A narrowly missed four-body resonant chain among the Galilean moons

The Galilean moons of Jupiter present one of the most peculiar orbital configurations in the solar system: a 4:2:1 three-body resonance between Io, Europa, and Ganymede, known as Laplace resonance. The outer moon, Callisto, is not involved in the Laplace resonance, but it is currently very close to a 7:3 resonance with Ganymede. Considering the estimated rate of the moons' orbital migration, this 7:3 resonance was encountered approximately two million years ago. To assess the effect of this resonance crossing, we performed accurate numerical simulations of the moons' orbital evolution over the last few million years. We found that the dynamical pathway that best reproduces the current configuration of the system requires that Ganymede and Callisto were not captured into the 7:3 resonance, not even temporarily. In this scenario, the moons started with eccentricities slightly larger than today's, which decreased due to the resonance crossing, eventually matching their present-day values. Under different initial conditions, capture into the 7:3 resonance would have been almost unavoidable, and today we could admire a four-body resonant chain between the Galilean satellites. However, had the moons been captured and subsequently exited the resonance, their eccentricities would have increased far too large, making such an evolutionary path incompatible with the observed system. Nevertheless, the crossing of the 7:3 resonance explains orbital features that have remained a mystery until now, such as the libration amplitude of the Laplace resonance. Moreover, we found evidence of a three-body resonance crossing among the three outer moons that occurred just 20,000 years ago.