Ripples testify to 14th century collision

During the 1300s, the Black Death was savaging Europe, England and France were locked in the Hundred Years' War and Chaucer was penning his Canterbury Tales. Meanwhile, more than a billion kilometers away, a comet careened toward Saturn and disintegrated, dropping dusty clouds of debris on the giant planet's iconic rings, creating rippled cometary footprints.

The ripples from that cataclysmic event can still be detected today, electrical engineer Essam Marouf reported October 4 during the joint meeting of the European Planetary Science Congress and the American Astronomical Society's Division for Planetary Sciences.

Marouf, a professor at San Jose State University in California and a member of the Cassini science team, described how the probe beamed radio waves back to Earth through the innermost part of Saturn's C ring, a tenuous inner band in the planet's ring system. The radio waves revealed what Marouf calls a "very unusual kind of addition" to the normal ring structure. "There were highly regular little wiggles that rippled over hundreds of kilometers in a very specific pattern," Marouf says.

The rippling region contains two different waves, one that repeats every 1.2 kilometers and another that repeats every 1.3 kilometers. Though curious, similar wiggles do appear elsewhere in the outer solar system. Scientists traced a similar structure in Jupiter's rings - spied by the Galileo probe - to debris littered by comet Shoemaker-Levy 9 as it crashed into the solar system's largest planet in 1994.

Saturn's C ring also features a longer rippling structure imaged by Cassini and reported earlier this year. Scientists think these longer undulations - between 30 and 50 kilometers - were caused by an impact event in 1983.

Using that information, Marouf and his team were able to determine how long ago the ripples were created, since wavelengths shrink predictably and elderly ripples are more closely packed together.

Rewinding that shrinking process revealed that the newly observed ripples are 600 years older than those born in the early 1980s. "They date back to about the late 1300s," Marouf says. "And there is very clear evidence for two events, not one, separated by about 50 years."

"This is such an amazing result," says Mark Showalter of the SETI Institute in Mountain View, Calif., who recently linked the Jovian ripples with the comet. "Two events is really a hint that this is a cometary kind of thing. Some object got captured into orbit, made two close passages. Survived the first, not totally damaged - then 50 years later it came back in and that was the end of it."