At noon on a sunny day, Enceladus sits at minus 198 degrees Celsius, yet this 500-kilometer moon is one of the most geologically active bodies we know. This episode follows Saturn's sixth-largest moon from William Herschel's 1789 discovery with a 40-foot telescope during a Saturnian equinox to the Cassini spacecraft's 2005 revelation of more than a hundred geysers erupting from tiger-stripe fractures at the south pole. We explain why fresh snow on the surface and the unstable E ring both demanded an active source, and how 250 kilograms of material per second at up to 360 miles per hour supplies them.

We then look beneath the ice: the 2:1 orbital resonance with Dione that keeps the orbit eccentric, tidal heating that squeezes the interior like a stress ball, and the gravity measurements that revealed a global ocean about 10 kilometers thick  ...