Detecting Europa’s water plumes with the Particle Environment Package on JUICE

The repeated eruptions of water plumes on Europa have been suggested based on Hubble observations, Keck observations and in-situ magnetic field data from Galileo (Roth et al., 2014; Sparks et al., 2016, 2017, 2019; Jia et al., 2018; Arnold et al., 2019; Paganini et al., 2019). The possibility that such plumes could transport material from Europa’s subsurface, or from water reservoirs contained in the ice layer (Vorbuger and Wurz 2021), far above the surface creates an unprecedented opportunity to sample Europa’s subsurface environment and investigate its habitability. The JUpiter ICy moon Explorer (JUICE) is scheduled to make two flybys of Europa, one over the Northern and one over the Southern hemisphere, with the closest approach at 400 km altitude.

In this work we investigate the detectability of such water plumes using the Neutral and Ion Mass Spectrometer (NIM) and the ion mass spectrometer Jovian Dynamics and Composition analyser (JDC) of the Particle Environment Package (PEP) on JUICE. Using a Monte Carlo particle tracing model we simulate the density distribution of the plume and simulate the measured signature with NIM and JDC along the two JUICE flyby trajectories.

Using a particle tracing model we show that H₂O molecules and H₂O⁺ ions of the plume, as well as possible minor constituents such as CO and CH_4, can be detected during the JUICE flybys. We find that the plume reported by Roth et al., 2014 is the most likely to be detected, even at the lowest mass fluxes, and that the southern-hemisphere JUICE flyby has the best coverage of all the presumptive plume sources. Lowering the altitude of the southern flyby will contribute to an increased chance of detecting the presumptive plume sources, and should be prioritized over lowering the other flybys if any deltaV is available.

Additionally, using a DSMC molecule and particle tracing model we investigate the effect of intermolecular collisions in the plume and demonstrate that such collisions will reduce the detectability of the plume. We also show that the JUICE flybys and the NIM characteristics will be suitable to discern the finer structure of the plume (e.g. shocks inside the plume), which will allow us to improve our understanding of the physics of Europa’s plumes.

Furthermore, we also investigate the separability of the plume from Europa’s asymmetric sputtered and sublimated water atmosphere and discuss the influence of the instrument pointing and operations on the plume detectability. We find that NIM’s operational constraints are not critical in terms of detecting H₂O molecules of a plume.