Resolved Microwave Observations of Ganymedes Ice Shell

On 7 June 2021, Juno had a close flyby of Jupiters moon Ganymede, flying within 1000 km of the surface. During the flyby, Junos Microwave Radiometer (MWR) observed Ganymede obtaining several swaths across Ganymede using Junos spin to partially map Ganymedes ice shell in six channels ranging from 600 MHz to 22 GHz. Ground-based interferometry at mm-cm wavelengths have helped characterize icy satellite surfaces with previous unresolved microwave and radar maps providing the basis for Ganymede models indicating surface temperature variations that correlate with surface albedo (Butler 2012). Previous observations at millimeter wavelengths probed the shallow sub-surface (~cm depths), and provided information on thermal properties such as emissivity and thermal inertia, show strong hemispheric differences in surface albedos, with large regions of warmer, darker terrain as well as cooler, ice-rich regions (de Kleer et al 2021). Previous full disk cm-wave observations have been hindered by the presence of Jupiters thermal and synchrotron emission. We present resolved brightness temperature maps and associated spectra of Ganymede with a spatial resolution of up to ~140 km (approximately 1/40th of Ganymedes diameter). The maps and spectra are sensitive to prominent localized thermal features in addition to the various types of terrain seen in visible and infrared images of Ganymede. Comparing the microwave spectra with maps of Ganymede reveal spectral differences corresponding to different types of terrain visible on Ganymede including bright and dark geological features. Junos wide range of wavelengths probe various depths providing information on porosity, water ice purity, thermal inertial and dielectric constants of the various ice regions as well as linear features thought to be associated with tectonic activity. Significant variation in the Juno MWR spectra with location suggest sub-surface ice properties are not uniform with location.