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

On June 7, 2021, and September 29, 2022, the NASA Juno spacecraft flew by Jupiter’s Galilean moons, Ganymede, and Europa, respectively.  The closest approach distance was only ~1000 km above Ganymede, and only ~350 km above Europa. More recently, on December 30, 2023, Juno passed by Io at a distance of 1500 km and is planned do so a second Io flyby on February 3, 2024 at a similar distance.  The close flybys were the first encounters with the moons in over two decades and provided the first opportunity to map the subsurface of the their shells 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.  The ice transparency at microwave frequencies is dependent on its purity; assuming pure ice, the observations probe depths ranging from meters to kilometers. The MWR observations represent the first resolved interrogation of Ganymede and Europa’s subsurface ice shell revealing new constraints on porosity, fracturing, differences in terrain type and possibly the thickness of the ice shell. These unprecedented measurements of Io, Europa and Ganymede will allow comparative studies of the surfaces and subsurface structures of the Jovian satellites. 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.