mid-1994 for Jupiter.
It is a forensic story that spans decades, as well as billions of kilometers of interplanetary space, and for which four spacecraft were called into service. NASA's Galileo spacecraft, orbiting Jupiter at the time, first noticed what turned out to be corrugations in the planet's main ring in 1996, but the nature of the ring features was somewhat unclear at the time. "Pretty much it was just one of those things where you stare at it and say, 'Huh, I hope we get more data,'" says Mark Showalter, a planetary astronomer at the SETI Institute in Mountain View, Calif., and the lead author of the study on Jupiter's rings.
It was not until 13 years later, when NASA's Cassini spacecraft noticed something similar as it orbited Saturn, that the pieces of the puzzle began to fit together. Cassini was exploring the Saturnian system during the planet's 2009 equinox, when the ring plane aligns with the center of the sun. With the sun at such a low angle, vertical corrugations within the rings cast long shadows that Cassini was able to identify. Some were just two meters high, about as tall as Los Angeles Lakers guard Kobe Bryant. They looked like a system of nearly concentric ripples, as would be formed if the rings had somehow been knocked out of alignment and then twisted into a tight, corrugated spiral by Saturn's gravity.
"This corrugation seemed to have formed at some discrete time in the past—1983," says Matt Hedman, a planetary scientist at Cornell University who led the Saturn study. "Something caused the rings to tilt, and we were seeing that tilted pattern wound up." But just what happened in 1983 remained unknown; one early hypothesis held that Saturn itself lurched somehow, throwing the planet and rings out of alignment.
The discovery of rippled rings at Saturn led to a reevaluation of the Jovian data, including a look at some later Galileo imagery from 2000 and images taken by the New Horizons probe in 2007 as it swung past Jupiter en route to Pluto. Showalter also dug up archival Hubble data of Jupiter, but its viewing angles did not allow for a good look at the ring structure. "One of the eureka moments was when we realized that if it was the same thing we were seeing at Saturn, it wouldn't look the same" as it did in 1996, Showalter says. "If that's the same physics that we see at Saturn, then we've been looking for the wrong pattern." As the tilt becomes more and more twisted into the ring, the spiral pattern gets tighter and the spacing between ripples shrinks; the corrugations that were almost 2,000 kilometers apart in 1996 were only 700 kilometers apart by 2000. In 2007, when New Horizons flew by, the spacing between corrugations was a mere 350 kilometers.
Tracing the evolution of the ripples backward in time, Showalter and his colleagues found that the ring had been tilted by about two kilometers at some time between July and October 1994. That was a huge clue pointing to a potential culprit in the interplanetary mystery—Comet Shoemaker-Levy 9 hit Jupiter in July 1994, bombarding the planet in a large, well-documented impact. Debris from the comet, which had broken up before it hit Jupiter in a number of fragments over the course of several days, seems have knocked Jupiter's rings out of whack. "That was basically our smoking gun," Showalter says.
That finding, in turn, shed light on what may have happened at Saturn. "Then we went, 'Oh, maybe this is the same sort of thing,'" Hedman says. "To tilt a broad region of the ring you need something kind of like an intense meteor shower," he says. A single, intact asteroid or comet would punch right through the ring, but a good-size comet, if broken apart as Shoemaker-Levy 9 was, could rain down on the rings with enough force to generate a sufficient tilt. (Saturn was sunward from Earth around the time of the presumed 1983 impact, so astronomers on Earth would not have been able to see it.)
The similarity between the ripple phenomena at the two planets—and the temporal connection between Shoemaker-Levy 9 and Jupiter's ripples—lends the research credibility with other ring scientists. "The fact that the biggest thing that we saw with our eyes agrees with the biggest influence in the rings of Jupiter is pretty compelling," says Jeff Cuzzi, a planetary scientist and Cassini team member at NASA Ames Research Center, Moffett Field, Calif., who did not contribute to the new studies.
"I think it's a very compelling argument, especially with the two papers together," says Linda Spilker, a planetary scientist and Cassini project scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif., who also did not contribute to the new studies. "I know we've been puzzling away on the Saturn side and looking at various explanations—linking to an event in the Jovian system really helps."
Spilker notes that Cassini has another several years of planned operation at Saturn; the spacecraft could document such a ring disruption if such debris strikes occur often enough. "Maybe if we can get lucky, we can be there and actually witness an event like this," she says.
For the time being, the puzzle seems to work as assembled. "We're taking pieces that nobody would have thought fit together," Showalter says. "And we're finding one fairly simple story—it's comets hitting the rings and tilting them, and the tilts turn into spirals. It's a fun thing to piece together."