NASA's Juno spacecraft has measured subsurface temperatures on Jupiter's moon Io for the first time, using its Microwave Radiometer instrument during two close flybys in December 2023 and February 2024. The findings, published Wednesday in the Journal of Geophysical Research: Planets, reveal significant heating within Io's shallow subsurface and show that the moon's surface is mostly smooth and composed of low-density material. This marks the first direct observation of heat below a rocky moon's surface and could inform how scientists study volcanic processes on Earth.
The MWR data came from passes within 930 miles of Io's surface. Temperature measurements showed gradients exceeding 40 degrees Fahrenheit just several feet down -- far steeper than solar heating alone can produce. The observations suggest either steady heat conduction through the crust, which would release energy 20 to 30 times Earth's average across the entire moon, or cooling lava flows buried under 30 to 35 feet of solidified crust covering about 10% of Io's surface at any time.
Io's extreme volcanism results from tidal heating as Jupiter's gravity constantly stretches and squeezes the moon in its slightly elliptical orbit. Until now, virtually all knowledge of that interior heat came from infrared observations, which detect only surface temperature. The new subsurface data provides a direct window into how tidal heating works in planetary systems, a process that fuels not only Io's volcanism but also the subsurface oceans beneath moons like Europa and Ganymede.
Away from its mountains, Io resembles the Great Plains of North America in its flatness, per the team's analysis. The MWR instrument was originally designed to probe Jupiter's deep atmosphere but has proven capable of characterizing subsurface structures of rocky and icy bodies. Juno's extended mission has now applied this technique to three Galilean moons: Ganymede, Europa, and Io.