Microwave Observations of Ganymede's Sub-Surface Ice: I. Ice Temperature and Structure

On 7 June 2021, Juno flew within 1,000 km of Ganymede's surface, partially mapping its ice shell at six frequencies ranging from 0.6 to 22 GHz. The radiance at these frequencies originates from successively deeper layers of the sub-surface and may reach depths of 24 km at 0.6 GHz. The MWR observations cover a latitude range from 20 degrees S to 60 degrees N and a longitude range from 120 degrees W to 60 degrees E. We present brightness temperature and derived reflectivity maps of Ganymede with a spatial resolution of up to similar to 140 km. The microwave brightness temperature at all MWR wavelengths is anti-correlated with the visible brightness of the terrain. Normalizing the MWR brightness temperatures using a thermal model for the ice shell reveals that the brightest regions are significantly more reflective in the microwave than the dark regions and that all terrain types are more reflective than is expected from a solid ice surface. We suggest that multiple reflections of the colder sky background at sub-surface interfaces (e.g., fractures) explain the depressed brightness temperatures observed in brighter terrain types. A thin silicate or salt contaminant surface layer, which is significantly more reflective than ice in the microwave, could explain the microwave reflectivity in the dark regions with little to no contribution from sub-surface fractures. The observed 0.6-1.2 GHz brightness temperature difference suggests an upper bound on the ice shell conducting layer depth of 150 km in the observation area.