A World of Constant Eruption
Imagine a world constantly wracked by volcanic eruptions, with lava plumes shooting hundreds of kilometres into space and a surface paved over with sulphur and cooling lava. That’s Io. It’s caught in a gravitational tug-of-war between the immense pull
of Jupiter and two of its other large moons, Europa and Ganymede. This relentless stretching and squeezing, known as tidal heating, generates tremendous internal heat, making Io a veritable planetary furnace. For decades, scientists have studied this activity from afar, relying on infrared instruments that could only measure the temperature of the very top layer of its surface. It gave us a picture of the hot spots, but it was only skin-deep. We could see where the eruptions were, but we couldn't understand the plumbing just below.
An Unexpected Tool for a New Job
Enter the Juno spacecraft. Launched in 2011 with the primary goal of studying Jupiter's deep atmosphere and powerful magnetic field, its mission was never intended to focus on the moons. However, during an extended mission phase, scientists repurposed one of its key instruments for a new task. Juno’s Microwave Radiometer (MWR) was designed to peer through Jupiter’s thick cloud tops. The team realised that if it could penetrate hundreds of kilometres of gas, it could likely probe a few metres into the rocky and icy surfaces of the moons. During two close flybys of Io in late 2023 and early 2024, they pointed the MWR at the volcanic moon, and the results were astonishing. For the first time, humanity was able to look beneath the crust of this alien world.
The First Subsurface Heat Map
The data returned by the MWR provided the first-ever measurement of Io's subsurface temperature profile. What it revealed was a world leaking heat everywhere. Scientists found that the temperature rose dramatically—by more than 20 degrees Celsius—just a few feet below the surface. This is a far steeper temperature gradient than sunlight alone could ever explain, proving that significant heat is rising from the moon's interior. The new maps add literal depth to our understanding, revealing localised hot spots and a general heat flow that was previously invisible. The readings suggest two possibilities: either heat is steadily rising through a conductive crust, or there are vast pockets of cooling lava trapped just below a thin surface layer. Either way, it’s a direct look at the engine driving Io’s volcanism.
A Surprisingly Smooth and Porous World
Beyond the heat, the MWR data held another surprise. While Io is known for its towering mountains—some taller than Mount Everest—much of its surface is remarkably smooth. The microwave readings suggest that the moon is covered in vast plains blanketed by porous, low-density material, likely layers of volcanic ash and other fine debris from constant eruptions. This continually resurfaces the moon, burying older terrain under fresh deposits. This finding helps explain the strange, mottled look of Io, often compared to a pizza. The smooth, ash-covered plains contrast with the rugged mountains and the dark spots of active volcanic calderas, creating the unique and chaotic landscape we see in images.
Why This Discovery Matters for Earth
Understanding Io is not just an academic exercise in planetary science. According to Scott Bolton, Juno's principal investigator, the techniques used to peer beneath Io's crust could have significant implications for studying volcanoes right here on Earth. By using similar microwave instruments, scientists might one day be able to detect the movement of heat and magma beneath terrestrial volcanoes, offering new data for eruption forecasting. Io serves as a unique natural laboratory for an extreme version of geological processes. Studying how tidal heating works there helps us understand a fundamental force that shapes worlds across the cosmos, from the subsurface oceans on other Jovian moons like Europa to distant exoplanets orbiting other stars.
