Enceladus is a mid-sized satellite of Saturn, about 500 km across, known since 1789. It is only one-tenth the size of the largest Saturnian moon, Titan. In 2005, the Cassini spacecraft discovered that this relatively small world is, surprisingly, geologically active. Geyser-like jets of water were seen venting from the moon's south-polar surface ices, vented by a process called cyrovolcanism ('cold volcanism'). Another satellite, Mimas, is seen on Saturn's ring plane.