Umeå University's logo

We present new model results for H2O, O2, H2, O, and H in the atmosphere of Ganymede. The results are obtained from a collision-less 3D Monte-Carlo model that includes sublimation, ion and electron sputtering, and ion and electron radiolysis. Because Ganymede has its own magnetic field, its immediate plasma environment is particularly complex. The interaction between Ganymede's and Jupiter's magnetospheres makes it highly variable in both space and time. The recent Juno Ganymede flyby provided us with new data on the electron local environment. Based on the electron measurements recorded by the Jovian Auroral Distributions Experiment (JADE), we implement two electron populations, one for the moon's polar regions and one for the moon's auroral regions. Comparing the atmospheric contribution of these newly defined electron populations to the overall source and loss processes is one of the main goals of this work. Our analysis shows that for H2O, sublimation remains the most important source process even after accounting for the new electron populations, delivering more than three orders of magnitude more H2O molecules to the atmosphere than all other source processes combined. The source fluxes for O2 and H2, on the other hand, are dominated by radiolysis induced by the auroral electrons, assuming that the electron fluxes JADE measured during Juno's transit of Ganymede's magnetopause current layer are representative of auroral electrons. Atomic O and H are mainly added to the atmosphere through the dissociation of O2 and H2, which is primarily induced by auroral electrons. Our understanding of Ganymede's atmosphere today is mainly based on spectroscopic observations. The interpretation of spectroscopic data strongly depends on assumptions taken, though. Our analysis shows that for a holistic understanding of Ganymede's atmosphere, simultaneous observations of the moon's surface, atmosphere, and full plasma environment (thermal and energetic ions and electrons) at different times and locations (both with respect to Ganymede and with respect to Jupiter) are particularly important. Such measurements are planned by ESA's Jupiter ICy moons Explorer (JUICE), in particular by the Particle Environment Package (PEP), which will greatly advance our understanding of Ganymede and its atmosphere and plasma environment.

In this paper we present ab initio 3D Monte-Carlo simulations of Ganymede's surface sputtered and sublimated H₂O exosphere. As inputs, we include surface water content maps and temperature distribution maps based on Galileo and Very Large Telescope (VLT) observations. For plasma precipitation, we use hybrid model results for thermal H⁺ and O⁺, energetic H⁺, O⁺⁺, S⁺⁺⁺, and electrons, with unprecedented energy resolution. Our results show that up to a solar zenith angle of ∼60° and up to ∼600 km altitude, sublimated H₂O dominates the atmosphere by up to four orders of magnitudes in number density, while sputtering dominates elsewhere. Sputtering is mainly induced by the impinging O⁺, O⁺⁺, and S⁺⁺⁺ ions, while protons (H⁺) and electrons only add about 1% of the total sputtered H₂O molecules to the atmosphere. Electrons are thus not important for the generation of the atmosphere, but they are important for spectroscopic observability of the atmosphere since they are the main inducer of the Lyman-α and O I emission lines. The extended H₂O atmosphere at altitudes ≳1 Ganymede radius is mainly the result of sputtering by thermal O⁺ ions, which is the only ion species with substantial fluxes in the low-energy range (10 eV–10 keV), i.e., is the only species that efficiently induces nuclear sputtering. Most released H₂O molecules return to the surface where they immediately adsorb, not forming a thermalized atmosphere. The morphology of Ganymede's magnetosphere, and the resulting dichotomies in the surface fluxes of the precipitating magnetospheric particles (polar fluxes > equatorial fluxes and leading equatorial fluxes > trailing equatorial fluxes), are thus well discernible in the sputtered atmosphere, persisting up to altitudes of a few thousand kilometers. In-situ measurements, as they are planned for the upcoming JUpiter ICy Moons Explorer (JUICE) mission, will mainly probe this sputtered atmosphere, except for encounters with the near-surface atmosphere on Ganymede's day-side, where the sublimated atmosphere will be probed instead. Finally, we compare our model results to the first observational evidence for a sublimated H₂O atmosphere on Ganymede, and find a very good agreement.

We study the dynamics of the thermal O+ and H+ ions at Ganymede's magnetopause when Ganymede is inside and outside of the Jovian plasma sheet using a three-dimensional hybrid model of plasma (kinetic ions, fluid electrons). We present the global structure of the electric fields and power density (E ⋅ J) in the magnetosphere of Ganymede and show that the power density at the magnetopause is mainly positive and on average is +0.95 and +0.75 nW/m3 when Ganymede is inside and outside the Jovian plasma sheet, respectively, but locally it reaches over +20 nW/m3. Our kinetic simulations show that ion velocity distributions at the vicinity of the upstream magnetopause of Ganymede are highly non-Maxwellian. We investigate the energization of the ions interacting with the magnetopause and find that the energy of those particles on average increases by a factor of 8 and 30 for the O+ and H+ ions, respectively. The energy of these ions is mostly within 1–100 keV for both species after interaction with the magnetopause, but a few percentages reach to 0.1–1 MeV. Our kinetic simulations show that a small fraction ((Formula presented.) 25%) of the corotating Jovian plasma reach the magnetopause, but among those >50% cross the high-power density regions at the magnetopause and gain energy. Finally, we compare our simulation results with Galileo observations of Ganymede's magnetopause crossings (i.e., G8 and G28 flybys). There is an excellent agreement between our simulations and observations, particularly our simulations fully capture the size and structure of the magnetosphere.

Plumes spewing water high above Europa's surface have been inferred from several observation campaigns in the past decade. Whereas the occasional existence of plumes on Europa has thus been confirmed, the origin of the plumes remains uncertain. Most notably, it is still unclear whether the inferred plumes are of oceanic origin, possibly containing information about the ocean's habitability, or if they are of surficial nature, offering information on the highly processed surface instead. In this study, we use a Monte-Carlo model to analyze three different plume models, two of which are surficial (near-surface liquid inclusion and diapir), and one of which originates in the sub-surface ocean (oceanic plume). We analyze all relevant Lyman- (Formula presented.) and OI 1304 (Formula presented.) emission mechanisms to determine the three models' emission profiles. These profiles are then compared to the Hubble Space Telescope/Space Telescope Imaging Spectograph measurements presented by Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) with the goal of determining which ab initio model fits these measurements best. Our analyses show that all three models investigated produce similar Lyman- (Formula presented.) and OI 130.4 nm emission profiles, with differences being perceivable only on scales well below the HST resolution and sensitivity. Since none of the models contradict the Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) observations, none of them can be ruled out as being the acting force behind the observed plumes. The currently available optical measurements are thus not sufficient to determine the physical nature of Europa's plumes, identification of which can only be achieved through higher resolution images or in situ measurements.