Satellite systems around Jupiter and Saturn have different characteristics. The Saturnian system is dominated by a giant moon, Titan, which holds over 95% of the total mass of Saturnian moons. On the other hand, the Jovian system has four large moons in a compact orbital area, and inner three, Io, Europa and Ganymede, are known to be in resonant orbits. Previous studies have shown that the specific resonance can be obtained if there is some structure, such as inner cavity, in the satellite forming circum-Jovian disk. However, the mechanism making such a structure is poorly understood. We estimated the strengths of planetary magnetic fields for Jupiter and Saturn that are generated by dynamo process by calculating the thermal evolution of them. Saturn’s magnetic field is too weak to open a gap between Saturn and the circum-Saturnian disk, thus the moons in the inner orbits fall onto Saturn due to inward orbital migration. Our models show there are cases where a moon in an outer orbit evacuated into “safety zone” and survived till the disk dispersal. In the case of Jupiter, its strong enough magnetic fields generate magnetosphere around it, and moons migrating toward Jupiter stop at the outer edge of the magnetosphere during a certain period of the disk evolution. Our results suggest that Io, Europa and Ganymede should be captured into resonance while there exists a cavity around Jupiter and evolve as a system after the magnetosphere has vanished. We believe our scenario can explain the difference of Jovian and Saturnian moon systems.
BIO
Yuri Fujii is an assistant professor at The Graduate School of Human and Environmental Studies in Kyoto University. She completed her PhD at Nagoya University, Japan in 2015, and moved to ELSI at Tokyo Tech as a postdoc before joining to the theoretical astrophysics group at the Niels Bohr Institute in Denmark. She continued her study as research assistant professor at Nagoya University. She was appointed to her current position in January 2021. Her research interest is dynamics of protoplanetary and circumplanetary disks that are the birthplaces of planets and moons.
