Ganymede is one of the Galilean moons orbiting Jupiter. Besides being the largest moon in the Solar System (and larger than Mercury), it is also the only one known to generate internally a dipole magnetic field that is strong enough to create a region of closed magnetic field lines at low latitudes. Therefore, Ganymede generates effectively its own magnetosphere embedded inside that of Jupiter: a unique environment within the Solar System. Moreover, there is observational evidence that the Galilean moons (except for Io) bear a subsurface ocean, which makes them potential habitats for extraterrestrial life.
Currently, our knowledge of Ganymede builds primarily upon from relatively recent remote-sensing observations and a limited set of in situ measurements acquired during flybys of the NASA spacecraft ‘Galileo’ between 1996 and 2000. Our understanding of this moon is far from complete. In fact, very little is known regarding its geological history, subsurface ocean, neutral atmosphere, ionosphere and interaction between the moon’s and planet’s magnetic fields. For this reason, the European Space Agency has selected a mission, called JUpiter ICy moon Explorer (JUICE), that will perform detailed measurements of this moon by orbiting it.
In preparation for the upcoming JUICE mission, I have developed a model aimed at advancing our understanding of the neutral and plasma environments around Ganymede, with a focus on the ionosphere. The model is based on 3–D test particle simulations of ion species present in the moon’s magnetosphere, including ions from Jupiter’s magnetosphere and Ganymede’s ionosphere, and en- ergetic neutrals formed from the interaction between the neutral atmosphere and ionosphere. In particular, these constitute the first 3–D simulations of Ganymede’s ionospheric ions. The model generates a 3–D mapping of the plasma moments around the moon, including number density, bulk velocity, thermal and average kinetic energy for all species. It records individual test particle trajectories, and it generates a surface mapping of the average impact energy, and impact and sputtering rates.
The simulated ionosphere agrees well with Galileo in situ data in terms of the energy spectrogram, but differs from it concerning the electron number density along the spacecraft trajectory. Such a discrepancy is resolved by adapting the input exospheric configuration used in the model, suggesting that our current picture of Ganymede’s exosphere is incorrect. According to the simulations, the exospheric column density should be at least one order of magnitude higher compared to previous estimates. Furthermore, the model finds that ionospheric ions provide a major contribution to the surface sputtering, which is a new finding that needs to be addressed in our theory of the moon’s exosphere. Hence, this source needs to be taken into account in exospheric models.
The results from this model will be used as a tool in preparation for the JUICE mission and the model itself will serve as a useful tool for the interpretation of data acquired by the payload instruments on board the spacecraft.