Enceladus

Enceladus is an icy moon orbiting Saturn, about 500 kilometres across. William Herschel observed it in 1789. Its bright surface and varied terrain make it distinctive among Saturn's moons, but spacecraft observations showed that it is not simply a static ice-covered body.

The Cassini spacecraft observed jets of water vapour and ice particles emerging from fractures near Enceladus's south pole. Analysis of the moon's gravity and shape, together with observations of the jets, provided evidence for liquid water beneath the ice. Subsequent accounts describe a global subsurface saltwater ocean. The plumes provide a way to sample material from this otherwise hidden environment without drilling through the crust.

Some plume material falls back onto the moon, while some escapes and contributes to Saturn's diffuse E ring. Cassini data also found silica nanograins in the ring; NASA describes these as evidence pointing to water–rock interaction at elevated temperatures within Enceladus. Combined evidence for liquid water, internal heat and chemical ingredients makes the moon a major target in studies of environments that might support life. These conditions indicate scientific interest, not evidence that life exists.

The Cassini mission, a collaboration involving NASA, the European Space Agency and the Italian Space Agency, transformed knowledge of Enceladus through repeated remote observations and close flybys. The spacecraft's observations of the plumes, gravity field and surrounding ring material helped researchers infer the moon's internal ocean and active processes. Cassini's final flyby of Enceladus took place in October 2015; its measurements remain an important basis for studying this ocean world.