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Callisto is often perceived as a boring and archaic satellite when compared to its seemingly more exciting neighbors: Io is the most volcanically active body in the solar system (Lopes, 2014 and references therein), Europa harbors a global subsurface ocean with features conducive to biology (Hand et al., 2009 and references therein), and Ganymede is the only known satellite in the solar system to possess an intrinsic magnetic field (Kivelson et al., 1996). Moreover, Io and Europa have extremely young surfaces, and Ganymede has many diverse terrains and landforms. In contrast, images of Callisto taken during Voyager flybys revealed a surface sculpted by impacts, making it the most heavily cratered Galilean satellite (Smith, Soderblom, Johnson, et al., 1979;Smith, Soderblom, Beebe, et al., 1979) and one of the oldest surfaces in the solar system. This implied that Callisto's surface was devoid of any active endogenic processes. The stigma of Callisto being an old, frozen, and geologically dead moon persisted until Galileo revealed that it is home to several exciting processes and yet unresolved mysteries of its own. For example, high-resolution images of Callisto taken by Galileo showed a surprisingly near absence of small craters on the surface, implying that, contrary to the aforementioned assumptions made about Callisto's endogenic activity, small crater degradation processes are occurring (Moore et al., 1999). Moore et al. (1999) demonstrated that these features were a result of sublimation-driven landform modification and mass wasting, and suggested Callisto possessed the most degraded surface of the icy Galilean satellites. A recent investigation by Stephan et al. (2020) of the size-distribution of water ice particles on the surface underlined the importance of temperature-driven processes for the physical properties of Callisto's surface, similar to its inner neighbor Ganymede.Although Callisto and Ganymede are of similar size and bulk composition, there are several dichotomies between the two bodies that make for an interesting comparison. While the surface of Ganymede features more diverse and complex terrains (Schenk, 1995), the surface of Callisto, in addition to being the oldest and most heavily

<p>The JUpiter ICy moons Explorer (JUICE) of the European Space Agency will investigate Jupiter and its icy moons Europa, Ganymede, and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of habitable worlds around gas giants. The Particle Environment Package (PEP) on JUICE is designed to measure neutrals and ions and electrons at thermal, suprathermal, and radiation belt energies (eV to MeV). </p><p>In the vicinity of Callisto, PEP will characterize the plasma environment, the outer parts of Callisto's atmosphere and ionosphere and their interaction with Jupiter's dynamic magnetosphere. About 20 Callisto flybys with closest approaches between 200 km and 5000 km altitude are<br>planned over the course of the JUICE mission. In this presentation, we review the state of knowledge regarding Callisto's ambient environment and magnetospheric interaction with recent modeling efforts for Callisto's atmosphere and ionosphere to identify science opportunities for the PEP observations and to optimize scientific insight gained from the foreseen JUICE flybys. These considerations inform science operation planning of PEP and JUICE and they will guide future model development for the atmosphere and ionosphere of Callisto and their interactions with the plasma environment.</p>

<p>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.</p> <p>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.</p> <p>Using a particle tracing model we show that H<sub>2</sub>O molecules and H<sub>2</sub>O<sup>+</sup> ions of the plume, as well as possible minor constituents such as CO and CH<sub>4,</sub> 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.</p> <p>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.</p> <p>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 H<sub>2</sub>O molecules of a plume.</p>

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