NASA’s Juno probe did something unexpected last year. It looked through Io.
For decades, scientists could only see the surface of Jupiter’s most volatile moon. They measured the heat blasting off the top. But they had no idea what was happening just a few feet down. That changed in late 2023.
During a series of close flybys, Juno pointed its Microwave Radiometer (MWR) at Io. The tool wasn’t built for this. It was designed to pierce Jupiter’s thick cloud cover. Instead, it gave us the first subsurface temperature map of a rocky moon.
Why does this matter? Because it changes how we read planetary crusts.
How Juno Saw Beneath Io’s Rocky Crust
The MWR instrument peered two to six meters (six to twenty feet) beneath the surface.
Infrared cameras miss this. They only catch surface emissions. Juno’s microwave technique is different. It detects the thermal gradient. The data showed temperatures rising by more than 40 degrees Fahrenheit (22°C) just below the crust.
Sunlight doesn’t do that.
The heat came from deep inside. Juno also spotted localized hotspots. Some areas were 18 to 36°F (10–20°C) warmer than the land around them. These aren’t random blips. They’re signatures of heat moving through the ground.
Scott Bolton, Juno’s principal investigator, called the finding surprising. He noted that if we put similar instruments near Earth’s volcanoes, we might see the same pattern. This could revolutionize volcano eruption forecasting methods by showing how magma moves before it breaks the surface.
Why Io Is So Hot And Smooth
Here is the twist.
Io is famous for towering peaks and erupting volcanoes. It is the most geologically active place in the solar system. You’d expect it to look jagged and old.
It doesn’t.
Juno’s data revealed a remarkably smooth surface. It is covered in low-density material. We think this is a thick blanket of porous volcanic ash, sulfur frost, and debris. Every eruption buries the old landscape under new stuff.
The heat detected by Juno likely rises from the moon’s molten interior. Or it might be trapped in cooling lava pockets just under that ash layer. Either way, the microwave data offers the clearest picture yet of Io subsurface heat flow.
What Io’s Tidal Heating Tells Us About Other Worlds
Earth’s volcanoes run on radioactive decay. Io is different.
Jupiter’s gravity is monstrous. As Io orbits, the planet stretches and squeezes the moon. This tidal flexing generates immense friction. That friction turns into heat. Hundreds of volcanoes stay active because of this constant mechanical pumping.
“Io provides a unique window into learning how tidal heating works throughout the cosmos.” — Scott Bolton
This isn’t just about Io.
This process powers subsurface oceans on other moons. Think Europa and Ganymede. If we understand how heat moves through Io’s crust, we get closer to understanding those icy shells. And if we understand the heat, we understand the habitability.
Microwave instruments can probe beneath ice too. Juno has already looked at Europa and Ganymede. Knowing how heat travels through those crusts tells us if those hidden oceans could support life.
The implications stretch further back in time. This technique helps explain ancient volcanism on Mars and Venus. Even Earth’s Moon. Their landscapes were shaped by eruptions billions of years ago. Now we have a better tool to reconstruct that history.
Juno didn’t just map a moon. It gave us a new lens for reading the interior of rocky worlds. We used to watch the eruptions. Now we can watch the heat moving toward them.
That’s a pretty big shift.
And we’re just starting to look.
