Constraining Ion Precipitation onto Ganymede’s Surface with Backscattered Energetic Neutral Atoms

Ion precipitation onto Ganymede, shaped by interaction between the Jovian plasma and Ganymede’s magnetosphere, has been connected to brightness patterns and radiolysis products on its surface [1,2]. JUICE will measure ion fluxes in-situ at around 500 km altitude, leaving uncertainties for the precipitation patterns on the surface [3]. At Earth’s Moon, backscattered Energetic Neutral Atoms (ENAs) have been shown to be suitable for studying the ion-surface interaction from an orbiting spacecraft [4]. We now present the first modeling of ENAs created by backscattered H, O and S ions at Ganymede, which will enable JUICE to remotely observe the ion impacts. Using the SDTrimSP code [5], which has been verified for backscattered ENAs at the Moon [6, 7], we account for inputs of magnetospheric plasma precipitation from hybrid simulations [8] and Ganymede’s surface composition from telescopic observations [9].Our simulation results support that backscattering is an important formation process mainly for atomic H and O populations, whose properties are directly related to the precipitation conditions. Especially backscattered H ENAs dominate over any sputtered ENAs [10] above at least around 1 keV, making them ideal candidates for observing the plasma-surface interaction at Ganymede. Compared to lunar ENAs, backscattering probabilities are lower, but extended high-energy tails occur due to energetic ion populations in the Jovian plasma. The backscattering process thus creates neutral atom contributions that are candidates for observation with both JUICE’s JNA and JENI instruments. [1]          S. Fatemi, et al., Geophys. Res. Lett. 43 (2016), 4745.[2]          S.K. Trumbo, et al., Sci. Adv. 9 (2023), eadg3724.[3]          C. Plainaki , et al., Astrophys. J. 940 (2022), 186.[4]          Y. Futaana, et al., Gephys. Res. Lett. 40 (2013), 262.[5]          A. Mutzke, et al., IPP Report 2019-02 (2019).[6]          P.S. Szabo, et al., Geophys. Res. Lett. 49 (2022), e2022GL101232.[7]          P.S. Szabo, et al., J. Geophys. Res.: Planets 128 (2023), e2023JE007911.[8]          A.R. Poppe, et al., J. Geophys. Space Phys. 123 (2018), 4614.[9]          N. Ligier, et al., Icarus 333 (2019), 496.[10]       A. Pontoni, et al., J. Geophys. Space Phys. 127 (2022), e2021JA029439.