A New Look at a Volcanic World
Io is a place of constant torment. Caught in a gravitational tug-of-war between the colossal Jupiter and other large moons, it is perpetually stretched and squeezed. This process, known as tidal heating, generates immense energy within the moon, forcing
its interior to melt and erupt through hundreds of volcanoes. For years, our understanding of this heat came from infrared cameras that could only see the surface temperature. That all changed thanks to NASA's Juno spacecraft. Originally sent to study Jupiter itself, its mission was extended to include daringly close flybys of its moons. On December 30, 2023, and February 3, 2024, Juno swooped within about 1,500 kilometres of Io, turning its powerful instruments to this tortured world.
How to Take a Moon's Temperature
The key to this breakthrough was an instrument that wasn't even designed for this task. Juno's Microwave Radiometer (MWR) was built to peer deep beneath Jupiter’s thick cloud tops. Scientists made a surprising discovery: it could also see below the rocky surface of a moon. The MWR uses multiple microwave frequencies to penetrate the ground to different depths, from a few centimetres to several metres. By measuring the natural thermal radiation coming from these layers, scientists can build a temperature profile of the subsurface. It's like giving the moon a medical check-up, but instead of an X-ray, you're using microwave emissions to see how heat is moving from the interior toward the surface.
Beneath the Fiery Surface
The results were stunning. Everywhere the MWR looked, it found the temperature rising sharply just below the surface. In the space of only a few feet, the ground became more than 22 degrees Celsius (40 degrees Fahrenheit) warmer. According to scientists, this temperature gradient is far too steep to be explained by sunlight alone. It is direct evidence of the immense internal heat of Io escaping. The readings suggest a heat flow up to 30 times greater than Earth's average, painting a picture of a world losing its primordial energy at a furious rate. This was the first time anyone had directly measured the movement of heat through Io's crust.
A Surprisingly Fluffy and Smooth World
The readings also revealed some unexpected physical properties. While known for its towering mountains, large swathes of Io's surface appear remarkably smooth when viewed with microwaves. More surprisingly, the top layer of ground seems to have a very low density. Instead of solid rock, the readings are consistent with a surface made of porous material like volcanic ash, pumice, or scoria. Scientists describe it as a 'fluffy' surface, likely built up from countless eruptions over aeons. It appears that while some parts of Io are hard and mountainous, much of it is covered in a deep, soft layer of volcanic debris.
The Bigger Picture
These discoveries do more than just add detail to our portrait of Io; they have profound implications. By understanding how heat moves through Io's crust, we can better model tidal heating—the engine that not only powers Io's volcanoes but may also sustain liquid water oceans inside icy moons like Europa. Furthermore, some analyses using Juno's data have challenged the long-held theory of a global magma ocean just beneath Io's crust, suggesting the volcanism may be fed by more localised magma chambers. The success of using the MWR on a rocky moon was itself a surprise, opening a new way to study volcanism not just on other worlds, but potentially on Earth as well.
