A quick tour of the solar system
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After at least 2½ weeks more travelling from Jupiter at a million miles an hour we reach Saturn, the lovely ringed planet. This may seem a long time, but the Cassini probe that is currently orbiting the planet took seven years to get there.
The most obvious thing about Saturn is the rings. They look solid, but actually they are made up from billions of chunks of ice and rock, all whirling round the planet. Sometimes we see the rings edge-on and at those times they almost disappear, because they are actually only a hundred metres or so thick. If you made a model of Saturn on a football pitch, the planet would be the size of the dome of St Paul’s Cathedral in London and the rings would be as wide as the pitch, but they would only be as thin as a sheet of paper. (This is intended for UK readers. Those elsewhere will have to find their own comparisons – the dome of St Paul’s is 33 m across and a football (soccer) pitch is 73 m wide.)
Like Jupiter, Saturn is made of hydrogen gas, though it is rather smaller, and it also has swirling bands of gas. We can’t land on Saturn, but it has several interesting moons. The Cassini spacecraft is still studying these moons, and only a few days ago it sent a space probe called Huygens down to land on the largest moon, Titan. This is the biggest moon in the solar system, and it is even bigger than the planet Mercury. Because it is surrounded by orange clouds, its surface was quite a mystery until Huygens landed there. Now we can see that there seem to be rivers flowing down to lakes – but they aren’t rivers of water but are probably methane, which on Earth is a gas. It’s so cold on Titan that the gas turns into a liquid. The surface is icy, and what look like rocks are probably chunks of ice a few centimetres across, sitting in a sort of methane mud. This view shows the surface from the probe itself, with the camera only 40 cm above the surface – from the height of a small dog. But there are no dogs or other animals on Titan. It’s probably too cold for life of any type to form.
The other moons of Saturn are also icy. This far out in the solar system, there is a great deal of ice. When the solar system formed about 4½ billion years ago, most of the rocky material stayed closer to the Sun and formed the rocky planets, but out here most moons are made of ice.
Look at Iapetus, for example. This looks like a Christmas pudding with icing on top.One half of it it very dark and the other half is bright, probably because of dark material thrown off another moon, Phoebe, which orbits beyond Iapetus.
Saturn has over 40 other moons, mostly very small. The number keeps on increasing as more are discovered.
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Saturn from the Cassini spacecraft
Artist’s impression of Saturn’s rings
Changing views of Saturn’s rings
Closeup of clouds on Saturn
Titan is completely cloud-covered
Shoreline on Titan
Surface of Titan
Iapetus – not a Christmas pudding!