Assessing JUICE's ability of in situ plume detection in Europa's atmosphere

Abstract

ESA's upcoming JUpiter Icy moons Explorer (JUICE) is scheduled to perform two flybys of Jupiter's moon Europa. Here, we present the results of our investigation into JUICE's capability to detect plumes and to separate them from the background water in the atmosphere as a function of their source's location and mass flux on the Europan surface using the neutral mass spectrometer NIM. For both of the currently planned flybys we evaluate different scenarios to estimate how feasible such a detection is, using a Monte Carlo particle tracing model of a plume. We also incorporate the putative plume candidates reported by Roth et al. (2014b), Sparks et al. (2016), Sparks et al. (2017), Jia et al. (2018), Arnold et al. (2019) and Sparks et al. (2019) into this investigation. We come to the conclusion that the first flyby trajectory is better suited to detect the potential plume candidates listed above as they are detectable down to mass fluxes of approx. 100 ​kg/s. The second flyby requires mass fluxes between 100 ​kg/s and 500 ​kg/s for all suggested plume candidates to be detectable. This discrepancy is caused by the first flyby's closer proximity to the locations of the suggested plume sources which lie mainly on the southern hemisphere. Separation of water plumes from the atmospheric water vapour density is dependent on the mass flux of the respective plume source. The only plume separable from the background water in the atmosphere at the lowest investigated plume source mass flux of 1 ​kg/s is the one suggested by Roth et al. (2014b). It is therefore the most likely plume to be detected by JUICE. Furthermore, we investigate the detectability of trace gas contents in a plume and analyse how these impact upon plume detection. The spacecraft will be able to detect whether lighter-than-water trace gas species are present within the plume down to trace gas source mass fluxes of 0.1 ​kg/s. Provided this is the case, such species constitute a means to identifying plumes in the Europan atmosphere. Finally, the effects of lowering the flyby altitude are examined. We find that lowering the altitude of the CA does not increase the region of separability. Therefore, lowering the second flyby trajectory will not suffice for the spacecraft to be able to identify plumes that originate from the locations of the candidates mentioned above. Considering that the second flyby is the most likely to be lowered, this case study provides an example of the importance of arranging the second trajectory in a manner such that its ground track lies in close proximity to the locations of the suggested plume candidates listed above. This approach to the mission planning maximises the chance of separating the presumptive plumes from the atmospheric background water even at the lowest plume source mass fluxes.