What are Saturn's rings actually made of?

Almost pure water ice, from grains of dust up to boulders the size of a house, orbiting in a sheet nearly 300,000 kilometres across and typically about ten metres thick. Scaled to a sheet of A4, they would be roughly the width of a football pitch and thinner than the paper.

They cannot last. Cassini measured material raining down onto the planet from them fast enough to drain the whole system in perhaps a few hundred million years, and the same data suggested they only formed something like 100 million years ago - so dinosaurs walked the Earth before Saturn had rings, and they will be gone long before the Sun does anything interesting. We are looking at a temporary feature, which is an odd piece of luck.

The gaps in them are moons. Small satellites orbiting inside the system sweep lanes clear and shepherd the edges of others sharply with their gravity, and resonances with larger moons further out clear the wider divisions. The rings are less a static object than an ongoing negotiation between ice and gravity.