NASA’s Juno spacecraft has produced the first measurements of temperature beneath the surface of Io, Jupiter’s innermost Galilean moon and the most volcanically active body in the solar system. The data, collected during two close flybys and published in the Journal of Geophysical Research: Planets on July 22, 2026, reveal a temperature increase of more than 20 degrees Celsius in the upper few meters of the moon’s crust and show that the surface is unexpectedly smooth and porous across vast areas.
The findings come from Juno’s Microwave Radiometer (MWR), an instrument with six antennas that simultaneously detect microwaves at wavelengths ranging from 1.3 to 51 centimeters. Originally designed to peer through Jupiter’s thick cloud layers, the MWR has been used during Juno’s extended mission to observe three of the planet’s Galilean moons: Ganymede, Europa, and Io. At Ganymede and Europa, the instrument probed tens of miles below icy surfaces, but the ability to see into volcanic rock at Io was an unexpected bonus.
During flybys on December 30, 2023, and February 3, 2024, Juno passed approximately 1,500 kilometers (930 miles) above Io’s surface. The MWR measured upwelling thermal emission at depths ranging from a few centimeters to several meters. Because each wavelength penetrates to a different depth, the six channels together produce a vertical temperature profile of the shallow subsurface.
A Temperature Gradient That Points to Internal Heating
The MWR data revealed a large spectral slope in the lowest-frequency channels at all latitudes sampled, indicating significant endogenic heating in the upper few meters. Temperatures rose by more than 20 degrees Celsius relative to the surface skin temperature, a gradient far too steep to be explained by sunlight warming an airless world.
The research team, led by Shannon Brown at NASA’s Jet Propulsion Laboratory, applied two simple models to explain the observed microwave spectra. Under the first model, heat rises steadily through a conductive near-surface layer, producing background heat flows of 1 to 3 watts per square meter. While modest at any single location, comparable to a small nightlight under every square yard, this flow integrated across Io’s entire surface represents a heat release up to 30 times Earth’s global average.
The second model proposes that the signal comes from relatively fresh lava flows or heat vents buried beneath a cooling crust. In this scenario, the hot material lies approximately 10 meters (33 feet) below the surface and covers about 10 percent of Io’s area at any given time. The lava would be less than five years old, suggesting a continuous cycle of eruption, burial, and cooling across the moon’s surface.
The MWR data alone cannot distinguish between these two scenarios, and both may be operating simultaneously across different regions of Io.
A Surface Like Pumice and Ash
Beyond the temperature profile, the MWR observations revealed unexpected properties of Io’s surface itself. The moon reflects microwaves in a specular, mirror-like fashion, indicating a surface that is remarkably smooth on scales of 100 kilometers, despite its visible topography of tall mountains and volcanic calderas. Brown noted that away from its mountains, the surface is more like the Great Plains of North America than a rugged volcanic world.
The microwave data also constrained the dielectric constant of the surface material, yielding values between 2 and 4. This corresponds to a density of 0.7 to 1.1 grams per cubic centimeter for the upper 10 centimeters of the surface layer. For comparison, solid volcanic rock has a density of roughly 2.5 to 3 grams per cubic centimeter. The low density indicates that Io’s surface consists of highly porous, fragmented material, more akin to pumice or fluffy volcanic ash than solid rock.
This porous surface layer probably results from the constant rain of volcanic ejecta that blankets Io. With hundreds of active volcanoes across its surface, the moon is continuously resurfaced by eruptions that deposit loose, airfall material.
Tidal Heating and the Broader Picture
Io’s extreme volcanism is powered by tidal heating. As Io orbits Jupiter, the planet’s immense gravity stretches and compresses the moon, while gravitational pulls from neighboring Europa and Ganymede keep its orbit slightly elliptical. This constant flexing generates enormous internal friction, producing far more heat than radioactive decay alone could supply for a moon of Io’s size.
The Juno results provide a new observational window into how tidal heat moves through a rocky crust. Until now, scientists could only observe heat escaping at Io’s surface through infrared measurements or direct observations of eruptions. The MWR data show where heat is stored and transported in the shallow subsurface, allowing researchers to characterize how tidal energy flows from the interior toward the surface.
The technique has implications beyond Io. The same tidal forces that drive Io’s volcanism are responsible for maintaining subsurface liquid water oceans on Europa and Enceladus. Understanding how tidal heating distributes energy through different crust types, rocky on Io and icy on Europa, helps planetary scientists build better models of how these moons work as systems. Scott Bolton, Juno’s principal investigator at Southwest Research Institute, noted that the discovery that microwave radiometry can see below a rocky moon’s surface has important implications for studying Earth’s volcanoes as well.
The paper is authored by Shannon Brown, Virgil Adumitroaie, Scott Bolton, Anton Ermakov, Jianqing Feng, Steven Levin, Matthew Siegler, and Zhimeng Zhang, affiliated with JPL, Caltech, Southwest Research Institute, Stanford University, the Planetary Science Institute, and the University of Hawaii. Juno launched in August 2011 as part of NASA’s New Frontiers program and entered orbit around Jupiter on July 6, 2016, completing its 85th perijove on July 5, 2026.