Following two flybys of Jupiter’s innermost Galilean moon Io, teams using data from NASA’s Juno spacecraft have revealed the first measurements of the temperature below Io’s surface. The flybys revealed substantial subsurface heating on the moon, as well as a surface that is extremely smooth and composed of low-density materials.
Io is the most volcanically active celestial body in the solar system, a characteristic caused by the immense tidal forces from its parent planet. As Io orbits Jupiter, the planet’s extreme gravity stretches and squeezes Io’s surface, generating an internal heat output significantly greater than that of Earth’s. However, until these new measurements, everything about Io’s thermal nature was learned through infrared observations of its surface.
To make the subsurface temperature measurements, Juno used its Microwave Radiometer (MWR) instrument. Designed to make observations deep into Jupiter’s thick atmosphere, the MWR is multi-wavelength, supporting radiation observations from 1.3 cm to 51 cm wavelengths (600 MHz to 22 GHz frequencies). MWR achieves these observations via six differently sized antennas mounted to the sides of Juno. The MWR has also conducted observations of Ganymede and Europa.
MWR data map showing where heat is rising from Io’s subsurface environment. The colors represent a temperature gradient across Io. (Credit: NASA/JPL-Caltech/SwRI/USGS)
“The Juno Microwave Radiometer directly observed Io’s heat output by looking below the surface. The surprising discovery that we could see below a rocky moon’s surface has important implications for studying Earth’s volcanoes. Juno has taught us that if we look with an MWR-type instrument near a volcano on Earth, we might see a similar signature in the subsurface temperature gradient, providing new information on how terrestrial volcanoes work,” said co-author and Juno principal investigator Scott Bolton of the Southwest Research Institute.
The team, led by Shannon Brown of NASA’s Jet Propulsion Laboratory, used Juno data from two flybys of Io: the first occurred on Dec. 30, 2023, and the second on Feb. 3, 2024. During both of these flybys, Juno came within approximately 1,500 km of Io’s surface. MWR data revealed that temperatures dramatically increase just feet into the moon’s surface.
“The instrument measured Io’s thermal emission at depths ranging from a few inches down to tens of feet. Everywhere we looked, we found the temperature rising by more than 40 degrees Fahrenheit just several feet into the surface — a gradient far steeper than solar heating alone can explain,” Brown said.
Brown et al. have two theories for the sharp increase in temperatures just below the surface. The first theory suggests that Io has a conductive crust, allowing heat to steadily rise to the surface. The measured background heat flow within the crust — approximately one to three watts per square meter — equates to an energy release almost 30 times Earth’s average across the entire moon, though localized measurements are far less energetic.
Io imaged by Juno during its Dec. 30, 2023, flyby. (Credit: NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstädt)
The second theory suggests that cooling lava flows could be heating Io’s subsurface environment. These lava flows, trapped under approximately nine to 11 meters of solid crust, are estimated to cover around 10% of Io’s surface at any time.
“Io provides a unique window into learning how tidal heating works throughout the cosmos, a fundamental process that provides energy and heat to worlds that are far from their parent star. This process can not only create the most volcanic body in the solar system, in the case of Io, but also fuels the subsurface oceans on the moons of giant planets, such as Europa and Ganymede. Up until this point, we could only observe the heat escaping at the surface or through eruptions. Now we can characterize how the heat is moving from the interior toward the surface,” Bolton said.
Io’s subsurface temperatures weren’t the only thing revealed from Juno’s flybys in 2023 and 2024.
MWR data also showed that the moon’s surface is remarkably smooth. While Io features many tall mountains, beneath them lay patches of smooth surface that stretch for 100 km or further. Brown et al. determined the smoothness of Io’s surface by mapping how it reflects microwaves, a measurement made possible by Juno flying over overlapping regions at different angles.
Map highlighting where Juno’s MWR instrument measured Io’s surface. (Credit: NASA/JPL-Caltech/SwRI/USGS)
“Away from its mountains, the surface is more like the Great Plains of North America, and even though Io is a rocky body, the surface material has a very low density — more like pumice or a fluffy volcanic ash than solid rock,” Brown said.
Juno, launched in August 2011, recently celebrated a decade at Jupiter, inserting itself into orbit around the planet on July 5, 2016. Launched as part of NASA’s New Frontiers program, Juno completed its most recent pass of Jupiter, called “perijoves,” on July 5, 2026 — the 85th of its mission.
(Lead image: Io imaged by Juno in October 2023. Credit: NASA/JPL-Caltech/SwRI/MSSS/Ted Stryk)
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