<Tr> <Th> Composition by volume </Th> <Td> 91% water vapor 4% nitrogen 3.2% carbon dioxide 1.7% methane </Td> </Tr> <P> Enceladus (/ ɛnˈsɛlədəs /; en - SEL - ə - dəs) is the sixth - largest moon of Saturn . It is about 500 kilometers (310 mi) in diameter, about a tenth of that of Saturn's largest moon, Titan . Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System . Consequently, its surface temperature at noon only reaches − 198 ° C (− 324 ° F), far colder than a light - absorbing body would be . Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago . </P> <P> Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, passed nearby in the early 1980s . In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail . In particular, Cassini discovered water - rich plumes venting from the south polar region . Cryovolcanoes near the south pole shoot geyser - like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling about 200 kg (440 lb) per second . Over 100 geysers have been identified . Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring . According to NASA scientists, the plumes are similar in composition to comets . In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mi). </P> <P> These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is currently geologically active . Like many other satellites in the extensive systems of the giant planets, Enceladus is trapped in an orbital resonance . Its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces, tidally heating its interior and driving the geological activity . </P>

Which one of saturn's moons has water