Plumes erupting from Enceladus. Earth is a few thousand kilometres from Enceladus, a mid-sized, icy moon of Saturn. Another of Saturn's moons, Tethys, can be seen at top right obscuring part of the Sun. In 2005, the Cassini spacecraft discovered that the relatively small world of Enceladus (only 500km across) is, surprisingly, geologically active. Geyser-like jets of water were seen venting from the moon's south-polar surface ices, produced by a process called cyrovolcanism (cold volcanism).