Juno Just Took Io’s Temperature Beneath the Surface

Io advertises its heat. Spacecraft have photographed lava flows, glowing volcanic hot spots, and plumes rising hundreds of kilometers above the surface. The latest Juno result comes from somewhere harder to see: a few meters down.

During two close flybys, Juno’s Microwave Radiometer measured thermal emission from beneath Io’s surface. The lowest-frequency channels can sense roughly 2 to 6 meters into the crust. A paper published July 22 in Journal of Geophysical Research: Planets reports the first spatially resolved microwave observations of Io and the first temperature measurements from below its surface.

The instrument was designed for Jupiter, not its moons. Juno carries six microwave antennas so researchers can look through Jupiter’s clouds at different depths in the atmosphere. On Io, the same long wavelengths pass through the upper layer of rock and volcanic material before escaping to space. The hardware did not need a new principle. It found a second use for the one it already had.

A few meters make a large difference

Infrared instruments are excellent at finding active eruptions and measuring surface heat. Microwave emission adds a shallow cross-section underneath. In the regions Juno sampled, the researchers found a temperature increase of more than 20 degrees Celsius within the upper few meters.

One broad anomaly near the Zal Montes Patera complex was about 10 to 20 degrees Celsius warmer than the surrounding subsurface. Another Juno instrument had observed an active lava flow in the same region. The microwave data therefore give researchers something more than another hot spot on a map: they provide information about heat that has not yet reached the visible surface.

The coverage is limited. These measurements came from close flybys in December 2023 and February 2024, both over the hemisphere of Io that faces Jupiter, and the radiometer’s footprints are large. There is no global subsurface thermal map yet.

The data leave two very different pictures open

The team tested two simple ways to account for the microwave spectrum. A steady conductive model gives heat flows of roughly 1 to 3 watts per square meter through the shallow crust. A second model allows much of the signal to come from relatively young lava flows or hot vents buried beneath about 10 meters of cooling material, with those hot areas covering perhaps 10 percent of the surface.

Either can reproduce the observations well enough that the present data do not choose between them. That matters because the two pictures describe different ways of moving Io’s enormous internal heat outward. The measurement has narrowed the problem without finishing it.

The same observations say something about the material above the heat. At microwave wavelengths, much of Io looks unexpectedly smooth at scales of around 100 kilometers. The inferred dielectric properties of the uppermost surface are also consistent with a density of only about 0.7 to 1.1 grams per cubic centimeter in the top 10 centimeters. That is far below solid rock and consistent with very porous material such as volcanic ash or pumice.

Visible images emphasize Io’s mountains, lava, and sulfurous color. The radiometer is sensitive to a different landscape: a light, porous upper layer with a steep thermal gradient underneath.

An instrument can outgrow its original target

Juno’s radiometer exists because Jupiter’s visible cloud tops hide most of the atmosphere beneath them. Different microwave wavelengths emerge from different depths, so the spacecraft can recover information that ordinary imaging cannot.

Io presents a different obstruction and rewards the same trick. The upper crust hides the thermal structure below it, and long-wavelength microwave emission carries some of that information out.

The researchers suggest that comparable microwave measurements could someday be useful around volcanoes on Earth, although that is still a proposed application rather than a demonstrated monitoring system. For now, Juno has already produced the stranger result: a sensor sent to look through Jupiter’s clouds has become a thermometer for the first few meters of a volcanic moon.

Sources

• Shannon Brown et al., Io Sub-Surface Temperature Profile Observed by the Juno Microwave Radiometer, Journal of Geophysical Research: Planets (July 22, 2026)
• NASA Jet Propulsion Laboratory, “NASA’s Juno Takes Temperature of Jupiter’s Fiery Moon Io”
• NASA Jet Propulsion Laboratory, “NASA’s Juno Peers Beneath Io’s Surface”
• NASA Science, “Mapping Io’s Hidden Heat With NASA’s Juno”