Io’s Sub-surface Heat Distribution Observed by the Juno Microwave Radiometer

The NASA Juno mission performed two close fly-bys of Jupiter’s moon Io on December 30, 2023 and February 3, 2024. Juno carries a 6-channel microwave radiometer (MWR) operating between 0.6-22 GHz. The first fly-by observed Io’s north pole and the 2^nd pass mapped latitudes within +/- 45^o on the Jovian facing hemisphere. The broad frequency range of the MWR probes successively deeper into the Io sub-surface with the 0.6GHz channel probing the deepest. The penetration depth into the sub-surface of the highest frequency channels is on the order of centimeters and the lowest frequency on the order of several 10s of meters. We find the surface of Io generally exhibits specular scattering properties over the 0.6-22 GHz frequency range. We use overlapping observations from the two fly-bys that observe the same areas at different incidence angles and polarizations to solve for the surface dielectric properties. We find the surface dielectric (real part) to be between 2-3, which is consistent with a low-density material. We use the MWR derived real part of the dielectric constant (reflection) with Earth analogs for the imaginary part (loss) to derive the sub-surface temperature profile by inverting the radiative transfer equation. We find the near-surface temperatures decrease with increasing latitude and are coldest at the north pole, consistent with prior infrared observations of the surface skin temperature. We find a strong sub-surface thermal gradient, on the order of 20-40K, over all regions observed by MWR. The sub-surface thermal anomaly is not spatially uniform. We fit several possible models to explain this gradient. One possible explanation are spatially distributed near-surface heat vents topped by a cooled crust, which fit the MWR spectra if they occupy 5-10% of the surface area. We will give an overview of the MWR observations and inferences about the sub-surface thermal and compositional properties.