NASA’s Juno spacecraft has produced the first measurements of temperature beneath the surface of Io, Jupiter’s innermost Galilean moon and the most volcanically active body in the solar system. The data, collected during two close flybys and published July 22, 2026 in the Journal of Geophysical Research: Planets, reveal a temperature increase of more than 20 degrees Celsius in the upper few meters of the moon’s crust and show that the surface is unexpectedly smooth and porous across vast areas.
The findings come from Juno’s Microwave Radiometer (MWR), an instrument with six antennas that detect microwaves at wavelengths from 1.3 to 51 centimeters. Originally designed to peer through Jupiter’s clouds, the MWR has been used during Juno’s extended mission to observe three Galilean moons: Ganymede, Europa, and Io. At Ganymede and Europa, the instrument probed miles below icy surfaces, but seeing into volcanic rock at Io was an unexpected bonus.
During flybys on December 30, 2023, and February 3, 2024, Juno passed about 1,500 kilometers above Io’s surface. The MWR measured upwelling thermal emission at depths ranging from a few centimeters to several meters. Because each wavelength penetrates to a different depth, the six channels together produce a vertical temperature profile of the shallow subsurface.
A Temperature Gradient That Points to Internal Heating
The MWR data revealed a large spectral slope in the lowest-frequency channels at all sampled latitudes, indicating significant endogenic heating in the upper few meters. Temperatures rose by more than 20 degrees Celsius relative to the surface skin temperature, a gradient far too steep to be explained by sunlight warming an airless world.
The research team, led by Shannon Brown at JPL, applied two models to explain the observed microwave spectra. Under the first model, heat rises steadily through a conductive near-surface layer, producing background heat flows of 1 to 3 watts per square meter. Modest at any single location, this flow integrated across Io’s entire surface represents a heat release up to 30 times Earth’s global average.
The second model proposes that the signal comes from relatively fresh lava flows or heat vents buried beneath a cooling crust. In this scenario, the hot material lies about 10 meters below the surface and covers about 10 percent of Io’s area at any given time. The lava would be less than five years old, suggesting a continuous cycle of eruption, burial, and cooling across the moon’s surface.
The MWR data cannot distinguish between these two scenarios; both may operate simultaneously across Io.
A Surface Like Pumice and Ash
Beyond the temperature profile, the MWR data revealed unexpected properties of Io’s surface. The moon reflects microwaves in a specular fashion, indicating a surface that is remarkably smooth on scales of 100 kilometers, despite its visible topography of tall mountains and volcanic calderas. Away from its mountains, the surface resembles the Great Plains more than a rugged volcanic world.
The microwave data also constrained the dielectric constant of the surface material, yielding values between 2 and 4, corresponding to a density of 0.7 to 1.1 grams per cubic centimeter for the upper 10 centimeters. For comparison, solid volcanic rock has a density of roughly 2.5 to 3 grams per cubic centimeter. The low density indicates that Io’s surface consists of highly porous, fragmented material, more akin to pumice or fluffy volcanic ash than solid rock. The instrument essentially saw a surface that is more loose, broken rubble than hard bedrock across vast areas of the moon.
This porous surface layer likely results from constant volcanic ejecta that blankets Io. With hundreds of active volcanoes, the moon is continuously resurfaced by eruptions depositing loose, airfall material.
Tidal Heating and the Broader Picture
Io’s extreme volcanism is powered by tidal heating, where Jupiter’s gravity and the gravitational pulls from Europa and Ganymede constantly flex the moon, generating internal friction far beyond what radioactive decay alone could produce.
The Juno results offer a new window into how tidal heat moves through a rocky crust. Scientists previously could only observe heat escaping at Io’s surface through infrared measurements or direct observations of eruptions. The MWR data show where heat is stored and transported in the shallow subsurface, allowing researchers to characterize how tidal energy flows from the interior to the surface.
The technique has implications beyond Io. The same tidal forces that drive Io’s volcanism maintain subsurface liquid water oceans on Europa and Enceladus. Understanding how tidal heating distributes energy through different crust types, rocky on Io and icy on Europa, helps build better models of how these moons work as systems. The discovery that microwave radiometry can see below a rocky moon’s surface also has implications for studying Earth’s volcanoes.
The paper is authored by Shannon Brown and colleagues at JPL, Caltech, SwRI, and other institutions.
Images
Images: None. Refer to NASA’s official Io imagery for a single composite view of the moon.
Clark – 1ban.news