Simulations of Energetic Neutral Atom sputtering from Ganymede in
preparation for the JUICE mission
Abstract
Jovian magnetospheric plasma irradiates the surface of Ganymede and is
postulated to be the primary agent that changes the surface brightness
of Ganymede, leading to asymmetries between polar and equatorial regions
as well as between the trailing and leading hemispheres. As impinging
ions sputter surface constituents as neutrals, ion precipitation
patterns can be remotely imaged using the Energetic Neutral Atoms (ENA)
measurement technique. Here we calculate the expected sputtered ENA flux
from the surface of Ganymede to help interpret future observations by
ENA instruments, particularly the Jovian Neutral Analyzer (JNA) onboard
the JUpiter ICy moon Explorer (JUICE) spacecraft. We use sputtering
models developed based on laboratory experiments to calculate sputtered
fluxes of H2, O2, and H2O. The input ion population used in this study
is the result of test particle simulations using electric and magnetic
fields from a hybrid simulation of Ganymede’s environment. This
population includes a thermal component (H+ and O+ from 10 eV to 10 keV)
and an energetic component (H+, O++, and S+++ from 10 keV to 10 MeV). We
find a global ENA sputtering rate from Ganymede of 1.42x10^27
s^-1, with contributions from H2, O2 and H2O of 34%, 17%, and 49%
respectively. We also calculate the energy distribution of sputtered
ENAs, give an estimate of a typical JNA count rate at Ganymede, and
investigate latitudinal variations of sputtered fluxes along a simulated
orbit track of the JUICE spacecraft. Our results demonstrate the
capability of the JNA sensor to remotely map ion precipitation at
Ganymede.