NASA’s Juno spacecraft has measured the temperature beneath the surface of Jupiter’s volcanic moon Io for the first time, revealing a temperature gradient rising by more than 40 degrees Fahrenheit every few feet. Researchers published these peer-reviewed findings in the journal JGR Planets on July 22, 2026.
Io holds the title of the most volcanically active body in the entire solar system. While its surface remains freezing and near-airless, its interior burns from immense tidal heating driven by Jupiter’s gravity. Until recently, scientists could only gauge that inner heat by observing thermal radiation escaping from surface eruptions or the top crust itself. That observational limit changed when the Juno mission turned its instruments downward during close approaches on December 30, 2023, and February 3, 2024, coming within approximately 930 miles of the moon’s surface.
Microwave Radiometer Probes the Subsurface Crust
The breakthrough came from Juno’s Microwave Radiometer, an instrument originally designed by scientists to peer beneath Jupiter’s dense cloud tops and examine the deep atmosphere of the gas giant. The system utilizes six microwave antennas operating simultaneously to detect radiation across wavelengths ranging from about half an inch to 20 inches.
During the extended phase of the mission, researchers redirected the hardware toward the planet’s Galilean moons, including Ganymede, Europa, and Io. While earlier flybys of icy worlds like Ganymede and Europa probed tens of miles into ice shells assumed to be mostly pure water, probing the volcanic rock at Io proved to be an unexpected discovery.
Bolton and his colleagues published the results, opening a new observational chapter for celestial bodies beyond Earth.
Steeper Gradients Than Solar Heating Can Explain
The instrument measured thermal emissions reaching from a few inches down to tens of feet beneath the moon’s exterior crust. Across the sampled areas, researchers documented a sharp thermal increase.
In addition to these subsurface thermal profiles, data from the flybys revealed that most of Io’s surface is remarkably smooth and consists of low-density material.
Evaluating Excess Heat Flow and Cooling Lava Flows
The subsurface temperature data points toward an unexpected extra background flow of heat originating within the moon’s interior. On a local scale, this background heat flow measures between 1 and 3 watts per square meter, comparable to a small nightlight glowing under every square yard. When calculated across the entire moon, however, it totals an energy release up to 30 times greater than Earth’s average.
Scientists are currently weighing two competing hypotheses to account for the signal. The first suggests that heat is steadily rising upward through a conductive crust. The second posits that the thermal output originates from cooling lava flows capped by roughly 30 to 35 feet of solidified crust, which would cover about 10% of the moon’s surface at any given moment.
Implications for Terrestrial Volcanoes and Tidal Heating
Beyond expanding our understanding of Jupiter’s system, researchers suggest the microwave technique has direct applications closer to home. By demonstrating that subsurface temperature gradients can be mapped on a rocky moon, the findings open a path for studying terrestrial volcanism using similar microwave instruments.
Furthermore, the data clarifies how tidal heating operates across the cosmos, supplying energy not only to volcanically extreme bodies like Io but also to the subsurface oceans of icy satellites such as Europa and Ganymede. Observers can now characterize how heat moves from planetary interiors toward the surface rather than solely measuring what escapes through eruptions.
Future Observational Windows and Unresolved Questions
As the Juno spacecraft continues its extended mission phase in the Jovian system, planetary scientists plan to refine these subsurface models and test whether similar thermal signatures can be detected across other targeted bodies. Whether the background heat flow is entirely driven by conductive transport or sustained by widespread cooling lava sheets remains to be definitively answered as analysts process the remaining telemetry.
