Enceladus (pronounced /ɛnˈsɛlədəs/) 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, reflecting almost all the sunlight that strikes it, making its surface temperature at noon reach only −198 °C (−324 °F). 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.

Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft 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, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling about 200 kilograms (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).

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