NASA's Juno reveals Io's inner secrets

• NASA's Juno spacecraft reveals the internal heat of Jupiter's moon Io
• Microwave data shows heat rises steeply just below the volcanic crust
• Findings suggest a solid mantle instead of a global magma ocean

For decades, we’ve known Jupiter's moon Io as the solar system's volcanic nightmare. But now, thanks to NASA's Juno spacecraft, we are looking past the fire and brimstone to see what lies beneath for the very first time.

Peering Beneath the Crust

Io is the most volcanically active body in our solar system, a world constantly stretched and squeezed by Jupiter's immense gravity, generating incredible heat. For years, scientists have studied its hundreds of erupting volcanoes, but what powers them from below has been a subject of intense debate. Was it a global ocean of magma, or something else entirely? Enter the Juno mission. During two extremely close flybys in late 2023 and early 2024, the spacecraft used its Microwave Radiometer (MWR) to do something unprecedented: measure the temperature below Io's surface. This instrument, designed to probe Jupiter's deep atmosphere, has given us our first look at the moon's internal plumbing.

Microwaves and Magma

Unlike infrared instruments, which only see the surface temperature, Juno's MWR can penetrate several feet into Io's crust. What it found was surprising. Scientists discovered that the temperature rises dramatically just a short distance below the surface, a gradient too steep to be explained by solar heating alone. This points to significant internal heat rising from below. The map generated from the MWR data revealed distinct hot spots. One massive anomaly, dozens of degrees warmer than its surroundings, was located near the Zal Montes Patera, a region where other Juno instruments observed an active lava flow. This provided a direct link between a surface feature and a subsurface heat source.

Rethinking a Magma Ocean

For a long time, the leading theory was that a shallow, global ocean of molten rock fueled Io's chaos. However, Juno's gravity data from the flybys challenged this long-held belief. By measuring how much Io's shape was deformed by Jupiter's pull, scientists determined that its interior was more rigid than a global magma ocean would allow. Instead of a single vast ocean, the evidence now points to a mostly solid mantle, perhaps like a rocky slushy, filled with pockets and chambers of molten rock that feed the volcanoes. This overturns a 44-year-old mystery, suggesting a much more complex and localized volcanic system than previously imagined.

A Surprisingly Smooth World

Beyond the heat, the MWR data revealed another of Io's secrets: its surface is remarkably smooth in many places. While known for its towering mountains, the microwave data indicates vast plains stretching for over 100 kilometers. This suggests that aside from the dramatic volcanic peaks and calderas, much of Io's surface is composed of very low-density material, constantly repaved by volcanic flows. The data from these flybys essentially give us a new, three-dimensional understanding. It’s not just a map of surface volcanoes, but a temperature map of the shallow subsurface, revealing how heat moves from the deep interior to the surface, much like a car radiator.