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© NASA/JPL/SSIInscrutable but not immutable
Saturn's rings seem almost immutable. These planetary jewels, carved by moonlets and shaped by gravity, could well have looked much the same now as they did billions of years ago - but only from afar.

Now it is emerging that an event around 25 years ago dramatically disrupted the rings - and all our telescopes and spacecraft missed it. This mysterious event suddenly warped the planet's innermost rings into a ridged spiral pattern, like the grooves on a vinyl record. The latest images reveal that the perturbation is so vast that only a profound change to the planet can have caused it.

The first hint that Saturn's rings had been perturbed came in 2006 from NASA's Cassini spacecraft, which took pictures of the planet's innermost ring, a tenuous, icy band called the D ring. Cassini found alternating light and dark bands that suggested the ring was not perfectly flat, but was composed of grooves, about a kilometre in amplitude.

Intrigued by these strange ripples, Matthew Hedman of Cornell University in Ithaca, New York, proposed an explanation. He built simulations that started with a simple assumption: that something had tilted the D ring slightly away from the plane of Saturn's equator - perhaps a comet or asteroid.

Since Saturn is not a perfect sphere, its gravity would pull unevenly on the tilted D ring, so that the ring "wobbles" over time relative to the planet, a bit like a spinning coin coming to rest. What's more, portions of the ring close to Saturn, where gravity is stronger, would wobble faster than parts further away. This difference in wobble rates would eventually create a spiralling pattern of ridges and troughs.

Hedman and colleagues were able to "unwind" the spiralling formation back in time to 1984, the point when the initial tilting of the D ring occurred. The bad news is that this date was more than two years after the Voyager spaceprobes took a close peek as they flew by the planet, and long before the Hubble Space Telescope came online or Cassini arrived.

The latest Cassini images deepen the mystery. They show the grooves are much more widespread than previously thought. In August, Saturn was orientated so that the sun illuminated the rings almost edge-on, which meant that even the most subtle features produced prominent shadows. This revealed that Saturn's vast C ring - the next ring out from the D ring - also has grooves. The C ring is about 17,000 kilometres wide from its inner to outer edge, which is more than twice the width of the D ring.

Though the C ring's ridge pattern is less dramatic - with an amplitude of 100 metres or less - such widespread corrugation probably rules out a rock or comet impact. "It's really hard to do with one object," says Hedman, who presented the results last week at a meeting of the American Astronomical Society's Division for Planetary Sciences in Fajardo, Puerto Rico.

One possible scenario is that a body much larger than a typical asteroid grazed and pulled on the rings. Or a dramatic change in weather might have shifted Saturn's gravity: if a band of clouds on the planet somehow sped up and were inclined to the equator, for example, it might lower pressure beneath, allowing more mass to accumulate there.

"The trouble is everything you come up with seems intrinsically unlikely," says Mark Showalter of the SETI Institute in Mountain View, California. It will require more observations and modelling to completely unravel Saturn's spiral mystery.