Revealing the Depth and Sub-surface Structure of Ganymede's Ice Shell from Juno Microwave Radiometer Observations

On 7 June 2021, Juno had a close flyby of Jupiter's moon Ganymede, flying within 1000 km of the surface. During the flyby, Juno's Microwave Radiometer (MWR) observed Ganymede obtaining several swaths across Ganymede using Juno's spin to partially map Ganymede's ice shell in six channels ranging from 600 MHz to 22 GHz. The emission at these frequencies originates from successively deeper layers of the sub-surface and may reach to depths of 20km at 0.6 GHz. The MWR observations cover a latitude range from 20S to 60N and an east longitude range from -120 to 60 degrees, roughly centered on the Perrine region. The local solar time varies from around noon to midnight over the longitude range. We present resolved brightness temperature maps and associated microwave spectra of Ganymede with a spatial resolution of up to ~140 km (approximately 1/40th of Ganymede's diameter). The microwave brightness temperature at all MWR wavelengths is anti-correlated with the visible brightness of the terrain, but is too large to be explained by albedo variations alone, suggesting sub-surface ice properties are not uniform with location. The dark regions tend to exhibit the warmest microwave spectra and brighter regions are observed to have a lower brightness temperature (up to half the blackbody temperature).The coldest microwave feature observed by MWR is the Tros crater and the immediate surrounding region. A radiative transfer algorithm and a thermal model for the conductive layer of Ganymede's ice shell are applied to the MWR spectra providing an estimate of the conductive shell thickness. The microwave observations are globally colder than would be expected for pure water ice alone, suggesting thin highly reflective layer, possibly silicate dust, covers much of the surface, although this interpretation is non-unique. We suggest that scattering at sub-surface interfaces (e.g. fractures) explains the depressed brightness temperatures observed in brighter terrain types. Juno will perform a close fly-by of Europa in September 2022, enabling a comparison of the sub-surface properties of these two icy satellites.