Jets of steam and icy grains erupt from deep fractures in the south polar terrain of Enceladus, making this tiny body one of only four places in the solar system known to have geologic activity in the present day. This artist's conception includes astronauts for scale.
Ron Miller

Key Concepts

• On the Saturnian moon Enceladus, jets of powdery snow and water vapor, laden with organic compounds, vent from the “tiger stripes,” warm gashes in the surface. How can a body just over 500 kilometers across sustain such vigorous activity?
• The answer may be the presence of underground fluids, perhaps a sea, which would increase the efficiency of heating by tidal effects. Support for this idea has come from recent flybys.
• If Enceladus has liquid water, it joins Mars and Jupiter’s moon Europa as one of the prime places in the solar system to look for extraterrestrial life.

When the Voyager 2 spacecraft sped through the Saturnian system more than a quarter of a century ago, it came within 90,000 kilometers of the moon Enceladus. Over the course of a few hours, its cameras returned a handful of images that confounded planetary scientists for years. Even by the diverse standards of Saturn’s satellites, Enceladus was an outlier. Its icy surface was as white and bright as fresh snow, and whereas the other airless moons were heavily pocked with craters, Enceladus was mantled in places with extensive plains of smooth, uncratered terrain, a clear sign of past internally driven geologic activity. At just over 500 kilometers across, Enceladus seemed far too small to generate much heat on its own. Yet something unusual had clearly happened to this body to erase vast tracts of its cratering rec­ord so completely.

Voyager’s brief encounter allowed no more than a cursory look, and, in hindsight, its imaging coverage of Enceladus was terribly unfortunate: a few medium-resolution images of the northern hemisphere, some low-resolution coverage in the south, and none of the south pole. We had no idea what we had missed.

   
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