Washington: The rings of Jupiter and Saturn contain ripples caused by comets that hit them decades ago, according to data from NASA`s Cassini, Galileo and New Horizons missions.
Jupiter`s ripple-producing culprit was comet Shoemaker-Levy 9.
The comet`s debris cloud hurtled through the thin Jupiter ring system on a collision course into the planet in July 1994.
Scientists attribute Saturn`s ripples to a similar object - likely another cloud of comet debris - plunging through the inner rings in 1983.
"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 on one of the papers, and a research associate at Cornell University in Ithaca, N.Y.
"The solar system is a much more dynamic place than we gave it credit for," he added.
Scientists learned about the patchy patterns in Jupiter`s rings in the late 1990s from Galileo`s visit to Jupiter.
Unfortunately, the images from that mission were fuzzy, and scientists didn`t understand why such patterns would occur.
Not until Cassini entered orbit around Saturn in 2004 and started sending back thousands of images did scientists have a better picture of the activity.
A 2007 science paper by Hedman and colleagues first noted corrugations in Saturn`s innermost ring, dubbed the D ring.
A group including Hedman and Mark Showalter, a Cassini co-investigator based at the SETI Institute in Mountain View, Calif., saw that the grooves in the D ring appeared to wind together more tightly over time.
Cassini imaging scientists received another clue around August 2009 when the sun shone directly along Saturn`s equator and lit the rings edge-on. The unique lighting conditions highlighted ripples not previously seen in another part of the ring system.
The collision tilted a region more than 19,000 km wide, covering part of the D ring and the next outermost ring, called the C ring.
Hedman and Showalter, the lead author on the second paper, wondered whether the long-forgotten pattern in Jupiter`s ring system might illuminate the mystery.
Using Galileo images from 1996 and 2000, Showalter confirmed a similar winding spiral pattern by applying the same math they had applied to Saturn and factoring in Jupiter`s gravitational influence.
Galileo was launched on a space shuttle in 1989 and studied Jupiter until 2003.
Unwinding the spiral pinpointed the date when Jupiter`s ring was tilted off its axis between June and September 1994.
Shoemaker-Levy plunged into the Jovian atmosphere in late July. The Galileo images also revealed a second spiral, which was calculated to have originated in 1990.
Images taken by New Horizons in 2007, when the spacecraft flew by Jupiter on its way to Pluto, showed two newer ripple patterns, in addition to the fading echo of the Shoemaker-Levy impact.
"We now know that collisions into the rings are very common - a few times per decade for Jupiter and a few times per century for Saturn," said Showalter.
"Now scientists know that the rings record these impacts like grooves in a vinyl record, and we can play back their history later," he added.
The findings are detailed in two papers published in the journal Science.