Animation of ice plumes erupting from Enceladus, the sixth-largest moon of Saturn. Enceladus is an icy moon, and in its south polar region huge jets of water ice particles erupt into space from ridges in its icy crust. This cryovolcanism is the source of the material that forms Saturn's large but diffuse E ring, in which Enceladus orbits. The heat that melts the ice and powers these eruptions is a combination of the radioactive decay of elements in Enceladus's rocky core, and tidal heating due to the varying effects of the gravity of Saturn during its orbit. Enceladus is also locked in a 2:1 resonance with the moon Dione (completing two orbits for Dione's one), which stabilises its eccentric orbit around Saturn and enhances the tidal heating effect. Chemical analysis of the plume material has revealed that the liquid subsurface ocean from which they erupt is salty, and Enceladus is considered one of the most likely places in the solar system to harbour extraterrestrial life.