Formation of Saturn's rings. Illustration of the theory that the rings of Saturn were formed by an impact (upper left) that shattered a medium-sized moon orbiting the planet. Debris spread out in a doughnut-shaped ring (upper right). Later collisions (lower left) evened out the particles into a ring plane. Jostling between the particles and gravitational effects produced circular orbits and gaps in the rings (lower centre). The final result is a flat and circular ring system (lower right). The rings, only about one kilometre thick, consist of millions of rock and ice chunks. The most visible rings extend outwards from Saturn for around 120,000 kilometres.