The Heat Hiding Just Below Io's Skin: Juno's First Subsurface Temperature Map

For decades, every thermometer pointed at Io read the same thing: its skin. Infrared cameras aboard Galileo, and more recently Juno's own JIRAM instrument, catch the glow of the surface and the lava that breaks through it. What they could never see was the layer underneath: the shallow crust where the moon's internal furnace either leaks its heat outward or hides it away. Now a team led by Shannon Brown at NASA's Jet Propulsion Laboratory has taken that reading, and the result is published in the Journal of Geophysical Research: Planets.

They did it with an instrument built for an entirely different job. Juno's Microwave Radiometer, or MWR, was designed to see through the clouds of Jupiter and gauge how much water and ammonia lie buried in the giant planet's atmosphere. Its six antennas listen across wavelengths from roughly half an inch to twenty inches. Longer microwaves come from deeper down, so a single sweep effectively stacks readings from a few inches beneath a surface to tens of feet below it. Pointed at Io during two close flybys (on Dec. 30, 2023, and again on Feb. 3, 2024, each passing about 930 miles above the moon), that same trick turned the radiometer into a probe of Io's crust.

What it found was a crust that heats up fast as you go down. Within the first several feet, temperatures climbed by more than 40 degrees Fahrenheit. Sunlight cannot do that. Solar warmth fades with depth rather than building; a gradient this steep has to be fed from inside. The background heat leaking out of Io measured between 1 and 3 watts per square meter, and across the whole moon the energy budget runs to something like 30 times Earth's average output. This is the signature of tidal heating: the relentless flexing Io suffers as Jupiter's gravity, and the tug of neighboring moons, knead its interior and turn friction into heat.

Two regions lit up more than the rest. One of them sits directly over Zal Montes, a mountain range flanked by an active lava field, which is roughly what you would expect from a place already known to be erupting. The finding is where the story gets interesting, because the paper does not settle on a single reason for the buried warmth.

Here the authors leave two doors open. In the first reading, the heat is simply the moon's internal energy conducting steadily upward through the crust, a continuous thermal flow rising from the tidally heated interior toward the surface. In the second, the warmth is more of a fading echo: lava that erupted, spread across the ground, and then cooled under a cap of solidified crust roughly 30 to 35 feet thick. Under that interpretation, something like a tenth of Io's surface at any given moment is quietly shedding the leftover heat of recent eruptions, sealed beneath its own hardened lid. The two pictures are not easy to tell apart from the data in hand, and the team is careful not to choose. Both would produce a crust that warms with depth; distinguishing steady conduction from capped, cooling flows will take more passes and more modeling.

That restraint matters, because Io sits at the center of a bigger unsettled question. How much of the moon's interior is actually molten? Is there a global magma ocean sloshing beneath the crust, or a hotter but still largely solid mantle? Knowing where the heat is stored, and how it moves through the outermost layers, is one of the few handles researchers have on that problem. A subsurface temperature map does not answer it outright. But it is the first direct measurement of the part of Io that had, until now, stayed hidden between the glowing surface and the deep unknown.