NASA’s Juno spacecraft has produced the first direct measurements of temperature below the surface of Io, Jupiter’s intensely volcanic moon. The findings use microwave observations from close flybys on December 30, 2023, and February 3, 2024, when Juno passed about 930 miles above the moon.\n\nThe spacecraft’s Microwave Radiometer measured thermal emissions from depths ranging from inches to tens of feet. Researchers reported temperatures rising by more than 40 degrees Fahrenheit within several feet of the surface — a gradient too steep to be explained by sunlight alone. The work was published July 22 in the Journal of Geophysical Research: Planets.\n\nScientists are considering at least two explanations. Heat may be moving steadily through Io’s crust, or the signal may come from cooling lava flows buried under roughly 30 to 35 feet of solidified material. The observations do not yet establish which mechanism dominates.\n\nJuno also found that much of Io’s surface material appears unusually smooth and low in density. NASA says the measurement technique could improve understanding of tidal heating on moons such as Io, Europa and Ganymede and may inform future studies of volcanic heat beneath Earth’s surface.\n\nSource: NASA Jet Propulsion Laboratory — https://www.nasa.gov/missions/juno/nasas-juno-takes-temperature-of-jupiters-fiery-moon-io/\n\nSource: Journal of Geophysical Research: Planets paper — https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JE009622