NASA's Juno spacecraft has measured temperature below the surface of Jupiter's moon Io for the first time. The readings show strong heating within the upper few metres, but they do not yet identify a single route by which that heat reaches the surface.

The measurements came from Juno's Microwave Radiometer during close flybys on December 30, 2023, and February 3, 2024. The spacecraft passed about 1,500 kilometres above Io on both occasions.

The study in Journal of Geophysical Research: Planets was published on July 22, 2026. Its central result is direct: temperatures rose by more than 22°C within the upper few metres of the ground, a gradient too steep for sunlight alone to explain.

Microwaves looked deeper than infrared

Io is the most volcanically active world in the solar system. Jupiter's gravity continually stretches and squeezes the moon along its slightly elliptical orbit, producing internal heat through tidal friction.

Researchers previously relied mainly on infrared observations to study that heat. Infrared instruments sense the outermost surface, whereas different microwave wavelengths can emerge from different depths.

Juno's radiometer uses six antennas spanning wavelengths from 1.3 to 51 centimetres. After accounting for microwave reflections from the sky, the team retrieved thermal emission from depths ranging from a few centimetres to tens of metres.

The data provide a temperature profile, not a direct image of underground lava or magma. That distinction matters because more than one physical model can fit the observed gradient.

Two explanations remain open

One model has heat moving steadily through a conductive near-surface layer. It implies a background heat flow of about 1 to 3 watts per square metre. On a local patch that is modest, but across Io it would be as much as 30 times Earth's global average heat flow.

Another model attributes the signal to relatively fresh lava flows or hot vents beneath roughly 9 to 11 metres of cooling crust. In that scenario, such areas would cover about 10% of Io's surface at a given time.

Both are modelled explanations. The flyby data do not establish which one dominates across Io, and the paper does not claim that Juno detected a global ocean of magma.

A summary from AGU's Eos notes that the fresh-lava or hot-vent explanation may fit Io's visible tall mountains better, while still treating the two models as alternatives rather than a settled conclusion.

Io's surface also looked unexpectedly smooth

The same observations suggest that, apart from visible mountains, Io reflects microwaves like a relatively smooth surface across scales of roughly 100 kilometres.

The inferred dielectric properties are consistent with a very low-density upper layer—about 0.7 to 1.1 grams per cubic centimetre in the top 10 centimetres. NASA researchers compare the material to pumice or fluffy volcanic ash rather than solid rock.

That result is an inference from the microwave response, not a sample returned from Io. It offers a new constraint on the moon's surface but does not by itself specify the exact mixture of materials.

The NASA mission account describes the technique as potentially useful beyond Io because similar radiometers can probe beneath rocky or icy surfaces. For now, the defensible advance is narrower: Juno has provided the first spatially resolved microwave temperature measurements beneath Io's surface and shown that its shallow subsurface is strongly heated. Explaining exactly how that heat escapes remains an open question.