Io looks like it should be one of the toughest, densest places in the solar system, a rocky moon torn apart by constant volcanic eruptions, its surface scarred by lava flows and sulfur deposits. NASA’s Juno spacecraft just measured what’s actually sitting beneath that violent surface, and the results paint a genuinely strange picture, most of Io isn’t dense rock at all. It’s closer to volcanic snow.
The First Look Below the Surface
Io’s extreme volcanism comes from tidal heating, the moon gets constantly squeezed and stretched by Jupiter’s enormous gravity as it travels its slightly elliptical orbit, generating far more internal heat than Earth produces on its own. Until now, everything scientists knew about that heat came from infrared observations, which only capture temperature at the very top layer. During two close flybys in December 2023 and February 2024, Juno came within about 930 miles of Io’s surface and turned its Microwave Radiometer instrument toward the moon, an instrument originally built to peer through Jupiter’s clouds, repurposed for something its designers never quite planned on.
“The ability to probe into the volcanic rock at Io was an unexpected discovery,” said Scott Bolton, Juno’s principal investigator at the Southwest Research Institute.
A Temperature Spike Nobody Expected
The instrument found something consistent everywhere it looked, temperatures rose more than 40 degrees Fahrenheit within just a few feet of the surface, a gradient far too steep to be explained by sunlight alone. “Everywhere we looked, we found the temperature rising by more than 40 degrees Fahrenheit just several feet into the surface,” said Shannon Brown, the study’s lead author at NASA’s Jet Propulsion Laboratory. Researchers propose two possible explanations, either heat is steadily conducting upward through Io’s crust at a rate that adds up to roughly 30 times Earth’s average heat flow across the whole moon, or the signal is coming from cooling lava flows buried under 30 to 35 feet of solidified crust, flows that cover roughly 10 percent of Io’s surface at any given moment.
Rock That Floats Like Pumice
The genuinely unexpected part came from measuring density rather than temperature. Away from Io’s tall mountains, vast stretches of the surface, some spanning 60 miles or more, turned out to be remarkably smooth and startlingly lightweight for solid rock. “Away from its mountains, the surface is more like the Great Plains of North America, and even though Io is a rocky body, the surface material has a very low density, more like pumice or a fluffy volcanic ash than solid rock,” Brown explained. Centuries of accumulated volcanic ash, falling and settling layer upon layer, appear to have built up a surface that behaves less like stone and more like packed snow.
If you’re curious about another world in the outer solar system reshaped almost entirely by internal heat rather than sunlight, our profile of what the Kuiper Belt actually is covers a very different, ice-dominated corner of the solar system driven by its own set of extreme processes.
A Technique With Uses Back Home
Beyond Io itself, Bolton sees a genuinely useful application closer to home. “The surprising discovery that we could see below a rocky moon’s surface has important implications for studying Earth’s volcanoes,” he said, suggesting the same microwave technique could eventually reveal subsurface temperature gradients near volcanoes right here on Earth. Juno previously used the same instrument to probe icy moons Ganymede and Europa, assuming their crusts were mostly pure water ice. Io, dry, rocky, and violently volcanic, presented an entirely different kind of subsurface to map, and the instrument handled it in stride.
For more on the discovery, check out the full release from NASA’s Jet Propulsion Laboratory and the published study in the Journal of Geophysical Research: Planets.