Episode Details

Mimas looks identical to the Death Star in photographs, and its heat signature from the Cassini orbiter glows in the shape of Pac-Man. This episode explores Saturn's innermost major moon, the smallest known body rounded by its own gravity at about 396 kilometers across, with a surface area smaller than Spain and a density of just 1.15 grams per cubic centimeter. The conversation examines the 139-kilometer Herschel crater that spans a third of the moon's diameter, why ice at 64 Kelvin behaves like granite and let Mimas survive an impact that should have shattered it, and how William Herschel first spotted the moon in 1789 with his 40-foot telescope.

The episode then unpacks the physics of orbital resonance that lets this tiny moon carve the 4,800-kilometer Cassini Division out of Saturn's rings, along with its 3:1, 7:6, and 2:1 resonances with other rings and the moon Tethys. Finally it traces the discovery that Mimas is not geologically dead at all. An anomalous libration spotted in 2014, a 2022 tidal heating model from the Southwest Research Institute, and a February 2024 Paris Observatory analysis of apsidal precession all point to a liquid water ocean beneath 20 to 30 kilometers of ice, an ocean so young, likely formed within the last 25 million years, that it has not yet cracked the surface.

• Why the 40-foot telescope name referred to focal length, not a 40-foot mirror
• How thermal inertia explains the Pac-Man shape in Cassini's temperature map
• The playground swing analogy for how a 2:1 resonance ejects ring particles
• Why scientists dismissed the ocean theory by 2017 and what changed their minds
• What a hidden ocean in a cratered, dead-looking moon means for the search for life