The origin of Saturn's rings, seen here by Voyager, is still unknown.

THE EVOLUTION OF SATURN

Now the diminished core of a gas giant, Saturn's rocky nucleus may once have been a sizeable planet in its own right. Saturn was formed as gases, (mostly hydrogen), were swept up by the planet as it grew in mass, becoming ever more gravitationally powerful. This cleared a broad swath through the nebulous early Solar System.

Cross-section through Saturn.

But how did the rings form? It was first thought that a meteorite or other body which strayed close to Saturn was broken up by tidal forces and the particles spread about the planet in a broad disk. The idea was proposed in the 19th century by a French mathematician called Edouard Roche. He defined a region about any planet (now called the "Roche limit") within which a body lacking internal friction is pulled apart by tides.

Since Voyager, it has been seen that very small moons moving close about the planets are not broken up. However, larger moons near the Roche limit, even if they are consolidated, can be broken apart by tidal forces. More recently, a popular idea has been that the particles which make up Saturn's rings were never part of a larger body at all, and they were simply swept into place about the planet as it evolved from the primitive solar nebula. The theory is supported by the large proportion of icy particles in some of the rings. It suggests that they formed in situ early on, when gas was available. The particles grew in size as vapour condensed on their surfaces.

The accretion theory is analogous to the formation of the Solar System itself. The rings are representative of the disk of particles moving about the Sun from which the planets formed. Another similar analogy to Solar System development is seen in the Jovian system. At Jupiter, the Galilean satellites with increasing density and a higher proportion of heavy elements and compounds have coalesced close to the planet. Whereas the Saturnian system is analogous to a young Solar System, the Jovian assemblage is analogous to a more evolved Solar System.

Although the rings could have formed in this way (via accretion), they may have been added to by the disruption of moons and the ejection of material from them by small meteorites hitting their surfaces and knocking pieces off.