Ocean inside Saturn's moon Enceladus, illustration. Enceladus is the sixth largest of Saturn's moons. 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 this ice to produce water which can rise to the surface through cracks in the ice. Polar jets (shown at bottom) have been observed on Enceladus, due to 'water volcanism' (cryovolcanism). Enceladus is 504 kilometres in diameter, with a mean surface temperature of 75 Kelvin (minus 198 degrees Celsius). This depiction is based on based on a gravity investigation by NASA's Cassini spacecraft and NASA's Deep Space Network. Image published in 2014.