NASA’s Juno spacecraft has achieved something never done before by measuring temperatures beneath the surface of Jupiter’s volcanic moon Io.
The new observations reveal that heat is rising from below the surface across much of the moon and show that Io’s outer layer is surprisingly smooth and made of lightweight material.
The findings, published in the Journal of Geophysical Research: Planets, offer scientists a new way to study not only volcanic worlds like Io but also volcanoes here on Earth.
Io is the most volcanically active object in the solar system. Unlike Earth, where volcanoes are powered mainly by heat from the planet’s interior, Io’s extreme activity is driven by Jupiter’s immense gravity.
As Io travels around the giant planet in a slightly oval-shaped orbit, Jupiter constantly stretches and squeezes the moon. This process, known as tidal heating, generates enormous amounts of internal heat that fuel hundreds of active volcanoes.
Until now, scientists could only study Io’s heat by observing infrared radiation coming from the surface. Juno has changed that by using its Microwave Radiometer, an instrument capable of detecting heat hidden beneath the ground.
Originally designed to study Jupiter’s deep atmosphere, the Microwave Radiometer uses six antennas that observe microwaves at different wavelengths. Because each wavelength penetrates to a different depth, the instrument can reveal temperatures ranging from just below the surface to several meters underground.
During close flybys on December 30, 2023, and February 3, 2024, Juno passed within about 1,500 kilometers of Io, giving scientists their first detailed look beneath the moon’s rocky surface.
The measurements showed that temperatures increase by more than 40 degrees Fahrenheit (about 22 degrees Celsius) within just a few feet below the surface. This temperature rise is much greater than sunlight alone could produce, indicating that heat from Io’s interior is constantly moving upward.
Researchers believe there are two possible explanations. One is that heat steadily travels upward through the moon’s crust. Although the heat flowing through any single area is relatively modest, when added together across the entire moon it represents an enormous amount of energy—up to 30 times greater than Earth’s average heat flow.
Another possibility is that about 10% of Io’s surface is covered by cooling lava flows hidden beneath a layer of solid rock roughly 9 to 11 meters thick. The buried lava would continue releasing heat long after the surface had hardened.
The discovery is important because tidal heating also powers hidden oceans beneath icy moons such as Europa and Ganymede. Understanding how heat moves through Io may help scientists better understand other worlds where life could potentially exist beneath frozen surfaces.
The flybys also revealed an unexpected feature of Io’s landscape. Although the moon is famous for its towering mountains and active volcanoes, much of its surface is surprisingly flat. Large smooth regions extend for more than 100 kilometers, resembling broad plains rather than rugged volcanic terrain.
Microwave measurements also suggest that the surface material is much less dense than solid rock. Instead, it appears to resemble lightweight volcanic ash or pumice, which contains many tiny air pockets.
The researchers say the success of Juno’s microwave observations opens new possibilities for studying rocky and icy worlds across the solar system. Similar instruments could even help scientists investigate how heat moves beneath active volcanoes on Earth, providing valuable insights into volcanic processes before eruptions occur.
The study was published in the Journal of Geophysical Research: Planets.
