History of the Saturnian System

We have seen already that Saturn formed in the same way that Jupiter did. A large icy planetesimal appeared first, which drew in gas from the Solar Nebula to create a disc around itself. Saturn grew at the centre of the disc. Since its birth, like Jupiter, it has been cooling down and contracting, and even now has a hot interior. However, the rings could not have been formed alongside the planet itself, for the heat would have evaporated them. And in any case, dynamical studies show that the rings are expected to last, at most, only one-tenth the age of the Solar System. Instead, Saturn's icy rings are the remains of large comets or moons that were broken up by the planet's gravity a few hundred million years ago - different in origin to Jupiter's rings.

Further out in the Saturn disc the material presumably lumped together to form many of the satellites. Most of the moons orbit the planet close to its equatorial plane and in the same direction - facts consistent with their formation in a disc. Yet there are irregularities with the Saturnian system that do not conform to a discal birth. The most important discrepancy is that the moons do not seem to be related to each other. Recall that the Galilean satellites have densities that decrease away from the planet Jupiter. This is what we expect: rocky worlds emerge close in where it is hot, and lightweight icy bodies lump together further out. The trouble is that all of the Saturnian satellites are very icy, and they do not show this pattern of decreasing density with distance from the planet. Also, only one of Saturn's satellites, Titan, is anywhere as near as large as Jupiter's big four. The rest are puny. The answer could be that many of the moons have suffered major impacts since their formation - impacts so devastating that the moons' surfaces now bear little testament to the past. The other difference compared with Jupiter is that even Saturn's smallest moons are icy. They are captured icy planetesimals, rather than the asteroids that Jupiter - being close to the asteroid belt - found it easier to net.

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