NASA's Juno used its Microwave Radiometer during late 2023 and early 2024 flybys to measure temperatures 6–20 feet (2–6 meters) below Io's surface. Subsurface temperatures rise by more than 40°F (22°C) in places, with local hotspots 18–36°F (10–20°C) warmer than surrounding terrain. The data suggest porous volcanic deposits blanket Io and reveal how tidal heating transports interior heat—insights that could help monitor Earth volcanoes and probe icy moons for habitability.
Juno’s Microwave Probe Reveals Heat Hiding Beneath Io’s Volcanoes
A pinkish-brown moon is seen (it's upper hemisphere). There are dark splotches on it.
Scientists using NASA's Juno spacecraft have, for the first time, measured temperatures several feet beneath the surface of Jupiter's volcanic moon Io — revealing hidden heat that helps power the most volcanically active world in the solar system.
How Juno Peered Below Io's Surface
During close flybys on Dec. 30, 2023 and Feb. 3, 2024, Juno pointed its Microwave Radiometer (MWR) at Io. Designed originally to probe Jupiter's dense atmosphere, the MWR proved capable of sensing thermal structure about six to 20 feet (two to six meters) beneath Io's crust. That subsurface reach allowed researchers to map temperature gradients below the surface for the first time.
Warmth That Sunlight Can't Explain
The MWR data show temperatures increasing by more than 40°F (22°C) just a few feet below the surface — a rise far larger than can be accounted for by sunlight alone. The measurements also uncovered localized warm patches roughly 18–36°F (10–20°C) hotter than surrounding terrain, indicating heterogenous subsurface heat sources or recent shallow lava deposits.
What Lies Beneath?
Researchers propose two main explanations for the subsurface warmth: steady conduction of heat from a molten interior through a solid crust, or shallow pockets of cooling lava trapped near the surface. Either scenario gives the clearest picture yet of how Io transports interior heat to its exterior.
The north polar region of Jupiter's volcanic moon Io was captured by NASA's Juno during the spacecraft's 57th close pass of the gas giant on Dec. 30, 2023. Data from that flyby and one on Feb. 3, 2024, is helping scientists understand Io's interior. | Credit: NASA/JPL-Caltech/SwRI/MSSS. Image processing by Gerald Eichstädt
"The surprising discovery that we could see below a rocky moon's surface has important implications for studying Earth's volcanoes," said Scott Bolton, study coauthor and Juno's principal investigator. "If we look with an MWR-type instrument near an Earth volcano, we might see a similar subsurface temperature gradient, providing new information on how terrestrial volcanoes work."
Surface Texture and Volcanic Blankets
Despite dramatic mountains and frequent eruptions, much of Io's surface appears unusually smooth and of low bulk density. Scientists infer that porous layers of volcanic ash, sulfur frost and other eruptive debris repeatedly blanket the moon, continuously resurfacing it and burying older landforms beneath fresh deposits.
Tidal Heating and Broader Implications
Io's internal heat is driven not by radioactive decay, as on Earth, but by tidal flexing: Jupiter's immense gravity stretches and squeezes Io as it orbits, generating powerful internal heating. Understanding how that heat moves through Io's crust provides a natural laboratory for tidal heating processes that operate elsewhere in the solar system and beyond.
Beyond Io, the microwave technique may improve volcano monitoring on Earth by revealing subsurface temperature gradients that precede eruptions. The same approach is already being applied by Juno to Jupiter's icy moons, Europa and Ganymede, where subsurface oceans may exist beneath thick ice shells; mapping heat flow there is crucial to assessing those moons' habitability potential.
Publication: The study describing these findings was published July 22 in the Journal of Geophysical Research: Planets.
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