What shook up Saturn’s rings in 1984?

Inscrutable but not immutable (Image: NASA/JPL/SSI)

14 October 2009 by Rachel Courtland, Puerto Rico

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 (see diagram).

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.

Something back in 1984 tilted the D ring slightly away from the plane of Saturn’s equator
The latest Cassini images deepen the mystery….

We highly reccommend New Scientist  magazine . Click on the link for the complete article.

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