Emission of energetic neutral atoms from water ice under Ganymede surface-like conditions

Abstract

a b s t r a c tThe co-rotating plasma around Jupiter precipitates on the surfaces of the jovian moons, where it is not hindered by a local magnetic field. Precipitating ions lead to the emission of energetic neutral atoms, which are produced via backscattering and sputtering processes, from the surface. The European Space Agency's JUICE mission to Jupiter carries as part of the Particle Environment Package experiment an imaging energetic neutral atom spectrometer called the jovian Neutrals Analyzer (JNA). When it is in orbit around Ganymede, JNA will measure the energetic neutral atom flux emitted from the surface of Ganymede in the energy range from 10 eV to 3300 eV. The surface of Ganymede consists of a large fraction of water ice. To characterize the expected energetic neutral atom fluxes from water ice due to precipitating jovian plasma, we impacted protons and singly charged oxygen ions with energies up to 33 keV on a salty water ice target kept at Ganymede surface conditions. Emitted energetic atoms were measured energy-and mass-resolved using the JNA prototype instrument. The data show high yields for energetic neutral atoms per incident ion in the JNA energy range. For incident protons, energetic neutral atom yields between 0.28 at 1 keV and ∼40 at 33 keV were observed. For incident singly charged oxygen ions, the observed energetic neutral atom yield ranged from 0.8 for at 3 keV to ∼170 at 23 keV.

Simulations of Energetic Neutral Atom Sputtering From Ganymede in Preparation for the JUICE Mission

“…Wieser et al. (2016) provides a good argument for the use of Thompson‐Sigmund in our case as they successfully used it to fit the energy spectra, measured using a JNA prototype, of molecules sputtered from water ice under Ganymede surface‐like conditions. Molecules with energies of up to 1.3 keV were observed under a 33 keV O + ion beam and followed a Thompson‐Sigmund distribution.…”

“…Backscattering yields are not well modeled, although studies by Wieser et al. (2016) and Futaana et al. (2012) suggest that a yield of about 0.1–0.2 can be applied for low (∼keV) energies.…”

“…Measurements both in laboratories and in space suggest that backscattered particles would have energies in the range that JNA can measure, but distributed according to a Maxwell-Boltzmann-like law rather than the Thompson-Sigmund law applicable to sputtering (Futaana et al, 2012;Wieser et al, 2016). Backscattering yields are not well modeled, although studies by Wieser et al (2016) and Futaana et al (2012) suggest that a yield of about 0.1-0.2 can be applied for low (∼keV) energies. The majority of ENAs in the 10 eV -1 keV range are produced by the sputtering process (by high energy particles), so the backscattered contribution to the total ENA spectra is expected to be small.…”

“…Thus ENA measurements can and have been used to remotely map ion precipitation on airless bodies, where in situ precipitating ion observation is not easily possible. For example, backscattered solar wind protons and sputtered oxygen atoms were observed at the Moon by the Interstellar Boundary EXplorer (IBEX) (Allegrini et al., 2013; McComas et al., 2009) and by CENA on Chandrayaan‐1 (Futaana et al., 2013; Vorburger et al., 2014; Wieser et al., 2016). Ganymede will also be visited by an ENA instrument: the Jovian Neutrals Analyzer (JNA) will fly on‐board the JUpiter ICy moon Explorer (JUICE) spacecraft.…”

Callisto's Atmosphere and Its Space Environment: Prospects for the Particle Environment Package on Board JUICE

“…The sputtering and backscattering processes are two main sources of ENA production. In the Jovian magnetic field, due to the high‐energy penetrating fluxes, the sputtering process is expected to be a dominant source (Wieser et al., 2016). The sputtered ENA emission from Ganymede was simulated (Pontoni et al., 2022), showing that the sputtered ENAs from the surface can be detected with JNA.…”