Juno’s microwave radiometer found temperatures rising more than 20°C within a few feet of Io’s surface, a gradient far too steep for sunlight alone to explain.

Io is the most volcanically active world in the solar system. The innermost of Jupiter’s four large moons hosts hundreds of active volcanoes, some of which hurl plumes of sulfur and other material hundreds of kilometres above its surface. Lava lakes, fountains and fresh flows constantly repave its sulfur-stained landscape.

Until now, nearly everything scientists knew about Io’s heat came from infrared observations, which measure only the temperature of the very surface. NASA’s Juno spacecraft has now looked deeper. Using its microwave radiometer, Juno made the first direct measurements of temperature beneath Io’s surface, revealing significant heating in the top few feet of the moon.

“The Juno microwave radiometer directly observed Io’s heat output by looking below the surface,” said Scott Bolton, Juno’s principal investigator at the Southwest Research Institute. “The discovery that we could see below the surface of a rocky moon was a surprise with potentially important implications for the study of Earth’s volcanoes.”

Squeezed by Jupiter

Io’s volcanism is driven by tidal heating. As the moon follows its slightly elliptical orbit, Jupiter’s immense gravity repeatedly stretches and squeezes it. Gravitational tugs from the neighbouring moons Europa and Ganymede, locked with Io in an orbital resonance, keep that orbit from becoming circular. The constant flexing generates enormous internal heat, many times what Earth produces.

Io’s volcanoes were discovered in 1979 when NASA’s Voyager 1 spacecraft photographed an eruption plume rising from its surface, confirming a prediction made just days earlier that tidal heating could melt its interior.

Io is slightly larger than Earth’s Moon. NASA’s Galileo spacecraft studied it from orbit around Jupiter between 1995 and 2003, documenting giant lava flows and features such as Loki Patera, a vast lava lake. Juno has added new views, including two close passes in late 2023 and early 2024 that brought it within about 1,500 kilometres of the surface.

A radiometer built for Jupiter

Juno arrived at Jupiter in 2016 to study the giant planet’s interior and atmosphere. Its microwave radiometer, or MWR, which Bolton invented and designed, was built to see beneath Jupiter’s cloud tops. It uses six antennas working as a single instrument, each sensitive to a different range of microwave wavelengths.

“The novel aspect of the instrument is that each wavelength explores a different depth to characterize the deep atmospheres of giant planets or the subsurface crusts of icy and rocky bodies,” Bolton said.

Juno’s extended mission brought it close to three of Jupiter’s large moons: Ganymede, Europa and Io. “At Ganymede and Europa, we probed tens of miles into their ice shells,” Bolton said. “At Io, the MWR measured thermal emissions at depths ranging from a few inches down to tens of feet.”

Heat rising from below

The new results, published in the Journal of Geophysical Research: Planets, come from two close flybys of Io.

At every location the instrument examined, temperature rose by more than 20 degrees Celsius (40 degrees Fahrenheit) within several feet of the surface. “That is a gradient far steeper than solar heating alone can explain,” said lead author Shannon Brown, a senior research scientist at NASA’s Jet Propulsion Laboratory.

The measurements also suggest that most of Io’s surface is smooth at large scales and has a density closer to volcanic ash or pumice than to solid rock.

Two explanations

The researchers offer two possible explanations for the steep gradient. The first is steady heat flowing upward through a conductive crust at about 1 to 3 watts per square metre, up to 30 times Earth’s average heat flow.

The second, which the release describes as more likely, is that the heat comes from cooling lava flows buried beneath many feet of solid crust. If that is right, such flows would cover roughly 10 percent of Io’s surface at any given time, a sign of just how relentlessly the moon is resurfacing itself.

Distinguishing between the two matters for understanding how Io loses its internal heat, a question that bears on how tidal heating works more broadly.

Io loses heat in two main ways: through its volcanoes and lava flows, which deliver hot material directly to the surface, and through slow conduction across its crust. How much heat travels by each route shapes models of the moon’s interior, including whether it harbors a global layer of molten or partly molten rock beneath the crust.

A laboratory for tidal heating

That broader process is a major theme in planetary science. Tidal heating is thought to keep liquid water oceans from freezing beneath the icy shells of moons such as Jupiter’s Europa and Saturn’s Enceladus, which are among the most promising places to search for life beyond Earth.

“Io provides a unique window into learning how tidal heating works throughout the cosmos, a fundamental process that provides energy and heat to worlds that are far from their parent star,” Bolton said. “This process can create both the most volcanic body in the solar system, in the case of Io, and the liquid subsurface oceans within the icy moons of giant planets, such as Europa and Enceladus.”

The release does not give the paper’s title, its full author list or detailed methods, and the dateline on the release lists a different year than the EurekAlert! posting date. The results rest on two flybys, so coverage of Io’s surface is necessarily limited. Still, by looking below the surface for the first time, Juno has added a new dimension to the study of the solar system’s most restless world.

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Notes

1. Brown, S., et al. Journal of Geophysical Research: Planets, published 22 July. Title and DOI not provided in the release.
2. Southwest Research Institute. “NASA’s Juno takes the subsurface temperature of Jupiter’s fiery moon.” EurekAlert!, 29 July 2026. https://www.eurekalert.org/news-releases/1137957.