NASA’s Juno mission has provided the first measurements of temperatures beneath the surface of Jupiter’s moon Io, revealing substantial heating within a shallow subsurface layer of the most volcanically active world in the Solar System. Data collected during two close flybys also show that most of Io’s surface is surprisingly smooth and composed of very low-density material. These discoveries open new avenues for observing both fiery and icy worlds beyond our planet.
Looking Beneath Io’s Surface
Io’s extreme volcanic activity is powered by tidal heating. The moon is constantly stretched and compressed by Jupiter’s enormous gravity as it moves along a slightly elliptical orbit, generating internal heat many times greater than that of Earth. Until now, almost everything known about this heat was based on infrared observations, which record only surface temperatures. According to phys.org, the latest discoveries are based on data collected by the spacecraft’s Microwave Radiometer instrument, or MWR.
“Juno’s Microwave Radiometer directly observed Io’s thermal emission by looking beneath the surface,” said Scott Bolton, a co-author of the study and principal investigator of the Juno mission at the Southwest Research Institute in San Antonio. “The unexpected discovery that we can see beneath the surface of a rocky moon has important implications for the study of volcanoes on Earth. Juno has shown us that, if we use an MWR-type instrument near a volcano on Earth, we may be able to detect a similar subsurface temperature gradient and obtain new information about how terrestrial volcanoes operate.”
A Microwave Method for Studying Moons
Juno’s Microwave Radiometer was developed by Bolton to look beneath Jupiter’s clouds and study the dynamics and composition of the gas giant’s deep atmosphere. The six microwave antennas of MWR operate as a single instrument, simultaneously detecting microwaves across a broad range of wavelengths, from approximately half an inch to 20 inches, or 1.3 to 51 centimeters. During the extended phase of the mission, MWR made it possible to observe three of the planet’s Galilean moons: Ganymede, Europa, and Io.
“The technique is new because each wavelength probes a different depth, providing a new way to study the deep atmospheres of giant planets and the subsurface layers of icy and rocky moons,” Bolton said. “At Ganymede and Europa, we examined tens of miles beneath the surface based on the assumption that their icy shells consist mainly of pure water, but the ability to study volcanic rock on Io was an unexpected discovery.”
During the flybys of December 30, 2023, and February 3, 2024, the solar-powered Juno spacecraft passed approximately 1,500 kilometers above the moon’s surface.
Two Explanations for the Heat
“The instrument measured Io’s thermal radiation at depths ranging from a few inches to tens of feet. Wherever we looked, we found that the temperature increased by more than 40 degrees Fahrenheit within only a few feet below the surface—a gradient far steeper than can be explained by solar heating alone,” said Shannon Brown, the lead author of the paper from NASA’s Jet Propulsion Laboratory in Southern California.
The data suggest two possible explanations. First, heat may gradually rise through a conductive crust. Although this background heat flow—measured at 1 to 3 watts per square meter—is relatively weak on a local scale, approximately equivalent to a small night-light glowing beneath every square yard, across the entire moon it represents an energy output up to 30 times greater than Earth’s average.
Alternatively, the signal may come from cooling lava flows covered by approximately 30–35 feet, or 9–11 meters, of solidified crust, which at any given time may cover about 10% of the moon’s surface.
Io provides a unique view of how tidal heating operates throughout the cosmos. It is a fundamental process that supplies energy and heat to worlds located far from their parent stars. This process may not only create the most volcanically active object in the Solar System, as in the case of Io, but may also sustain subsurface oceans on the moons of giant planets, such as Europa and Ganymede.
Until now, astronomers could observe only the heat reaching the surface or escaping through eruptions. They can now characterize how heat moves from the interior toward the surface.
A Smooth Surface Made of Low-Density Material
Another important discovery from the two flybys was just how smooth Io is. Before the latest findings, the moon was known for its towering mountains, but MWR data show that, beyond the visible topography, its surface contains extensive smooth areas stretching for 60 miles, or 100 kilometers, or more.
Because Juno passed over overlapping regions of Io at different angles, the team was able to map how the surface reflects microwaves, much as an airline passenger may see sunlight glinting off the ocean only at certain angles.
“Away from its mountains, the surface looks more like the Great Plains of North America, and although Io is a rocky body, the surface material has a very low density—more like pumice or loose volcanic ash than solid rock,” Brown said.
