Image caption Alternating light and dark bands in Saturn's C ring captured by the Cassini spacecraft in 2009

Scientists say that strange ripples observed in the ring systems of Saturn and Jupiter were caused by comets.

The ripples, which the researchers say resemble the undulations of corrugated metal, were detected in both Saturn's rings and in Jupiter's lesser-known rings.

The ripples in Jupiter's rings are believed to have been caused by the comet Shoemaker-Levy 9, which struck the planet in 1994. Details are published in two separate papers in the journal Science.

The researchers analysed images of Jupiter's rings taken by the Galileo spacecraft in 1996 and 2000 and by the New Horizons probe in 2007. They also looked at images of Saturn's rings taken by the Cassini spacecraft during 2009.

What they found were undulations that the researchers liken to a corrugated tin roof, which when lit from a low angle, appear as alternating dark and light bands.

This corrugation was found across Saturn's entire C ring, stretching for thousands of kilometres. It appeared to be part of a similar pattern observed previously in the fainter D ring. At least two separate spirals were meanwhile detected in Jupiter's rings.

There was the eureka moment when we realised we had a smoking gun Dr Mark Showalter, SETI Institute

The researchers believe they were caused by debris, most likely from a comet, striking the rings, and tilting them.

"The material passes through the ring and basically causes the entire ring to be slightly tilted with respect to the planet's equatorial plane, and then it shears out to form this spiral pattern," said Dr Matthew Hedman of Cornell University in New York.

Over time, the spiral becomes more tightly wound, and it may be decades before the rings flatten out again, say the researchers.

The team was then able to rewind the process using mathematical models to give an estimated date of the impact event. For Saturn, they arrived at a point in 1983, but have not yet found a possible candidate comet.

Media playback is unsupported on your device Media caption How a comet impact produces ripples in a planet's rings over time

With Jupiter, they detected at least two spirals and so possibly two impact events. When they wound back the process for one of the spirals, they reached a point in 1994, the same year of the Shoemaker-Levy 9 impact.

Dr Mark Showalter of California's SETI Institute remembers the moment he and Dr Hedman realised the significance of the date.

"Matt and I were talking, saying now wait a second doesn't that date sound a little bit familiar and honestly we had to run off to Wikipedia to ask 'When did Shoemaker-Levy 9 hit Jupiter?' and there was the eureka moment when we realised we had a smoking gun," he told the BBC.

The second spiral led them to an estimated impact event in 1990, which the scientists hypothesise may have been caused by debris from a previous close encounter with Shoemaker-Levy 9.

Images taken by the New Horizons spacecraft also suggested further impacts may have occurred across Jupiter's rings in September 2001 and December 2003, but additional detections will be needed to confirm these results.

Image caption A scar left behind by Shoemaker-Levy 9's impact with Jupiter

Dr Showalter said ripples like those studied in the new papers could provide clues to the frequency of such events in the outer Solar System.

"Rings are comet catchers," he says. "We probably already realised that comets hit rings periodically but we never realised before that every comet puts its own signature into the rings when it comes by and that that history gets recorded into these spiral patterns.

"Decades later, you can actually look at the same ring, find these ripple patterns, and each of them then tells you something about what hit the ring and when it did so."

Commenting on the work, Professor Alan Fitzsimmons, a comet researcher at Queen's University Belfast, said: "The papers show exactly what you can do when you have these beautifully functioning spacecraft observing the outer planets close up."

He said the gravitational forces of the giant planets were enough to tear apart fragile comets, creating fields of debris that could cause the effects seen in the ring systems.

Asteroids too could be torn apart by what are called tidal forces, he said, but this was more likely to be seen in the case of Jupiter, close to the Asteroid Belt.

He said the work would give researchers the ability to "directly calculate how often objects do hit the giant planets, which we've been struggling to pin down".