Io Sub‐Surface Temperature Profile Observed by the Juno Microwave Radiometer

Abstract We report on new observations of Io upwelling thermal emission over the range of 0.6–22 GHz acquired with the Juno Microwave Radiometer in December 2023 and February 2024. The microwave emission spectrum from the surface of Io is retrieved from the calibrated brightness temperatures by characterizing and removing reflections from the sky. The surface appears to exhibit specular reflection in the microwave, suggesting a relatively smooth surface on 100 km spatial scales (apart from visible topography), like Earth's land and ocean surfaces. The real part of the dielectric constant is found to be in the range of 2–4; constrained by overlapping observations on the surface from two viewing geometries. This relatively low value is consistent with a density of 0.7–1.1 g/cm 3 of the surface layer (