Published online 31 March 2011 | Nature | doi:10.1038/news.2011.198

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Wobbles in the rings of Saturn and Jupiter preserve a record of past impacts.

Comet impacts give the rings of Saturn a distinctive corrugated pattern that can be used to date when the comet struck. NASA/JPL/Space Science Institute

The rings of Saturn and Jupiter contain ripples caused by comets that hit them decades ago. Monitoring how the rings wobble could reveal how common comet impacts are – and may also help astronomers map the planets' cores.

Matthew Hedman, an astronomer at Cornell University in Ithaca, New York, and his colleagues spotted the ripple in one of Saturn's rings in images taken by the Cassini spacecraft in 2009. Sunlight striking the rings edge-on revealed previously unseen bright and dark bands in the planet's C ring, which lies between about 74,600 and 92,000 kilometres from the planet's centre. "It's the same thing you see when sunlight races across a corrugated tin roof," says Hedman.

The undulations appear because particles in the rings are moving up and down, like people in a Mexican wave. The ripple formed when part of the ring was knocked out of kilter. As time passed, this tilt has become a progressively tighter spiral, meaning that the shorter the ripple's wavelength, the longer ago it was formed.

Using this relationship, Hedman and his colleagues calculated that the ripple began in 1983 and reasoned that it was caused by an unseen comet impact. As the comet broke up, they argue, its cloud of debris – with a total mass between 1011 and 1013 kilograms – bashed into the ring particles, making the ring tilt and wobble. Earlier Cassini images also showed signs of a ripple in the adjacent D Ring, probably caused during the same strike, Hedman says.

The more time that has elapsed since the impact, the tighter the ripple in the rings will be. Courtesy of Science/AAAS

Strike two

Jupiter provides further evidence that comets make rings ripple. A team led by Mark Showalter at the SETI Institute in Mountain View, California, found two sets of ripples in images of Jupiter's rings taken by the Galileo spacecraft in 1996 and 2000 and by the New Horizons spacecraft in 2007.

The team calculated that debris from two comets had struck Jupiter's rings, the first between January and June 1990 and the second between July and October 1994. The latter date matches comet Shoemaker-Levy's plunge into Jupiter in July 1994.

"It just seemed like too good a match to be a coincidence," says Hedman, who worked on both sets of calculations. Both the Saturn and Jupiter findings appear in Science today1,2.

It's remarkable that ring systems can preserve a record of comet impacts stretching back decades, says Carl Murray, an astronomer at Queen Mary University of London, UK. "Essentially, they've developed an entirely new way of dating comet impacts," he says.

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Hedman plans to examine future higher-resolution images of the rings still to come from Cassini and any future missions for signs of earlier collisions. "We'll poke around to see if we can get a picture of how frequent collision events are," he says.

Murray notes that monitoring the ripples will help astronomers discover more about the planets' innards - such as the size and shape of their cores, which may not be perfectly spherical. The ripples are slowly spiraling towards to the planets' centres at a rate controlled by gravity, which, in turn, depends on the planets' internal structure.

"It's tough for astronomers to find out much about that structure otherwise, but these particles could reveal all sorts of new information," Murray says.