Exploring the Collisionality of the Icy Galilean Moon Atmospheres.

In this study, we use the Direct Simulation Monte Carlo (DSMC) method 1 to investigate the collisional fraction of the atmospheres of Europa, Ganymede, and Callisto. The extent of the collisional atmosphere of the icy moons is still subject to ongoing debate. While Europas atmosphere is tenuous and effectively collisionless, the exobase for Ganymede and Callisto is expected to be located above a thin collision-dominated atmospheric layer 2. In the 2030s, both ESA's Jupiter Icy Moons Explorer (JUICE) and NASA's Europa Clipper mission are set to explore Jupiter's icy moons from up close, using high-resolution mass spectrometers to sample their atmospheres. The Neutral gas and Ion Mass spectrometer (NIM) of the Particle Environment Package (PEP) onboard JUICE 3 and the MAss Spectrometer for Planetary EXploration (MASPEX) onboard Europa Clipper 4 will determine the atmospheric composition of the moons and potentially sample plume material on Europa. The collisional fraction of their atmospheres affects the abundances of the various species that will be measured, and hence also the deduction of the underlying surface composition. Therefore, obtaining a comprehensive understanding of atmospheric structures, including the collisional fraction, is imperative for both missions. This knowledge is essential to ensure the correct interpretation of the measured data once it becomes available. The DSMC method is a computation technique, where rarefied gas flows are simulated by tracking the motion of individual particles, including their collisions and interactions, to provide insight into the macroscopic gas dynamics. Therefore, this method is ideal for studying thin atmospheres that transition from being collisional near the surface to ballistic at higher altitudes, such as the atmospheres of the icy Galilean moons. The model 5, 6 used herein includes different physical and chemical processes that create the atmospheres of the icy moons, such as sputtering due to interactions with Jupiter's magnetosphere, the sublimation of surface ice, and photochemical reactions. 1 Bird, G. A. (1994). Molecular gas dynamics and the direct simulation of gas flows.2 Schlarmann, L., et al. (2024), in preparation.3 Grasset, O., et al. (2013). Planetary and Space Science, 78, 1-21.4 Phillips, C. B., and Pappalardo, R. T. (2014). Eos, Transactions AGU, 95(20), 165-167.5 Carberry Mogan, S. R., et al. (2021). Icarus, 368, 114597.6 Carberry Mogan, S. R., et al. (2022). Journal of Geophysical Research: Planets, 127(11).