3D Monte Carlo simulation of Ganymede's atmosphere - lessons learned from Juno's Ganymede flyby

Among Jupiter's satellites, Ganymede undoubtedly has one of the most complex atmospheres. This is primarily due to the fact that Ganymede has its own magnetic field, which forms a small magnetosphere within the much larger magnetosphere of Jupiter. This interaction not only results in atmospheric auroral emissions in the UV range but also strongly influences Ganymede’s space environment.

With the recent Ganymede flyby by the Juno spacecraft, new information on Ganymede’s environment has become available. We have included these measurements into our 3D Monte Carlo model, determining Ganymede’s resulting H₂O, O₂, H₂, O, and H atmosphere. Our simulations show that accounting for all major source and loss processes, sublimation is still the dominating source process for the water in Ganymede’s atmosphere, delivering more than three orders of magnitude more molecules to the atmosphere than all other source processes combined. For the non-condensing atmospheric species (O₂ and H₂), on the other hand, it is the auroral electrons that mainly govern the atmospheric structure and density. The auroral electrons also govern the structure and density of the atomic species O and H, which are mainly added to the atmosphere by electron-impact dissociation of O₂ and H₂ in the auroral belts. Comparison with available spectroscopic observations of Ganymede’s atmospheric constituents shows that our results agree well with the results inferred from these observations, with the exception of H, where our derived line-of-sight column density is about one order of magnitude lower than the column density inferred from Lyman-α measurements.

Our analysis shows that for a complete understanding of Ganymede's atmosphere, simultaneous observations of Ganymede's surface, atmosphere, and plasma environment at different times and locations are particularly important. Such measurements are planned with the Jupiter ICy moons Explorer, in particular with the Particle Environment Package (PEP). In this presentation we will show how PEP will help us learn more about Ganymede’s complex atmosphere, providing simultaneous in-situ electron, ion, and neutral gas measurements.