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,  ...