Orbital Modification of the Himalia Family during an Early Solar System Dynamical Instability

Among the irregular satellites orbiting Jupiter, the Himalia family is characterized by a high velocity dispersion delta nu of several hundred m s(-1) among its members, inconsistent with a collisional origin. Efforts to account for this through internecine gravitational interactions do not readily reproduce this feature. Here, we revisit the problem in the context of recent cosmogonical models, where the giant planets migrated significantly through interaction with a planetesimal disk and suffered encounters with planetesimals and planet-sized objects. Our starting assumption is that family formation either predated this phase or occurred soon after its onset. We simulate numerically the diffusive effect of three distinct populations of perturbers on a set of test particles representing the family: Moon-sized (MPT) and Pluto-sized (PPT) planetesimals, and planetary-mass objects (PMO) with masses typical of icegiant planets. We find that PPT flybys are inefficient, but encounters with MPTs raise the delta nu of similar to 60% of our test particles to > 200 m s(-1) with respect to Himalia, in agreement with observations. As MPTs may not have been abundant in the disk, we simulate encounters between Jupiter and PMOs. We find that too few encounters generate less dispersion than MPTs while too many essentially destroy the family. For PMO masses in the range 5-20 m(circle plus), the family orbital distribution is reproduced by a few tens of encounters.