NASA’s Juno spacecraft makes the first measurements of temperature below the surface of Jupiter’s moon Io. Data from two close flybys indicate significant heating in Io’s shallow subsurface, highlighting active internal energy sources beneath the moon’s crust. The observations focus on thermal conditions below the surface rather than surface temperatures alone, providing new constraints on how Io’s intense volcanism is powered and how heat is distributed near the top of the moon. The results also describe the broader character of Io’s exterior: most of the moon’s surface appears remarkably smooth, and the material covering it is reported to have very low density. By combining subsurface temperature estimates with surface properties, the measurements offer a more complete picture of the relationship between Io’s internal heating and the nature of its uppermost layers. The findings are based on Juno’s targeted passes during which the spacecraft gathers the relevant thermal data and scans conditions across much of Io.