Juno took the temperature of Io’s furious volcanic surface

NASA’s Juno spacecraft has used microwave observations to measure heat beneath the surface of Io, Jupiter’s intensely volcanic moon. The data show temperatures climbing sharply just below parts of the crust, giving researchers a new way to examine how lava and internal heat reshape the world.

The measurements came from two close flybys rather than a lander or surface probe. Juno’s instrument detected thermal energy at several depths, allowing scientists to compare the temperature of the visible surface with material buried feet below it.

Juno flew within 930 miles of Io

Juno passed within roughly 930 miles of Io on Dec. 30, 2023, and again on Feb. 3, 2024. The spacecraft was built to investigate Jupiter, but mission controllers aimed its instruments at the moon during those encounters. According to NASA’s account of the findings, the passes supplied the first measurements of how temperature changes beneath broad areas of Io’s surface.

The work used Juno’s Microwave Radiometer, or MWR. Its six channels respond to different microwave wavelengths, which emerge from different depths. That design lets the instrument sense material from inches to tens of feet below the surface rather than simply photographing hot lava exposed to space.

Infrared cameras are highly effective at spotting exposed hot surfaces, but microwave observations answer a different question. Longer wavelengths can emerge through cooler overlying material, carrying information about temperatures that visible and infrared images cannot directly sample. Combining these techniques helps distinguish a newly erupted surface from older lava that remains warm underneath.

The buried material was substantially warmer

Researchers found that temperatures rose by more than 40 degrees Fahrenheit several feet below the surface in some regions. The gradient matters because sunlight mainly warms the topmost layer. Heat detected farther down can reveal cooling lava, conductive heat escaping from the interior or a combination of both.

Io is not uniformly hot. Much of the visible surface is cold enough for sulfur dioxide frost, while active vents and lava can be extremely hot. A broad microwave measurement averages material across a large footprint, so the result does not mean every point under Juno’s path shared the same temperature. It instead constrains the mixture of warm and cold terrain within each observed area.

Two explanations fit the heat profile

The research team modeled two plausible ways to produce the observed microwave signal. One possibility is steady heat conducted through the crust at about one to three watts per square meter. That would imply widespread upward heat flow even where no fresh eruption is visible.

A second possibility involves still-warm lava flows concealed below a crust 30 to 35 feet thick. The team estimated that such buried flows would need to cover roughly 10% of the surface to match the observations. The peer-reviewed study in the Journal of Geophysical Research: Planets presents the measurements and thermal models, but the available data do not yet select a single explanation for every region.

Smooth plains may hide porous deposits

Juno also detected unusually low-density material in smooth patches that can stretch as far as 60 miles. Researchers proposed that these areas could contain porous deposits or fractured lava. A material’s density and structure affect how deeply microwaves can travel, so those properties are part of interpreting the temperatures.

The smooth terrain offers a contrast with Io’s familiar volcanoes, mountains and colorful sulfur deposits. If widespread porous or cooling material lies beneath apparently quiet plains, visible imagery alone may understate the amount of recent geological activity. Additional passes at different angles and wavelengths would help separate composition, density and temperature effects.

Jupiter supplies the energy behind the eruptions

Io’s extreme volcanism comes from tidal heating. Jupiter’s gravity repeatedly flexes the moon as it travels through a slightly eccentric orbit, while gravitational interactions with Europa and Ganymede help maintain that orbital shape. Friction inside Io converts the flexing into heat, powering hundreds of volcanoes and resurfacing the moon.

Subsurface temperature maps can test where that energy travels after it is generated. Competing models place more heat in a deep mantle, a shallow partially molten layer or localized volcanic systems. The Juno observations cannot by themselves map the entire interior, but they add a direct constraint near the surface where interior heat finally escapes.

The result extends a mission designed for Jupiter

The Juno mission entered orbit around Jupiter in 2016 and has used repeated close passes to study the planet’s atmosphere, magnetic field and deep structure. Its extended mission added targeted encounters with the large moons, turning existing instruments toward questions that were not the spacecraft’s original focus.

These measurements are a snapshot from two flybys, not a continuous global thermometer. Changing eruption patterns, limited coverage and assumptions about the crust all affect the interpretation. Even with those limits, sensing heat beneath Io’s surface gives planetary scientists a rare look at an active volcanic system that cannot be sampled directly.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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