Page last updated at 13:20 GMT, Friday, 12 March 2010

Saturn's beauty and power

By Michael Lachmann
Producer, Wonders of the Solar System


Why Saturn's rings are like an Icelandic lagoon

It's exactly 400 years since Galileo first pointed a telescope at the planet Saturn. He immediately realised he had discovered something remarkable: the planet appeared to have ears.

Today, we know the mysterious bulges he described are a magnificent network of rings.

Their intricate beauty is captivating but Saturn's rings are more than cosmetic wonders.

Many astronomers now believe the patterns encoded in the rings tell us something fundamental about the forces that formed the entire Solar System.

And that is part of the story we wanted to tell in the second episode of BBC2's Wonders of the Solar System - Order Out of Chaos.

Seen from a telescope on Earth, the rings of Saturn appear to be a solid disk that encircles the planet.

But we know they cannot be solid - the gravitational forces around the planet would tear them apart. The rings are actually made of billions of chunks of ice all independently orbiting Saturn in a belt that is over 70,000km wide, but as little as 3m thick.

And, although the rings are a billion kilometres away, you don't have to travel far to get a sense of what they must be like.

A glacial lagoon in Iceland, packed with icebergs was the perfect filming location for the series, which left a lasting impression on the programme's presenter, Brian Cox.

"I climbed up a floating iceberg in the lagoon and was left there alone whilst we did a series of sweeping helicopter shots," he recalls.

"It was a magical and quiet place (until the helicopter flew past) and I allowed my mind to drift. The rings are magnificently pure water-ice, just like the icebergs, and I genuinely felt transported three-quarters of a billion miles away to the rings of Saturn.

"I make a comment on the show to that effect in an astonishingly broad Oldham accent, which shows that my mind was elsewhere!"

Saturn (Nasa)
Saturn - the "supermodel of the Solar System"

It's by studying the dynamics of the icebergs in the rings that scientists hope to understand how the whole Solar System formed.

One of the enduring mysteries of the Solar System is how the ordered network of planets orbiting the Sun spontaneously emerged form the chaos of space. In Saturn's rings, we can see similar ordered patterns and structures assembling themselves from the chaos of the ring particles.

Images sent back from the Cassini spacecraft (which has been in orbit around Saturn since 2004) have revealed how the fine structure of the rings is broken up into intricate patterns of thousands of ringlets and gaps.

Carl Murray, from Queen Mary, University of London, is one of the senior British astronomers on the Cassini mission.

He believes the whole Saturn system is an excellent model for the early Solar System. "In the rings, we're learning something about our own origins," he says.

Because the physical processes that go on in the rings are probably similar to what went on in the early Solar System and we've got this incredible opportunity to observe those processes happening in front of our eyes."

One of the key discoveries has been the role Saturn's moons play in maintaining the order and structure of the rings. Saturn has a network of over 60 moons and as they orbit the planet their gravitational influence sweeps through the rings.

In some of the most breathtaking pictures from Cassini we can actually watch the moons at work. As the moons pass close to - or even through the rings, their gravitational pull tugs waves of the ring particles towards them, twisting and shaping the ring and sweeping paths through them.

These close-range effects are wonderful to watch. But the most significant influence is wielded by the moons that orbit thousands of km outside the rings. Even at this distance, they carve delicate patterns in the rings through a phenomenon called orbital resonance.


The effect of the moons on the rings (Imagery courtesy of Nasa/JPL/SSI)

The particles of ice in the rings orbit Saturn like cars speeding along a road.

Particles nearest to the planet move more quickly than those further away. Beyond the rings, the moons orbit even more slowly.

Resonances occur between the moons and ring particles whose orbital rate is a simple ratio of the moon's orbit. For instance if a ring particle orbits the planet in exactly half the time that the moon does, it will always line up with the moon in exactly the same place in orbit and will repeatedly get a gravitational tug in exactly the same direction.

Over many orbits, these tiny nudges add up and shepherd the particles into one of Saturn's countless ringlets.

But orbital resonances can do much more than gently sculpt Saturn's rings.

It's now believed orbital resonances between planets played a crucial role in the evolution of the whole Solar System - and may ultimately have helped make Earth a habitable planet.

Influence on Earth

It is thought that the giant planets did not form where they are now - instead their orbits drifted for hundreds of million of years. Eventually, the two giants of the Solar System, Jupiter and Saturn, fell into a resonant order - where Jupiter was orbiting exactly twice in the time it took Saturn to complete one lap of the Sun.

This resonance had such a powerful gravitational surge that it distorted the orbits of all the outer planets.

It's believed that Neptune was catapulted from its orbit into the heart of the Kuiper Belt, the ring of icy comets surrounding the Solar System, where it caused chaos.

For a hundred million years, a rain of comets was fired from the Kuiper Belt into the inner Solar System. It was called the Late Heavy Bombardment.

The comets left their scars. The craters they caused can still be seen on the moon - and the Earth would have received similar damage.

But the effects weren't all bad. It's believed that up to half the water we have on Earth today was delivered by that rain of comets.

The same forces of orbital resonance we see carving the delicate patterns in Saturn's rings helped to make our planet the place it is today.

You can see the second part of Wonders of the Solar System with Professor Brian Cox on BBC Two this Sunday (2100 GMT)

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