Flight over Saturn showing its rings. Saturn's extensive system of rings is formed of countless small particles of water ice, up to a few metres across at most. The bright main rings have a diameter of some 270,000 kilometres, although fainter rings extend much further. Despite their great extent, the rings are extremely thin - the thickest ring is less than 30 metres deep, and many regions are thinner than that. One effect of this is that the rings are invisible when they are edge-on to Earth. The formation of the rings is not fully understood. It is thought that they are the remains of an icy moon that was either ripped apart by tidal forces, or destroyed in a collision with another body.