NASA's Juno spacecraft has measured subsurface temperatures on Jupiter's moon Io using its Microwave Radiometer, marking the first direct observation of heat beneath the surface of the solar system's most volcanically active world. During flybys in late 2023 and early 2024, the instrument probed six to twenty feet underground, revealing heat signatures that infrared observations alone could never detect. The finding opens a new way to study volcanism across planetary bodies and could eventually improve eruption forecasting on Earth.
Juno detected temperatures rising more than 40 degrees Fahrenheit just a few feet below Io's surface, far exceeding what sunlight could account for. The data also mapped localized hot zones between 18 and 36 degrees Fahrenheit warmer than surrounding terrain. Researchers believe this heat either conducts steadily from Io's molten interior or comes from cooling lava pockets trapped beneath the crust, though distinguishing between the two mechanisms remains uncertain.
Io's extreme volcanism stems from tidal heating: Jupiter's immense gravity continuously stretches and squeezes the moon as it orbits, generating internal heat that powers hundreds of active volcanoes. This is fundamentally different from Earth, where radioactive decay drives most volcanic activity. The same microwave technique could help scientists understand ancient volcanism on Mars, Venus, and the Moon, and could eventually aid searches for subsurface oceans on Europa and Ganymede where conditions might support life. Juno has already used the instrument to observe those icy moons. The findings were published July 22 in the Journal of Geophysical Research: Planets.