Modeling Ganymede's Surface Charging in Preparation for the JUICE Mission

Abstract The European Space Agency's (ESA) JUICE mission (JUpiter ICy moons Explorer) is en route to the Jovian system to characterize Ganymede's subsurface ocean. Determining the ocean's conductivity and depth requires precise measurements of its induced magnetic field at the position of JUICE. Electron reflectometry provides additional constraints for the surface magnetic field, but this would require knowledge of Ganymede's surface electric potential. Here, we model the global electrostatic surface potential distribution on Ganymede using a semi‐analytical charging formulation, by feeding it with surface plasma environment parameters derived from different magnetospheric interaction simulations. We further contrast these estimates against self‐consistent charging simulations using the Spacecraft Plasma Interaction Software (SPIS). Our results indicate that surface potentials should be mostly negative, ranging down to about −100 V. All model predictions exhibit a consistent morphological trend: closed field line regions are more negatively charged than open field line ones. Depending on the input used, the driver behind this trend can either be the spatial variations of plasma density alone or combined density and electron temperature patterns, especially in the ionosphere; the electron current is the dominant driver of surface potential, with the relative influence of secondary electron, ion, and photoelectron currents varying depending on the location at Ganymede. While estimates of absolute surface potential are consistent with aspects of Juno flyby data, they are heavily dependent on how simulated plasma parameters are extrapolated to Ganymede's surface. Consequently, advanced modeling is required to achieve more reliable constraints of Ganymede's near‐surface environment description.