Gravitational scattering within the Himalia group of jovian prograde irregular satellites

We test the hypothesis that gravitational scattering within the Himalia prograde irregular satellite group is responsible for the large velocity dispersion identified by Nesvorný et al. (2003, Astron. J. 126, 398-429). We carry out numerical simulations of the dynamical evolution of this group under several scenarios for Himalia's mass for 108yr. We find that modification of the satellite orbits is significant, 10-15% of their semimajor axes differences but <5% of the eccentricities. Additionally, the inclination of the satellite Lysithea may have been modified by a secular resonance with Himalia. We scale the observed orbital element diffusion to the age of the Solar System using a power law approximation. The projected changes in a and e can reduce the observable ejection speeds of Elara and Lysithea with respect to Himalia under the 100 m s−1 mark, rendering them consistent with hydrocode simulation results (Michel et al., 2002, Icarus 160, 10-23). The dispersion magnitude required to migrate Elara to its present orbit implies that Himalia's size has been underestimated by Cassini (Porco et al., 2003, Science 299, 1541-1547) and/or that its density is significantly higher than that of extensively fractured or “rubble-pile” bodies. In addition, Lysithea could have been created as recently as the last 500 Myr of the Solar System's history. Simple calculations indicate that such diffusion would also be evident in groups of satellites genetically related to Phoebe at Saturn, Caliban and Sycorax at Uranus and Nereid at Neptune.