Enceladus is the sixth-largest moon of Saturn. It was discovered in 1789 by William Herschel, though little was known about Enceladus until the two Voyager spacecraft passed near it 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 of the sunlight that strikes it. Voyager 1 found that the moon orbited in the densest part of Saturn's diffuse E ring, indicating a possible association between the two. Voyager 2 revealed that, despite the moon's small size, it has a wide range of terrains ranging from old, heavily cratered surfaces to young, tectonically deformed terrain, with some regions with surface ages as young as 100 million years old.
In 2005, the Cassini spacecraft performed several close flybys of Enceladus, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows 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; proximity to Saturn could then lead to tidal heating of Enceladus's interior, offering a possible explanation for the activity.
In 2014, NASA reported that evidence for a large south polar underground ocean of liquid water within Enceladus with a thickness of around 10 km had been found by Cassini. Cryovolcanoes near the south pole shoot large jets of water vapor, other volatiles, and some solid particles like NaCl crystals and ice particles into space, totaling approximately 200 kilograms (440 lb) per second. Some of this water falls back onto the moon as "snow", some of it adds to Saturn's rings, and some of it reaches Saturn. The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E ring.