New data from NASA’s Juno spacecraft show temperatures spike more than 20 degrees Celsius just a few meters under Io’s surface, providing the first look at what’s happening below the crust of the Solar System’s most volcanic world.
The north polar region of Io was captured by NASA’s Juno spacecraft on December 30, 2023. Image credit: NASA / JPL-Caltech / SwRI / MSSS / Gerald Eichstädt.
Jupiter’s moon Io is the most volcanically active world in our Solar System.
It is constantly stretched and squeezed by Jupiter’s gravity, generating enormous amounts of internal heat.
Until now, almost all observations of this heat came from infrared measurements that sense only the surface skin temperature.
“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,” said Juno’s principal investigator Dr. Scott Bolton, a researcher at Southwest Research Institute.
“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.”
To make the first-ever measurements of the temperature below the surface of Io, Dr. Bolton and his colleagues used data from the Microwave Radiometer (MWR) instrument on NASA’s Juno spacecraft.
These data allowed the team to see several meters below the surface, revealing a significant temperature increase — more than 20 degrees Celsius — in the upper few meters.
The observations showed that Io’s surface is smooth when viewed at long microwave wavelengths.
The microwave signal also revealed that the upper layer of Io’s surface has a low density compared to solid rock, similar to volcanic ash or pumice.
This map represents data from the Microwave Radiometer (MWR) instrument on NASA’s Juno spacecraft, indicating heat rising from just beneath the surface of Io; the colors illustrate a distinct temperature gradient across the moon, with the most extreme, localized heat output in red. Image credit: NASA / JPL-Caltech / SwRI / USGS.
The researchers also detected strong endogenic heating within the upper tens of meters.
Two possible explanations match their data: heat rising steadily through a conducting crust, or it may escape through scattered patches of thin crust above lava or hot vents covering about 10% of the surface.
“The MWR instrument directly observed Io’s heat output by looking below the surface,” Dr. Bolton said.
“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.”
The results appear in the Journal of Geophysical Research: Planets.
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Shannon Brown et al. 2026. Io Sub-Surface Temperature Profile Observed by the Juno Microwave Radiometer. Journal of Geophysical Research: Planets 131 (7): e2025JE009622; doi: 10.1029/2025JE009622
