NASA’s Juno spacecraft has made a groundbreaking discovery by measuring temperatures beneath the surface of Jupiter’s volcanic moon, Io—a feat never accomplished before. The new data shows that heat rises from deep within the moon across much of its surface, revealing that Io’s outer shell is surprisingly smooth and composed of lightweight materials.
The findings, published in the Journal of Geophysical Research: Planets, give scientists a fresh perspective not only on volcanic worlds like Io but also on how volcanoes work here on Earth.
Io is the most volcanically active body in our solar system. Unlike Earth, where volcanoes are primarily fueled by internal heat, Io’s intense volcanic activity results from Jupiter’s powerful gravity. As Io orbits the giant planet in a slightly elongated path, Jupiter’s gravity bends and squeezes it. This process, called tidal heating, generates vast amounts of internal heat, fueling hundreds of active volcanoes.
Until now, scientists could only observe Io’s heat by detecting infrared radiation emitted from its surface. Juno’s Microwave Radiometer changed that by probing beneath the surface to detect hidden heat. Originally designed to analyze Jupiter’s deep atmosphere, the radiometer features six antennas that measure microwaves at different wavelengths. Since each wavelength penetrates to a different depth, the instrument can determine temperatures from just below the surface down to several meters underground.
During two close flybys—on December 30, 2023, and February 3, 2024—Juno came within approximately 1,500 kilometers of Io, allowing scientists to peer beneath the moon’s rocky exterior for the first time. These measurements detected temperature increases of over 40°F (roughly 22°C) just a few feet beneath the surface. Such a steep temperature rise far exceeds what solar heating alone could produce, indicating that heat from Io’s interior is constantly rising.
Researchers suggest two main explanations. One is that heat continually moves upward through the crust, spreading across the entire moon. While the heat flow per small area is modest, collectively it amounts to an immense energy—up to 30 times the average heat flow seen on Earth. The second possibility is that about 10% of Io’s surface is covered by cooled lava flows hidden beneath a layer of solid rock about 9 to 11 meters thick, which continues to release heat even after the surface cools.
This discovery is significant because tidal heating—driving Io’s internal activity—also energizes subsurface oceans beneath icy moons such as Europa and Ganymede. Understanding heat transfer in Io may help scientists better grasp the potential habitability of environments beneath frozen surfaces on other worlds.
Flyby data also revealed a surprising aspect of Io’s landscape. Although it’s known for towering mountains and active volcanoes, much of its terrain is surprisingly flat. Large, smooth plains stretch over distances exceeding 100 kilometers, resembling broad volcanic plains rather than rugged mountain regions. Microwave readings also suggest the surface material is much less dense than solid rock, likely similar to volcanic pumice or ash filled with tiny air pockets.
Overall, the success of Juno’s microwave observations opens new doors for studying both rocky and icy worlds throughout the solar system. Future missions might utilize similar instruments to understand heat movement beneath active volcanoes on Earth, offering insights that could improve volcanic hazard assessments long before eruptions happen.
This research was published in the Journal of Geophysical Research: Planets.
