Microwave Observations of Ganymede's Sub‐surface Ice: 2. Reflected Radiation

Juno's microwave radiometer experiment (MWR) provided the first spatially resolved observations beneath the surface of Ganymede's ice shell. The results indicate that scattering is a significant component of the observed brightness temperature, which is a combination of the upwelling ice emission and reflected emission from the sky and from Jupiter's synchrotron emission (Brown et al., 2023). Retrieval of the sub‐surface ice temperature profile requires that these confounding signals are estimated and removed to isolate the thermal signature of the ice. We present data analysis and model results to estimate the reflected synchrotron emission component. Our results indicate reflected emission over a broad range of observed angles, due to surface roughness and internal scattering. Based on viewing geometry, direct specular reflection from a smooth surface at a narrow angle is not observed. A microwave‐reflective medium is indicated, that is, a very rough surface and/or non‐homogeneous subsurface. Plain Language Summary On 7 June 2021, Juno had a close flyby of Jupiter's moon Ganymede, flying approximately 1,000 km above the surface. During the flyby, Juno's six channel Microwave Radiometer (MWR) mapped a portion of Ganymede, providing the first resolved observations of Ganymede's sub‐surface ice shell. The observed brightness temperature is composed of upwelling thermal emission from the ice shell and reflected radiation from the sky and from Jupiter's synchrotron emission. To study the sub‐surface ice shell temperature profile, we present data analysis and model results to estimate the reflected radiation component. The radiation is reflected diffusively by a very rough surface and/or non‐homogeneous subsurface. Key Points Reflected radiation from the sky and from Jupiter's synchrotron is an important component for Juno microwave radiometer experiment (MWR) observations at 0.6 and 1.2 GHz Absence of specular reflection indicating that Ganymede has a rough surface Reflections originate mostly from internal scattering