Juno measures Io subsurface heat

United States — NASA's Juno spacecraft has provided the first measurements of temperatures beneath the surface of Jupiter's volcanic moon Io, revealing significant heating within its shallow subsurface. Using the Microwave Radiometer (MWR) instrument during two close flybys in December 2023 and February 2024, Juno detected temperatures rising by more than 40 degrees Fahrenheit (about 22 degrees Celsius) just a few feet below the surface, a gradient far steeper than solar heating alone can explain. The findings, published in the Journal of Geophysical Research: Planets, also show that most of Io's surface is remarkably smooth and composed of low-density material resembling pumice or fluffy volcanic ash. The heat is attributed to tidal heating, as Jupiter's gravity stretches and squeezes Io, generating internal heat output up to 30 times Earth's average. The data suggest either heat rising steadily through a conductive crust or cooling lava flows capped by solidified crust covering about 10% of the surface. This discovery provides new insights into tidal heating processes that also fuel subsurface oceans on icy moons like Europa and Ganymede, and could help scientists study terrestrial volcanoes using similar microwave techniques.