Juno Takes the First Temperature Readings Beneath the Surface of Io
Juno's microwave radiometer found Io warms by more than 22 °C just a few feet down, pointing to strong internal heat or buried cooling lava.
NASA’s Juno spacecraft has made the first measurements of temperature below the surface of Io, Jupiter’s moon and the most volcanically active world in the Solar System. The study, led by Shannon Brown of NASA’s Jet Propulsion Laboratory, is published in JGR: Planets, and NASA reported it on July 22, 2026.
Juno’s Microwave Radiometer can sense heat from a short depth below the surface, not just the surface itself. The data come from two close flybys, on December 30, 2023 and February 3, 2024, when Juno passed within about 1,500 km of Io. Everywhere the instrument looked, the temperature rose by more than 40 °F (about 22 °C) just a few feet below the surface. That is far steeper than sunlight alone can explain.
The team offers two readings. One is a steady background flow of heat from inside Io, 1 to 3 watts per square meter, which across the whole moon would be up to 30 times Earth’s average. The other is cooling lava flows buried under about 9 to 11 meters of solid crust and covering about 10% of the surface. Both are possible with the current data.
Io is heated by tides: Jupiter’s gravity, and the regular tugs of neighboring moons, flex its interior and generate heat. That makes it the clearest natural laboratory for tidal heating, the same process that keeps oceans liquid inside Europa and Ganymede, two of the most promising places to search for life (see our astrobiology guide). More on Jupiter’s moons in our Solar System guide.
What to watch next: further analysis of Juno’s data should help decide between internal heat flow and buried lava as the main explanation.
Primary source
This summary is based on NASA and the peer-reviewed paper. Read the original for full details, data and caveats.
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