A World of Fire and Fury
Jupiter's moon Io is a place of beautiful violence. Covered in hundreds of active volcanoes, it constantly paves over its own surface with fountains of lava and sulfurous snow. This geological rage isn't powered by the same internal heat as Earth's volcanoes.
Instead, Io is caught in a gravitational tug-of-war between the immense pull of Jupiter and the rhythmic nudges of its neighbouring moons, Europa and Ganymede. This constant stretching and squeezing, known as tidal heating, generates tremendous friction and heat inside the moon, causing its very surface to bulge up and down by as much as 100 metres. For decades, scientists could only study the heat on Io's surface using infrared telescopes, getting a snapshot of the hot lava and cold plains. But what happens just below the crust has remained a mystery.
Peeking Beneath the Crust
Enter NASA's Juno spacecraft. Originally designed to study Jupiter's deep atmosphere, Juno's mission was extended to include close flybys of the planet's fascinating moons. During two daring passes in December 2023 and February 2024, the spacecraft flew within about 1,500 kilometres of Io's surface. It turned its Microwave Radiometer (MWR) instrument, an instrument built to peer through Jupiter's clouds, toward the rocky moon. Scientists weren't even sure it would work on a rocky body, but the MWR successfully penetrated several feet beneath Io's surface, giving humanity its first-ever direct measurement of temperature below the crust.
Decoding the Hidden Heat
The results, recently published in the Journal of Geophysical Research: Planets, were surprising. The data revealed a steep temperature gradient just beneath the surface. In the areas Juno studied, the temperature rose by more than 22 degrees Celsius (40 degrees Fahrenheit) within just a few feet of depth. This is a much sharper increase than sunlight alone could ever cause, indicating a huge amount of heat is escaping from Io's interior. This isn't just happening at a few volcanic hotspots; the findings suggest this heat is leaking out across vast regions of the moon. Scientists have two main ideas for what's happening: either heat is steadily moving through a conductive crust, or the signal is coming from massive, still-cooling lava flows buried just under a thin layer of solidified rock.
A Surprisingly Smooth and Fluffy World
The microwave data delivered another shock. Despite being a world defined by towering mountains and violent eruptions, large parts of Io's surface appear remarkably smooth to the MWR's sensors. Some of these smooth plains may stretch for 100 kilometres or more. The data also suggests the surface material has a very low density, more like lightweight pumice or fluffy volcanic ash than solid rock. It seems that Io's constant volcanic activity has blanketed the world in a porous, lightweight layer of debris, which smooths over the underlying terrain like a fresh snowfall.
Why This Discovery Matters
This new ability to see beneath a rocky moon's surface is a game-changer. It provides the first real data on how heat moves through a tidally heated world, a process fundamental to understanding moons across the cosmos. According to Scott Bolton, Juno's principal investigator, these findings have important implications for studying volcanoes right here on Earth. He suggests that a similar microwave instrument could be used to detect subsurface temperature changes near terrestrial volcanoes, potentially offering new clues about how they work and providing a new tool for monitoring their activity. By understanding the engine that drives the solar system's most volcanic body, we gain a powerful new perspective on the geological forces that shape worlds, including our own.
