In 1671 Giovanni Domenico Cassini discovered a moon of Saturn that seemed to vanish on one side of its orbit. It took until 1705 and a better telescope to see Iapetus on the eastern side, two magnitudes dimmer, and to deduce that the moon is tidally locked with one bright hemisphere and one dark one. This episode explores Saturn’s third largest moon: the most distant tidally locked moon in the solar system, orbiting 2.4 times farther out than Hyperion on an inclination of 5 to 21 degrees that gives it the only top down view of the rings.

We explain the two step mechanism behind the dichotomy. Dark dust shed by the retrograde moon Phoebe, confirmed by the Spitzer Space Telescope, spirals inward and coats the leading hemisphere, Cassini Regio, which has an albedo of just 0.03 to 0.05. Then a 79 day rotation bakes the dark side to 129 Kelvin, just warm enough for ice to sublimate and hop to the brighter, colder trailing side, leaving a lag deposit only tens of centimeters thick and eliminating any gray transition zones. We also cover the walnut shape, the 1,300 kilometer equatorial ridge with peaks up to 20 kilometers, the 580 kilometer Turgis basin with its 15 kilometer scarp, low gravity ice landslides, competing ridge theories from collapsed rings to a frozen 16 hour spin, and Cassini’s 2007 flyby at 1,227 kilometers.

• How Cassini deduced tidal locking and a two toned surface from a moon that disappeared for decades
• Phoebe’s dust ring and why the leading hemisphere acts like a windshield collecting bugs
• The 16 Kelvin difference that drives a runaway ice migration and explains the absence of gray
• The equatorial ridge, the Turgis basin scarp, and sturzstrom avalanches in two percent gravity
• Why the half ridge on a globally spun moon remains unexplained, and why Iapetus may be a primordial relic