The migration of the Galilean satellites in the circumjovian disk

Abstract

Various physical and orbital properties of the Galilean satellites, such as the decreasing density with distance and the Laplace resonance, imply an intriguing formation history of the system. With prograde orbits of low inclinations and eccentricities, the satellites are thought to be formed in a circumjovian disk composed of gas and solids. Satellites grew by accreting solids in the disk, and migrated due to disk-satellite interaction, both of which depend on disk properties. In this thesis, the migration of the Galilean satellites was studied, and how satellite migration can possibly help in assembling the satellites into their current Laplace configuration was also investigated.

The gas-starved subnebula model by Canup & Ward (2002) and its modified version proposed by Turner et al. (2013) (TLS13) were adopted in this study. Supplied by inflowing materials from the ambient solar nebula, the disk profiles are determined by disk viscosity, disk opacity and the inflow timescale of the supply. Including a temperature-dependent opacity model, the TLS13 disk has non-smooth disk profiles, producing sharp changes in the profile gradients. The disk was assumed to deplete with the dissipation of the solar nebula.

Non-isothermal type I regime was adopted in the migration of the satellites. Potential saturation of the corotation torque was also taken into account in the study. It was found that outward migration is possible in our disks, which contrasts with the solely inward migration in the isothermal regime, and that exist positions where the disk torque is zero and satellite migration is stalled. These zero-torque positions evolve with the disk. The non-smooth profile gradients of the TLS13 disk produce multiple zero-torque positions, resulting in interesting migration of the satellites.

Using the SyMBA symplectic integrator, simulations for the migration of the satellites were conducted for various disk models and migration regimes. The effect of satellite growth and an inner cavity in the disk on satellite migration was included in this study. To begin with, single-satellite runs were conducted to study the migration behaviours of the individual satellites. Initial locations of the satellites were searched from the results of single-satellite runs, in which Europa has to cross Io’s orbit in most cases. Simulations with multiple satellites were then conducted to search for ways for the satellites to arrive at their current orbital configuration. Various satellite interactions were investigated, including the convergence of Europa and Ganymede into a 2:1 resonance and the effects of earlier-generation satellites. At last, the migration of multiple satellites in a disk with an inner cavity was explored, in which systems with similar orbital configuration as the real Galilean satellites could be reconstructed. Despite this, assembling the satellites into the Laplace resonance by migration due to interaction with a circumjovian disk was found to be difficult, probably due to the intrinsically faster migration of Io over Europa arising from differences in mass and location between the two satellites.