Juno’s Microwave Radiometer Peers Below Io’s Surface

During two close flybys in late 2023 and early 2024, NASA’s Juno spacecraft came within about 930 miles (1,500 kilometers) of Io, using its Microwave Radiometer (MWR) to take the first-ever subsurface temperature readings of a rocky moon. The instrument, originally designed to look deep into Jupiter’s turbulent atmosphere, detected heat rising from just beneath Io’s volcanic crust.

Scientists found that within a few feet of the surface, temperatures rose by more than 40 degrees Fahrenheit — a gradient far steeper than solar heating alone can explain. The measurements suggest either steady heat conduction through a thin crust, equivalent to a small nightlight glowing under every square yard across the entire moon, or cooling lava flows capped by solid rock about 30 to 35 feet thick covering roughly 10 percent of the surface at any given time.

In addition to the heat signature, the MWR revealed that away from its towering mountains, Io’s terrain is remarkably smooth — “more like the Great Plains of North America,” according to lead author Shannon Brown of JPL. The material there has an unexpectedly low density, similar to pumice or fluffy volcanic ash, a contrast to what scientists had assumed about such a rocky body.

What Tidal Heating Tells Us About Worlds Beyond Earth

A New Window Into Tidal Heating

Io’s extreme volcanism has long been attributed to tidal heating: Jupiter’s immense gravity stretches and squeezes the moon as it moves along its slightly elliptical orbit, generating enormous internal heat. Until now, however, all direct temperature data came from infrared observations that only sensed the top surface. The MWR’s multiple wavelengths, which probe different depths from a few inches to tens of feet, finally allowed researchers to map how that heat moves from the interior outward — a fundamental step in understanding volcanic worlds both inside and beyond our solar system.

Principal investigator Scott Bolton noted that the technique could be applied to Earth’s volcanoes: “If we look with an MWR-type instrument near a volcano on Earth, we might see a similar signature in the subsurface temperature gradient.” That opens a path for studying terrestrial volcanism in fresh detail.

From Fiery Io to Icy Moons

The same instrument had already explored the icy shells of Ganymede and Europa, probing tens of miles below the surface. Bolton emphasized that tidal heating fuels subsurface oceans on those moons, and the ability to characterize heat flow at Io provides a missing link. “Io provides a unique window into learning how tidal heating works throughout the cosmos,” he said. By tracing the energy from deep within a moon to its surface, scientists can better estimate the habitability potential of ocean-harboring worlds that are far from their parent star.