3D Monte-Carlo Simulations of Ganymede's Water Atmosphere - Predictions for JUICE/PEP/NIM

We present 3D Monte-Carlo simulation results for the surface-sputtered and sublimated H₂O molecules in Ganymede's atmosphere. To calculate particle fluxes onto Ganymede's surface, we use test particle model results for electrons, thermal H⁺ and O⁺, energetic H⁺, O⁺⁺, and S⁺⁺⁺, with unprecedented energy resolution. In addition, besides a thermal model based on Galileo measurements, we use recently published surface water content maps and recently measured water sputter yields.

Our simulations show that for the sputtered atmosphere, it is mainly the impinging O⁺, O⁺⁺, and S⁺⁺ ions that deliver H₂O to the atmosphere, while electrons and protons only play a minor role in comparison. With Ganymede's surface temperature ranging from 80 K to 150 K (the latter being an upper bound), most returning H₂O molecules stick to the surface. As a consequence of this, the morphology of Ganymede's magnetosphere, and the resulting patterns in the precipitation maps, are well preserved in the exosphere up to altitudes of a few thousand kilometers.

In the sub-solar region, it is the sublimated H₂O that dominates the atmosphere by up to four orders of magnitude. The sublimated atmosphere quickly decreases with altitude, though, and sputtering becomes the dominant release process for H₂O molecules reaching beyond a few hundred kilometers altitude. The sublimated H₂O atmosphere is thus quite substantial but highly limited in spatial extent.

In addition to our most important modeling results concerning Ganymede's H₂O atmosphere, we will also discuss their implications for spacecraft observability. Using the recently updated JUICE trajectories (CREMA 5), we will show which atmospheric populations (sublimated and/or sputtered H₂O) will be encountered during the different Ganymede orbit phases (elliptical, high polar, and low polar). Finally, we will present expected measurement results for the Neutral and Ion Mass spectrometer (NIM), part of the Particle and Environment Package (PEP) onboard JUICE / ESA.