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NASA’s Juno mission has provided the first measurements of the temperature beneath the surface of Jupiter’s moon Io, revealing significant warming within the shallow subsurface of the solar system’s most volcanically active world. Data collected during two close flybys also show that most of Io’s surface is remarkably smooth and composed of very low-density material.

Published Wednesday in the Journal of Geophysical Research: Planets, these findings open new avenues of observation for fiery, icy worlds beyond our planet.

Io’s extreme volcanism is due to tidal heating. Jupiter’s immense gravity constantly stretches and compresses the moon as it travels its slightly elliptical orbit, generating internal heat production many times greater than that of Earth. Until now, virtually everything that was known about that heat came from infrared observations, which only detect the temperature of the upper surface. The latest findings are derived from data collected by the spacecraft’s Microwave Radiometer (MWR) instrument.

“Juno’s microwave radiometer directly observed Io’s heat production by looking beneath the surface,” said Scott Bolton, study co-author and Juno principal investigator at the Southwest Research Institute in San Antonio. “The surprising discovery we were able to see beneath the surface of a rocky moon has important implications for the study of Earth’s volcanoes. Juno has taught us that if we look with an MWR-type instrument near a volcano on Earth, we could see a similar signature in the subsurface temperature gradient, providing new information about how Earth’s volcanoes work.”

Juno’s microwave radiometer was designed by Bolton to look beneath Jupiter’s cloud tops and 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 over a wide range of wavelengths, from about half an inch to 20 inches (1.3 to 51 centimeters). During the extended phase of the mission, the MWR instrument provided the opportunity to observe three of the planet’s Galilean moons: Ganymede, Europa and Io.

“The technique is novel because each wavelength scans different depths, providing a new way to characterize the deep atmosphere of giant planets and the subterranean crusts of rocky, icy moons,” Bolton said. “On Ganymede and Europa, we explored tens of kilometers beneath the surface, assuming their ice sheets were mostly pure water, but the ability to probe volcanic rock on Io was an unexpected discovery.”

During flybys on December 30, 2023, and February 3, 2024, the solar-powered Juno spacecraft came within about 930 miles (1,500 kilometers) of the moon’s surface.

“The instrument measured Io’s thermal emission at depths ranging from a few centimeters to tens of feet. Everywhere we looked, we found that the temperature increased by more than 40 degrees Fahrenheit just several feet from the surface, a much steeper gradient than solar heating alone can explain,” said Shannon Brown, lead author of the paper 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 flux, measured at 1 to 3 watts per square meter, is relatively mild on a local scale (roughly equivalent to a small night light shining under each square meter), across the Moon it represents an energy release up to 30 times the Earth average. Alternatively, the signal could come from cooling lava flows, covered by about 30 to 35 feet (9 to 11 meters) of solidified crust, covering about 10% of the moon’s surface at any given 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 far from their parent star,” Bolton said. “This process can not only create the most volcanic body in the solar system, in the case of Io, but also feeds the subterranean oceans on the moons of giant planets, such as Europa and Ganymede. Until now we could only observe heat escaping at the surface or through eruptions. Now we can characterize how heat moves from the interior to the surface.”

Another great insight gained from the two flybys is how fluid Io is. Before the recent findings, the moon was known for its high mountains, but the MWR indicates that apart from this visible topography, the surface features extensive smooth patches extending for 60 miles (100 kilometers) or more. Because Juno flew over overlapping regions of Io at different angles, the team was able to map how the surface reflects microwaves, much like an airplane passenger might see the ocean glow with sunlight only at specific angles.

“Far from its mountains, the surface looks more like the Great Plains of North America, and although Io is a rock body, the surface material is very low in density, more like pumice or fluffy volcanic ash than solid rock,” Brown said.

JPL, a division of Caltech in Pasadena, California, manages the Juno mission for principal investigator Scott Bolton of the Southwest Research Institute. Juno is part of NASA’s New Frontiers Program, which is managed at the agency’s Marshall Space Flight Center in Huntsville, Alabama, for NASA’s Science Mission Directorate in Washington. Lockheed Martin Space in Denver built and operates the spacecraft. More information about Juno is at:

https://science.nasa.gov/mission/juno

News Media Contacts

DC Eagle
Jet propulsion laboratory
818-393-9011
agle@jpl.nasa.gov

Karen Fox/Molly Wasser
NASA Headquarters, Washington
202-358-1600
karen.c.fox@nasa.gov / molly.l.wasser@nasa.gov

Deb Schmid
Southwest Research Institute, San Antonio
210-522-2254
dschmid@swri.org

2026-050