Callisto: Jupiter’s Mysterious Moon Resists Resonance

Scientists have proposed a new hypothesis explaining why Callisto, the outermost Galilean moon of Jupiter, does not participate in the Laplace resonance that links Io, Europa, and Ganymede. Traditionally, it is believed that the Laplace resonance (a 4:2:1 orbital period ratio among Io, Europa, and Ganymede) arose due to the migration of the moons under the influence of the gas disk that surrounded Jupiter during their formation. Callisto’s absence from this chain is usually explained by its late or slow formation, which hindered its migration. The new study offers an alternative explanation: the structure of the ‘jovian disk’ could have created a unique migration trap, isolating Callisto.

The authors conducted a series of N-body simulations, considering the interaction of satellites with the disk.

The simulation results showed that the presence of a high-pressure zone in the disk could function as a barrier, keeping Callisto on a specific orbit and preventing it from joining the resonance. At the same time, the parameters of the high-pressure zone must be within a certain range: a barrier that is too steep or flat will not yield the desired effect. The authors note that their work may shed light on uncertainties related to Callisto’s internal structure. For instance, data obtained by the Galileo spacecraft, which cannot definitively determine the contribution of non-hydrostatic factors to the satellite’s shape and gravitational field, remains a critical area of study.