What are the rings made of, according to scientists?

Saturn's rings are among the most beautiful sights in the solar system, and one of its mysteries. Scientists have ideas about why the planet has rings, but no one knows for sure.

What we do know comes largely from four robotic spacecraft that have visited Saturn: Pioneer 11, Voyager 1, Voyager 2 and Cassini. They showed that what looks like one flat ring from the Earth is really hundreds of rings, perhaps five hundred to a thousand, with gaps between them.

The rings are enormous in one direction and astonishingly thin in the other. From edge to edge they stretch about 400,000 km, roughly the distance from the Earth to the Moon. Yet in places they are as little as 100 metres thick.

Nor are they solid. They are made of countless separate pieces, ranging from particles too small to see to pieces the size of a bus. Scientists think these are icy snowballs, or rocks covered in ice, all circling the planet together.

The longest visit was Cassini's. It arrived in July 2004 and orbited Saturn for thirteen years, studying the planet, its rings and its moons.