Illustration of a cryovolcano erupting from the surface of Saturns moon Enceladus. The surface of Enceladus is thought to be a thick layer of water ice covering a hard rocky interior. Geologic activity and tidal heating melts the ice to produce water, which can rise to the surface through cracks in the ice. Polar jets have been observed on Enceladus, due to water volcanism (cryovolcanism).