Saturn's small, snow and ice–covered moon, Enceladus, only 310 miles (500 kilometers) across, has made a big impact on astronomers. On a series of close flybys in 2004, the Cassini spacecraft revealed a great deal of unexpected activity bursting forth from this frozen world, which travels with 33 other named satellites in Saturn's domain over 740 million miles from Earth.

But none, save Titan, Saturn's largest moon, have proved so enigmatic: The Cassini spacecraft has imaged jets feeding an active plume of water vapor spouting into space from "tiger stripes," or gashes, on Enceladus's south pole, signaling perhaps underground liquid seas stirred by enough internal heat to drive surface venting. Add to the mix organic compounds, and the remarkable thing is that this little moon has joined Mars, Jupiter's satellite Europa, and Titan as one of the most promising candidates scientists have for finding life elsewhere in the solar system.