NASA's Juno maps Io's subsurface heat

• NASA's Juno spacecraft maps heat beneath Io's volcanic surface
• Subsurface temperatures jump 22°C just feet below the crust
• Data could help scientists improve eruption forecasting on Earth

Io, Jupiter's fiery moon, is the most volcanically active world in our solar system. Now, for the first time, NASA's Juno spacecraft has peered beneath its chaotic surface, mapping a hidden world of heat that powers its extreme geology.

A Groundbreaking Look Beneath the Crust

For decades, our understanding of Io's intense heat has come from infrared observations, which can only measure the temperature of the very top layer of its surface. But during two close flybys in late 2023 and early 2024, NASA's Juno spacecraft used its Microwave Radiometer (MWR) to change the game. Originally designed to peer through Jupiter's thick clouds, the instrument was repurposed to probe Io's subsurface. This gave scientists their first-ever measurements of temperature from a few inches to tens of feet below the moon's surface, providing an unprecedented glimpse into the engine driving its volcanism.

The Surprising Heat Gradient

The data from Juno's flybys, which came within about 1,500 kilometers of Io, revealed a startling discovery. Scientists found that the temperature rises dramatically just a few feet underground. In some areas, the temperature jumped by more than 40 degrees Fahrenheit (22 degrees Celsius) in a very short depth—a gradient far too steep to be explained by sunlight alone. This points to a powerful internal heat source making its way up through the crust. The findings suggest two main possibilities: either heat is rising steadily through a conductive crust, or it's escaping from pockets of cooling lava trapped just below a thin surface layer.

A Smoother and Fluffier World Than Expected

While famous for its towering mountains, some taller than Mount Everest, the new data reveals another side of Io. The MWR readings indicate that, away from the rugged peaks, Io has vast, smooth plains stretching for 100 kilometers or more. Lead author Shannon Brown compared these areas to the Great Plains of North America. Furthermore, the data suggests that the surface material itself is surprisingly low-density. Instead of solid rock, much of the upper crust appears to be made of porous material similar to pumice or fluffy volcanic ash, likely built up over eons of constant eruptions.

Why This Matters for Earth and Beyond

This new understanding of Io has significant implications that reach far beyond Jupiter. Io serves as a natural laboratory for studying tidal heating, a fundamental process that generates energy and heat on worlds far from their star. This same process is thought to fuel the subsurface oceans on other moons like Europa and Ganymede. According to Scott Bolton, Juno's principal investigator, the techniques used to peer into Io's rocky crust could one day be applied to volcanoes on Earth. An instrument like the MWR might detect similar subsurface temperature gradients near terrestrial volcanoes, potentially providing new information to help improve eruption forecasting.