Among Jupiter’s four Galilean moons, Callisto orbits at the greatest distance from the giant planet. With its ancient, heavily cratered surface, its still poorly understood internal structure, and probable subsurface ocean, Callisto is undoubtedly the most mysterious of the Galilean satellites to date. The flybys of the icy moon performed by the Galileo spacecraft, supplemented by remote observations, have provided insights into its characteristics, notably its tenuous atmosphere, dominated by O2 and CO2, its relatively dense ionosphere and its subsurface ocean. As it orbits through Jupiter’s magnetosphere, Callisto encounters varying magnetospheric environments with which it interacts through processes such as surface erosion and the formation of Alfvén wings. This variability in the magnetospheric environment at Callisto’s orbit is primarily driven by the tilt of the Jovian magnetic dipole relative to the gas giant’s rotation axis, resulting in significant excursions in magnetic latitude. Temporal variations in the magnetic field surrounding the Galilean moon trigger electromagnetic induction within its conductive layers, specifically its ionosphere and subsurface ocean. Measurements of the magnetic field induced in the latter by the Galileo mission’s magnetometer confirmed the existence of a liquid water layer beneath the icy crust.

With ESA’s JUICE and NASA’s Europa Clipper missions currently en route to Jupiter and its icy moons, and scheduled to perform numerous flybys of Callisto, it is essential to prepare for their arrival by combining observations and simulations. This approach will help advance our understanding of the icy moon and its interaction with Jupiter’s magnetosphere.

Following the Galileo mission in the late 1990s, the ongoing Juno mission became the second to orbit the giant planet. Although it did not perform any flybys of Callisto, unlike its predecessor, it crossed the moon’s orbit numerous times in its absence. These crossings, often referred to as “ghost flybys”, provide an opportunity to probe the pristine magnetospheric conditions the moon encounters along its orbital path. By combining measurements from magnetometers and particle detectors (electrons, heavy ions) on both missions, it is possible to quantify plasma and magnetic field parameters and study their variability. This work has highlighted significant variability linked to magnetic position, itself modulated by local time, with increased variability observed on Jupiter’s dawnside.

Simulations describing the interaction between Callisto’s neutral and ionized environments and the Jovian magnetosphere have also been developed. These use the parallelized LatHyS code, enabling 3D multi-species hybrid simulations of the interaction. Given the large gyroradii of ionospheric ions captured by the Jovian field at Callisto, a kinetic description of the plasma’s ionic component is essential to capture the asymmetries of the plasma interaction. Two key developments were achieved: a self-consistent model of ionospheric resistivity was developed, along with a coupling established between LatHyS and an exospheric model. Simulations performed with LatHyS highlight the diverse interaction modes experienced by Callisto depending on its magnetic position within the Jovian magnetosphere. The developed model will effectively support preparations for the upcoming flybys planned for the JUICE and Europa Clipper missions.

The presentation will be in English.

You are also warmly invited to a reception that will be held after the jury deliberation, in the IRAP-Roche cafeteria.

Hope to see you there or online!

Lieu
IRAP-Roche conference room (Toulouse)

Visio
https://visio.numerique.gouv.fr/mvd-xekl-lgt

Karine ISSAUTIER, Reviewer, Laboratoire d’Instrumentation et de Recherche en Astrophysique
Roch SMETS, Reviewer, Laboratoire de Physique des Plasmas
Vincent GENOT, Examiner, Institut de Recherche en Astrophysique et Planétologie
Mika HOLMBERG, Examiner, Dublin Institute for Advanced Studies
Lucas LIUZZO, Examiner, UC Berkeley Space Sciences Laboratory
Nicolas ANDRE, PhD supervisor, Institut Supérieur de l’Aéronautique et de l’Espace
Ronan MODOLO, PhD co-supervisor, Laboratoire Atmosphères, Observations Spatiales