Juno Measures First Below‑Surface Temperatures on Io, Finds Intense Shallow Heating

NASA says its Juno spacecraft has made the first measurements of temperatures below the surface of Jupiter’s moon Io, revealing shallow underground heating far stronger than sunlight alone can explain.

The agency announced the results Tuesday, and said the findings were published in Journal of Geophysical Research: Planets. The work is based on data from Juno’s Microwave Radiometer, or MWR, collected during two close flybys of Io on Dec. 30, 2023, and Feb. 3, 2024, when the spacecraft passed within about 930 miles, or 1,500 kilometers, of the moon’s surface.

The result matters because Io is the most volcanically active world in the solar system, and scientists have long tried to understand how it sheds the immense internal heat that powers its eruptions. By probing below the surface rather than just measuring hotspots at the top, Juno offers a new way to track how that heat moves through the moon’s crust.

NASA said the microwave instrument measured thermal emission from depths ranging from a few inches to tens of feet below the surface. According to the agency’s summary of the paper, the team found temperatures rising by more than 40 degrees Fahrenheit within just several feet below the surface.

That increase is too steep to be explained by solar heating alone, NASA said.

One possible explanation is a background conductive heat flow of about 1 to 3 watts per square meter. If that level were representative across Io, NASA said, it would amount to as much as roughly 30 times Earth’s average geothermal heat flow.

NASA also outlined a second possible explanation: widespread cooling lava flows buried beneath about 30 to 35 feet, or 9 to 11 meters, of solid crust. In that interpretation, cooling lava could cover about 10% of Io’s surface at any given time.

The new measurements do not settle which of those explanations is correct. But they give researchers a way to test how heat is being released on Io using data from below the surface, not only from the surface itself.

Io’s extreme volcanism is driven by tidal heating, a process in which Jupiter’s gravity — along with orbital interactions with the neighboring moons Europa and Ganymede — continually flexes Io’s interior. That kneading generates heat that must escape through the crust. Previous estimates of Io’s heat output came mostly from infrared observations, which mainly show surface temperatures and the hottest active regions.

“The instrument measured Io’s thermal emission at depths ranging from a few inches down 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.

Scott Bolton, a study co-author and Juno’s principal investigator at Southwest Research Institute in San Antonio, said the instrument offers a different view of the moon’s energy budget. “The Juno Microwave Radiometer directly observed Io’s heat output by looking below the surface,” he said.

Juno launched in 2011 and entered orbit around Jupiter in 2016. Although it was built primarily to study the giant planet, its extended mission has included close flybys of major moons, including Io. The new result suggests those passes are doing more than adding to pictures of a violent world — they are opening a new window into how that violence is powered underneath.