Io is the most geologically active object in the solar system, a world with over 400 active volcanoes that repaves its own surface in sulfur and lava. This episode explains how a small rocky moon five times farther from the Sun than Earth stays molten. We begin with the 1610 discovery dispute between Galileo and Simon Marius, Ole Romer's use of Io's eclipses to measure the speed of light in 1676, and the shock of the 1979 Voyager 1 flyby when Linda Morabito spotted a plume rising from a crater-free surface.

We then dig into the 4:2:1 Laplace resonance with Europa and Ganymede that forces Io into an eccentric orbit and stretches its crust by up to 100 meters, generating internal friction. We cover the sulfur chemistry behind Io's colors, Everest-sized mountains formed by subsidence rather than volcanism, Juno's December 2024 detection of an  ...