Investigating influences of collisions on the icy Galilean moon atmospheres

Jupiter’s icy Galilean satellites Europa, Ganymede, and Callisto possess tenuous atmospheres that are mainly sourced from the surface. However, the extent of the collisional atmosphere of the icy moons is still subject to ongoing debate. In this study, we model the atmospheres of the three icy satellites using the Direct Simulation Monte Carlo (DSMC) method [1] to locate the exobases at the icy moon atmospheres and to investigate the influences of collisions among the different atmospheric species.

Our model [2, 3] includes the main physical and chemical processes that create the atmospheres of the icy moons, such as the sublimation of surface ice (H₂O) and the radiolytic production and sputtering of molecular oxygen (O₂) and hydrogen (H₂). Furthermore, we include photochemical reactions and electron impacts, that can ionise and dissociate species in the atmosphere. To determine the location of the exobase, we use the Knudsen number (Kn), defined as the ratio of the mean free path (λ) to the scale height (H). The exobase, marking the boundary between collision-dominated and collision-free atmospheric regions, is typically considered to be at Kn = 1. In addition, we compare the results of the collisional DSMC model with a ballistic model, where the particles do not interact with each other, to investigate the influences of collisions on the abundances and escape rates of the included species. 

In this effort of comparative selenology [4], we find that the exobases of all three icy moon atmospheres are located above the surface (see Figure 1). For Ganymede and Callisto, our model shows that the collisionality is significantly affected by the abundance of sublimated H₂O near the subsolar point. In contrast, for Europa, the abundance of recycled O₂ results in the exobase being located above the surface, which differs from the assumptions made in previous exospheric models. Furthermore, we find that a collisional atmosphere leads to reduced escape rates for most species, as particles that would have escaped on ballistic trajectories lose their energies via collisions. However, collisions with dissociation products can also significantly increase the escape of heavy species, such as sublimated H₂O and recycled O₂, that would not be expected to escape in a non-collisional atmosphere, as their thermal velocities are significantly smaller than the escape velocities of the icy moons.

Figure 1: Knudsen number as a function of the solar zenith angle and altitude (in km and satellite radii) for Europa (left), Ganymede (middle), and Callisto (right). A solar zenith angle of 0° corresponds to the subsolar point. The exobase (Kn = 1, solid line), and the boundary between the quasi- and fully collisional regime (Kn=0.1, dashed line) are also shown.

In the 2030s, both ESA's Jupiter Icy Moons Explorer (JUICE) and NASA's Europa Clipper mission are set to conduct close-up explorations of the three satellites. Equipped with high-resolution mass spectrometers, more specifically the Neutral gas and Ion Mass spectrometer (NIM) aboard JUICE [2] and the MAss Spectrometer for Planetary EXploration (MASPEX) on Europa Clipper [3], they will measure the atmospheric composition. In this study, we show that the collisionality of the icy Galilean moon atmospheres affects the abundances and escape rates of the different species that will be measured, consequently impacting the determination of the moons' underlying surface composition.

Acknowledgements:
This work has been carried out within the framework of the National Centre of Competence in Research PlanetS supported by the Swiss National Science Foundation under grant 51NF40_205606. The authors acknowledge the financial support of the SNSF. Calculations were performed on UBELIX (http://www.id.unibe.ch/hpc), the HPC cluster at the University of Bern.

References:
[1] Bird, G. A. (1994). Molecular gas dynamics and the direct simulation of gas flows.
[2] Carberry Mogan, S. R., et al. (2021). Icarus, 368, 114597.
[3] Carberry Mogan, S. R., et al. (2022). Journal of Geophysical Research: Planets, 127(11).
[4] Schlarmann, L., et al. (2024), in preparation.
[5] Föhn, M., et al. (2021), IEEE Aerospace Conference (50100). IEEE, 1-14.
[6] Waite Jr, J. H., et al. (2024). Space Science Reviews, 220.3, 30.