Science Puzzle
Why Does Saturn Have Rings?
Saturn's rings stretch about 280,000 kilometres from edge to edge, yet in most places they're only about 10 metres thick. They're made of countless lumps of ice, from grains like dust to chunks as big as a house, each one orbiting Saturn like a tiny moon.
Further out, Saturn has plenty of real moons, some of them made from the same kind of ice. Why does the material close to Saturn stay spread out in rings instead of clumping together into another moon?
The Answer
Close to Saturn, the planet's gravity pulls so much harder on the near side of any clump than on its far side that the clump can't hold itself together. Inside that distance, called the Roche limit, a moon can't form, and a moon that strays inside it gets torn apart.
Two lumps of ice can stick together by their own weak gravity, but Saturn is pulling on them too. Gravity weakens with distance, so Saturn pulls the nearer lump a little harder than the farther one, which tends to drag them apart. Far from Saturn the difference is tiny and the lumps can gather into a moon. Close in, the difference beats the lumps' own gravity, and every attempt to clump is undone. Saturn's bright rings lie inside this limit; its big moons orbit outside it.
The rings are so thin because the lumps keep bumping into each other. Any lump moving up or down out of the plane soon collides with others and loses that motion, so the whole ring settles into a flat sheet.
Nobody is sure where the ring material came from. One idea is an icy moon or comet that wandered too close and was shredded. Measurements by the Cassini spacecraft suggested the rings could be young, perhaps only a few hundred million years old, though that's still argued over. Jupiter, Uranus and Neptune have rings too, just much fainter ones.
The principle: The Roche limit. Close to a planet, the difference in its pull across an object is stronger than the object's own gravity, so loose material can't gather into a moon and stays spread out as rings.