Enceladus is the sixth-largest moon of Saturn. It was discovered in 1789 by William Herschel, but little was known about it until the two Voyager spacecraft passed nearby in the early 1980s. The Voyagers showed that the diameter of Enceladus is only 500 kilometers (310 mi), about a tenth of that of Saturn's largest moon, Titan, and that it reflects almost all the sunlight that strikes it. Enceladus has a wide range of surfaces ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago, despite its small size.
In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered a water-rich plume venting from Enceladus's 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 approximately 200 kilograms (440 lb) per second. Over 100 geysers have been identified. Some of the water vapor falls back as "snow" and the rest escapes, which supplies most of the material making up Saturn's E ring.
|S/2009 S 1 - (nameless moonlets) - Pan - Daphnis - Atlas - Prometheus - Pandora - Epimetheus - Janus - Aegaeon - Mimas - Methone - Anthe - Pallene - Enceladus - Tethys - Telesto - Calypso - Dione - Helene - Polydeuces - Rhea - Titan - Hyperion - Iapetus - Kiviuq - Ijiraq - Phoebe - Paaliaq - Skathi - Albiorix - S/2007 S 2 - Bebhionn - Erriapus - Skoll - Siarnaq - Tarqeq - - S/2004 S 13 - Greip - Hyrrokin - Jarnsaxa - Tarvos - Mundilfari - S/2006 S 1 - S/2004 S 17 - Bergelmir - Narvi -Suttungr - Hati - S/2004 S 12 - Farbauti -Thrymr - Aegir - S/2007 S 3 - Bestla - S/2004 S 7 - S/2006 S 3 - Fenrir - Surtur - Kari - Ymir - Loge - Fornjot|
These 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 geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity. Enceladus is in such a resonance with Saturn's fourth largest moon, Dione. Enceladus's proximity to Saturn leads to tidal heating of its interior, offering a possible explanation for the activity. In 2014, NASA reported that evidence for a large south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km had been found by Cassini.