A World of Fire and Fury
Io is the most volcanically active body in our solar system. It is a world constantly in turmoil, with over 400 active volcanoes spewing plumes of sulfur hundreds of kilometres into space. This extreme activity isn't driven by the same internal radioactive
decay that fuels Earth's geology. Instead, Io is caught in a gravitational tug-of-war. As it orbits the colossal planet Jupiter, it is relentlessly stretched and squeezed, generating immense friction and heat inside the moon. This process, known as tidal heating, is so intense that it melts rock deep within Io, powering its ceaseless eruptions. Until now, our understanding of this heat has been skin-deep, based on infrared instruments that could only measure the temperature of the very top surface.
An Unexpected Tool for the Job
The breakthrough came from an instrument on the Juno spacecraft that wasn't originally intended for this purpose. The Microwave Radiometer (MWR) was designed to peer deep beneath the thick clouds of Jupiter's atmosphere. During two close flybys of Io in late 2023 and early 2024, scientists turned this powerful instrument toward the rocky moon. Unlike infrared light, the MWR's microwaves can penetrate solid surfaces. This allowed Juno to probe between two and six meters below Io's crust, effectively giving it subsurface vision. The mission team was surprised it worked so well on a rocky body, a discovery that opens up new ways to study volcanic worlds, including our own.
What the Heat Map Reveals
The data returned by the MWR was used to create the first-ever subsurface temperature map of Io. The map shows that temperatures rise significantly just a few meters below the surface—in some places by more than 20 degrees Celsius. This sharp temperature gradient is far more than what could be explained by sunlight alone and is clear evidence of heat rising from the interior. The map also revealed massive, localized hot spots, with one major heat source coinciding with the Zal Montes Patera volcanic complex. Interestingly, the data also suggested that away from its towering mountains, much of Io's surface is remarkably smooth and covered in a low-density material, perhaps like volcanic ash or pumice.
A New Window Into Volcanoes
This new ability to map subsurface heat is a game-changer. Scientists now have two main theories for how the heat is moving: it could be rising steadily through a conductive crust, or it may be escaping through scattered patches of thin crust over pockets of lava. Either way, the data provides the clearest picture yet of the plumbing that drives Io's volcanic engine. The discovery has profound implications not just for understanding Io, but for planetary science as a whole. The same tidal heating process that makes Io a volcanic inferno is also thought to fuel the subsurface oceans on other moons like Europa and Ganymede, which are prime targets in the search for extraterrestrial life. Furthermore, scientists believe a similar microwave technique could be used to study volcanoes on Earth, potentially providing new insights into how they work and offering new ways to monitor their activity.
