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NASA's Juno mission has provided the first measurements of the temperature below the surface of Jupiter's moon Io, revealing significant heating within the shallow subsurface of the most volcanically active world in the solar system. Collected during two close flybys, the data also show that most of Io's surface is remarkably smooth and composed of material with very low density.
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Published in the Journal of Geophysical Research: Planets, these findings break new observational ground for both fiery and icy worlds beyond our planet.
Io's extreme volcanism is powered by tidal heating. The moon is constantly stretched and squeezed by Jupiter's immense gravity as it travels in a slightly elliptical orbit, generating internal heat output many times greater than Earth's. Until now, virtually everything known about that heat came from infrared observations, which sense only the temperature of the surface. The latest findings are based on data collected by the spacecraft's Microwave Radiometer (MWR) instrument.
"The Juno Microwave Radiometer directly observed Io's heat output by looking below the surface," said Scott Bolton, study co-author and Juno's principal investigator at Southwest Research Institute in San Antonio. "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."
How Juno looked underground
Juno's Microwave Radiometer was designed by Bolton to peer beneath Jupiter's cloud tops to investigate the dynamics and composition of the gas giant's deep atmosphere. The MWR's six microwave antennas serve as a single instrument, simultaneously detecting microwaves at a wide range of wavelengths, from about half an inch to 20 inches (1.3 to 51 centimeters). During the mission's extended phase, the MWR instrument has provided the opportunity to observe three of the planet's Galilean moons: Ganymede, Europa and Io.
"The technique is novel in that each wavelength explores different depths, providing a new way to characterize the deep atmosphere of giant planets and the subsurface crusts of icy and rocky moons," said Bolton. "At Ganymede and Europa, we explored tens of miles below the surface, assuming their ice shells were mostly pure water, but the ability to probe volcanic rock on Io was an unexpected discovery."
During flybys on Dec. 30, 2023, and Feb. 3, 2024, the solar-powered Juno spacecraft came within about 930 miles (1,500 kilometers) of the moon's surface.
Two explanations for the heat
"The instrument measured Io's thermal emission at depths ranging from a few inches 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," said Shannon Brown, the paper's lead author at NASA's Jet Propulsion Laboratory in Southern California.
The data suggest two possible explanations. First, heat could be rising steadily through a conductive crust. While this background heat flow—measured at 1 to 3 watts per square meter—is relatively gentle on a local scale (roughly equivalent to a small nightlight glowing under every square yard), across the entire moon it represents a release of energy up to 30 times Earth's average. Alternatively, the signal could be coming from cooling lava flows, capped by roughly 30 to 35 feet (9 to 11 meters) of solidified crust, that cover about 10% of the moon's surface at any given time.
"Io provides a unique window into 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 Bolton. "This process can not only create the most volcanic body in the solar system, in the case of Io, but also fuel the subsurface oceans on the moons of giant planets, such as Europa and Ganymede. 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."
A surprisingly plain-like surface
Another major insight gained from the two flybys is just how smooth Io is. Before the recent findings, the moon was known for its tall mountains, but the MWR indicates that apart from this visible topography, the surface features expansive smooth patches that stretch for 60 miles (100 kilometers) or more. Because Juno flew by overlapping regions of Io at different angles, the team was able to map how the surface reflects microwaves, much like an airline passenger might see the ocean flash with sunlight only at specific angles.
"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 fluffy volcanic ash than solid rock," said Brown.
More information: Shannon Brown et al, Io Sub‐Surface Temperature Profile Observed by the Juno Microwave Radiometer, Journal of Geophysical Research: Planets (2026). DOI: 10.1029/2025je009622
Provided by NASA
This story was originally published on Phys.org.
