NASA/JPL/Space Science Institute

The year is 2005, and the Cassini spacecraft is closing in on Saturn's moon Enceladus for the very first time. Both NASA's Voyager probes had visited the distant moon about two decades earlier, and from those observations, NASA was convinced that Enceladus was nothing more than a solid ball of ice.

On February 17, Cassini's magnetometer instrument noticed something rather odd. Enceladus lies within the Saturn's magnetic field, and the magnetometer detected a disturbance in the planet's magnetosphere around Enceladus's south pole. The perturbations in the magnetic field were similar to what astronomers have observed around objects with thin atmospheres. It seemed that something was restoring the thin air around Enceladus—something was moving underneath the ice.

"Enceladus was so exciting that, instead of just three close flybys planned for our four-year primary mission, we added 20 more, including seven that went right through the geysers at the south pole," Linda Spilker, Cassini project scientist at JPL, said in a press release.

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