Scott Bolton’s first encounter with Io took place in the summer of 1980, right after he graduated from college and started a job at NASA. The Voyager 1 spacecraft had flown past this moon of Jupiter, catching the first glimpse of active volcanism on a world other than Earth. During recent flybys of the volcanic moon by NASA’s Juno spacecraft, scientists measured Io’s gravitational effect on Juno, using the spacecraft’s tiniest wobbles to determine the moon’s mass distribution and therefore its internal structure. Io’s volcanism is powered by a gravity-driven mechanism called tidal heating, which melts the rock into magma that erupts from the surface. NASA launched the $5 billion Clipper spacecraft to search Europa’s sky for signs of life in the proposed underground ocean.