In 2005, NASA's Cassini Saturn orbiter discovered geysers blasting particles of water ice into space from "tiger stripe" fractures near Enceladus' south pole. That material, which forms a plume that feeds Saturn's E ring (the planet's second-outermost ring), is thought to come from a huge ocean of liquid water that sloshes beneath the moon's icy shell.

And there's more than just water ice in the plume. During numerous close flybys of the 313-mile-wide (504 kilometers) Enceladus, Cassini spotted many other compounds as well — for example, dihydrogen (H2) and a variety of carbon-containing organic compounds, including methane (CH4).