A small icy moon of Saturn sprays an ocean into space
Near the south pole of Enceladus, more than 100 geysers blast water vapour, salt crystals and ice grains off the moon at around 200 kilograms every second. Some of it snows back down; the rest feeds one of Saturn's rings. Underneath lies an ocean of liquid water roughly 10 kilometres deep.
Enceladus is only about 500 kilometres across, a tenth the width of Titan, making it Saturn's sixth-largest moon. Its surface is blanketed in clean, fresh snow hundreds of metres deep, so it bounces back most of the sunlight that hits it; as a result, even at noon it stays around minus 198 degrees Celsius. Despite its size it shows great variety, from ancient cratered plains to young, buckled terrain.
William Herschel found it on 28 August 1789, on the first night he used his new 40-foot telescope at Slough, then the largest in the world. Faint and lost in Saturn's glare, it remained little more than a dot until the Voyager flybys of 1980 and 1981. Herschel's son John later named it after a giant from Greek myth, part of a scheme honouring the Titans led by Cronus, the Greek Saturn. Its craters and valleys carry names from Richard Burton's 1885 translation of the Arabian Nights; the IAU has named 85 features so far.
The real surprises came from Cassini, which began close passes in 2005. It found the south polar jets, which contain molecular hydrogen alongside water and resemble comets in composition, and supply most of Saturn's E ring. Escaping internal heat and a near-absence of craters in the south showed the moon is active right now. The engine is an orbital resonance with the moon Dione that keeps stretching Enceladus's orbit; tides fight the stretch and heat the interior. In 2014 NASA reported evidence for the buried southern ocean, whose existence has since been modelled mathematically.
Chemical sampling of the plumes pointed to hydrothermal activity beneath the ice, possibly driving complex chemistry. Research still under way suggests such an environment could suit some of the microbes that live at Earth's deep-sea vents, and that methane found in the spray might even be produced by organisms of that kind.
Source: Enceladus
