The original version of this story appeared in Quanta Magazine. 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. Umbrella-shaped outbursts of magmatic matter rocketed into space from all over Io’s surface. “They looked amazingly beautiful,” said Bolton, who is now based at the Southwest Research Institute in Texas. “It was like an artist drew it. I was amazed at how exotic it looked compared to our moon.” Scientists like Bolton have been trying to understand Io’s exuberant volcanism ever since. A leading theory has been that just below the moon’s crust hides a global magma ocean, a vast contiguous cache of liquid rock. This theory dovetails neatly with several observations, including ones showing a roughly uniform distribution of Io’s volcanoes, which seem to be tapping the same omnipresent, hellish source of melt. But now, it appears that Io’s hell has vanished—or rather, it was never there to begin with. 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. The scientists reported in Nature that nothing significant is sloshing about just beneath Io’s crust. “There is no shallow ocean,” said Bolton, who leads the Juno mission. Independent scientists can find no fault with the study. “The results and the work are totally solid and pretty convincing,” said Katherine de Kleer, a planetary scientist at the California Institute of Technology. The data has reopened a mystery that spills over into other rocky worlds. Io’s volcanism is powered by a gravity-driven mechanism called tidal heating, which melts the rock into magma that erupts from the surface. Whereas Io is the poster child for this mechanism, tidal heating also heats many other worlds, including Io’s neighbor, the icy moon Europa, where the heat is thought to sustain a subterranean saltwater ocean. NASA launched the $5 billion Clipper spacecraft to search Europa’s sky for signs of life in the proposed underground ocean.