Juno Microwave Radiometer Observations into the Subsurface of the Ice Shells of Io, Europa and Ganymede

During the Juno extended mission, the spacecraft passed Jupiter’s Galilean moons, Ganymede, and Europa, and then Io respectively.  The flyby of Ganymede was in June 2021, at a distance of ~1000 km, and in September 2022, the spacecraft flew by Europa at a distance of ~350 km.  Two flybys of Io at a distance of 1500 km occurred in December 2023 and February 2024. The close flybys were the first encounters with the moons in over two decades and provided the first opportunity to probe their subsurface at multiple microwave frequencies using Juno’s Microwave Radiometer (MWR).  The observations provided several swaths across the moons at six frequencies, ranging from 600 MHz to 22 GHz.

Early radar results of Jupiter’s icy moons dating back several decades identified the moons as extremely bright objects with significant radar scattering (Ostro et al., 1980).  Comparisons of the radar properties of Europa & Ganymede indicated important differences in the radar signatures from each other and our Moon (Ostro et al., 1992).  Possible explanations included modulations in porosity (Ostro and Shoemaker, 1990), random facets, larger than the observed wavelengths (Goldstein and Green, 1980), and the idea that the top meters of ice covering their surfaces may be crazed, fissured, and/or filled with jagged ice boulders (Goldstein and Green 1980).

The Juno MWR observations represent the first resolved interrogation of the moons Ganymede, Europa and Io’s subsurface structure.  For icy bodies such as Ganymede and Europa, the MWR observed brightness temperature, T_B,  is dependent on such ice shell parameters as ice purity, the thermal structure of the icy shell (providing a constraint on the global heat flux) as well as the distribution of internal microwave scattering, thus allowing MWR to provide integral constraints on these shell properties. The MWR observations of Ganymede showed T_B generally increases with depth, has a significant reflected synchrotron radiation component at the lowest MWR frequency, 600 MHz, and was well correlated with terrain type.  The T_B was generally anticorrelated with visible reflectivity (albedo).  hermal gradient from deepest channels constrains heat flux (and thickness of conductive ice shell).  Analysis of the MWR results at Ganymede provided a new constraint on Ganymede’s heat flux and shell thickness (conductive and total).  Using a thermal model based on modified Mixing Length Theory from Kamata et al. (2018) and a Radiative transfer model accounts for microwave radiation propagation through the ice shell including the effect of synchrotron and galactic reflected radiation the results suggest a heat flux of  mW/m² assuming pure ice.  The model suggests a thickness of conductive ice shell of   km

The surface of Jupiter’s moon Europa mapped by MWR covers a latitude range from ~20^oS to ~50^oN and a longitude range from 70^oW to 50^oE.  At these frequencies, the emission originates well beneath the nearly-transparent surface, probing from as deep as 28 km (at 0.6 GHz) and less than 20 m (at 22 GHz), depending on the purity of the ice.  At these frequencies, the emission originates well beneath the nearly-transparent surface, probing from as deep as 28 km (at 0.6 GHz) and less than 20 m (at 22 GHz), depending on the purity of the ice.  Microwave reflection plays an important role, and MWR data suggest the presence of small (radius a few cm) scatterers at depths of many meters.  Spatial variation is dominated by reflection, especially for the higher-frequency channels, and correlates with terrain type.  T_B is generally lower than the expected physical temperature of the ice, indicating strong microwave reflection of the cold sky. 

The slope of the spectra indicates successively more reflection with decreasing frequency, down to ~2.5 GHz.  T_B varies across Europa, much more than the expected variation in ice temperature, implying the amount of reflection must also vary.  The spectra from different locations are nearly parallel, suggesting all frequencies see a common set of reflectors.  No evidence of diurnal variation is observed in the MWR data, indicating even the highest frequency channel is probing beneath the diurnal layer.  Using a simple model, and minimal assumptions the MWR data suggest the presence of volume scattering small reflectors extending to ~1 km depth. 

At Io, 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 sub-surface temperature, dielectric and surface roughness properties are encoded in the spectra obtained by the MWR. Here we report on the first spatially resolved observations of Io at frequencies below 22 GHz.  We find the brightness temperatures decrease with increasing latitude and are coldest at the north pole, consistent with prior infrared observations of the surface skin temperature. We observe a strong spectral gradient in the lowest frequency channels (increasing with depth) reflecting the sub-surface temperature profile from which we can infer endogenic heat flow. A large specular component of the surface microwave reflection, constrains the surface roughness. MWR provides first maps of sub-surface thermal profile and constraints on surface material properties.  Prior infrared thermal measurements only sense very top surface (skin layer).  Results indicate a surface that is very close to specular (in microwave) – meaning the surface is relatively flat (on ~meter scales) as viewed by MWR (100 km spatial scale).  The surface dielectric constant is close to 3, suggesting a relatively low density surface layer. Preliminary analysis suggests evidence that that MWR may be able to constrain the sub-surface thermal gradient (internal heat from Io).

These unprecedented measurements of Io, Europa and Ganymede will allow comparative studies of their surfaces and subsurface structures. The Juno MWR measurements complement previous ground-based radar and microwave radiometry observations, which provided early characterization of these surfaces.  A comparison of the microwave spectra for all three satellites will be presented, as well as a detailed analysis and interpretation of the Ganymede MWR data that provide new constraints on ice subsurface properties.