On the in-situ detectability of Europa's water vapour plumes from a flyby mission

We investigate the feasibility of detecting water molecules (H₂O) and water ions (H₂O⁺) from the Europa plumes from a flyby mission. A Monte Carlo particle tracing method is used to simulate the trajectories of neutral particles under the influence of Europa's gravity field and ionized particles under the influence of Jupiter's magnetic field and the convectional electric field. As an example mission case we investigate the detection of neutral and ionized molecules using the Particle Environment Package (PEP), which is part of the scientific payload of the future JUpiter ICy moon Explorer mission (JUICE). We consider plumes that have a mass flux that is three orders of magnitude lower than what has been inferred from recent Hubble observations (Roth et al., 2014a). We demonstrate that the in-situ detection of H₂O and H₂O⁺ from these low mass flux plumes is possible by the instruments with large margins with respect to background and instrument noise. The signal to noise ratio for neutrals is up to ∼5700 and ∼33 for ions. We also show that the geometry of the plume source, either a point source or 1000km-long crack, does not influence the density distributions, and thus, their detectability. Furthermore, we discuss how to separate the plume-originating H₂O and H₂O⁺ from exospheric H₂O and H₂O⁺. The separation depends strongly on knowledge of the density distribution of Europa's exosphere.