Scientists working with data from NASA's Cassini, Galileo and New Horizons missions have traced telltale ripples in the rings of Saturn and Jupiter back to collisions with cometary fragments 10-30 years ago.

Cassini scientists first noticed spiral ripples in a 2000km wide swath of Saturn's D ring (the ring closest to the planet) in 2005, said Matt Hedman, Cornell research associate and lead author on one of the papers. In 2009, when the ring plane swept through the sun's position, the astronomers recognized more extensive corrugations covering the entire 17000 km wide C ring and began looking for clues about their origin.

By comparing the 2005 and 2009 data, Hedman, co-author Joseph A. Burns, Cornell professor of astronomy and the Irving Porter Church Professor of Engineering, and colleagues were able to chart how the ripples changed over time and space.

They found that the spiral formation was getting tighter as it "wound up" around the planet, like a piece of cloth being twisted around a central point. They then worked backward to "unwind" the spiral back to a flat sheet whose orbit was tilted relative to its normal path around Saturn's equatorial plane.

"You could explain this corrugation as if the entire ring had become tilted at some time in the past," Hedman said. That time, according to the calculations, was the fall of 1983.

No one knows what happened in 1983 to cause such a tilt. But as Hedman and colleagues worked on the Cassini data, former Cornell graduate student Mark Showalter, Ph.D. '85, now a scientist at SETI and lead author of the second paper with Hedman and Burns, was examining similar corrugations in the main ring around Jupiter.

Applying the same model, calculations indicated that Jupiter's ring plane was originally tilted in the summer of 1994 -- exactly when the comet Shoemaker-Levy 9 crashed into the planet. This suggested that cometary impacts could tilt rings.

A single solid object, however, would not be enough to knock the rings into a tilted orbit; it would simply plow through the rings and into the planet. But if the object -- an average-size comet, for example -- had been pulled apart into a diffuse cloud of particles, then the story was different: In that case, the smallest particles could rain down on a broader region and knock the ring out of whack.

And the evidence these collisions leave behind in the rings can offer clues about the formation and evolution of the solar system as a whole, Burns said.
"What's cool is we're finding evidence that a planet's rings can be affected by specific, traceable events that happened in the last 30 years, rather than a hundred million years ago," said Matthew Hedman, a Cassini imaging team associate, lead author of one of the papers, and a research associate at Cornell University "The solar system is a much more dynamic place than we gave it credit for."

Source: NASA/JPL  and Cornell University