Observations of Ganymede’s Sub-Surface Ice Temperature and Structure from the Juno Microwave Radiometer: Part I

On 7 June 2021, Juno flew within 1000 km of the surface partially mapping Ganymede’s ice shell in six channels ranging from 0.6 GHz to 22 GHz. The radiance at these frequencies originates from successively deeper layers of the sub-surface and may reach to depths of 24km 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 mid-night over the longitude range. We present brightness temperature and derived reflectivity maps 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. Globally, the observed spectra are consistent with an ice shell conducting layer depth of approximately 100km. We suggest that scattering at sub-surface interfaces (e.g. fractures) explains the depressed brightness temperatures and higher reflectivities observed in brighter terrain types.