Three-Body Capture of Jupiter's Irregular Satellites and Resonant History of the Galilean Satellites
Philpott, Catherine Mary
Hamilton, Douglas P.
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We examine the capture of small, irregular satellites, which, with their distant, eccentric, and inclined paths, must have originated in heliocentric orbits. We investigate a new theory: capture of one member of a pair of ∼100-km asteroids after tidal disruption. The energy loss from disruption is sufficient for capture, but it cannot deliver the bodies directly to the currently observed orbits. Instead, the long-lived capture orbits must evolve inward after capture, perhaps due to interactions with a tenuous circumplanetary gas disk. We find that at Jupiter, binaries offer an increase of a factor of ∼10 in the capture rate of 100-km objects as compared to single bodies, for objects separated by tens of radii that approach the planet on relatively low-energy trajectories. These bodies are at risk of collision with Callisto, but may be preserved by gas drag if their pericenters are raised quickly enough. We conclude that our mechanism is as capable of producing large irregular satellites as previous suggestions, and it avoids several problems faced by alternative models. To investigate possible source populations for these captured satellites, we simulated escaping asteroids from Jupiter's Trojan region and the outer main belt, calculating the Jacobi constant during close approaches and comparing with three-body capture statistics. We found that Trojans' high approach speeds make them unlikely source bodies, but asteroids from the outer main belt, especially those interior to Jupiter's 4:3 resonance, approach with low speeds that favor capture. Unlike irregular satellites, regular satellites formed with their planets. Gravitational resonances are important for these bodies, and we study the most famous of them. Io, Europa, and Ganymede are in the Laplace resonance, meaning that they have orbital periods in the ratio of 1:2:4. We focused our work on Io and Europa's orbital lock and modeled passage through the 2:1 resonances. We discovered cases where damping from satellite tides led to stable equilibria prior to capturing into the resonances. The mean-motion ratio at which this occurs matches that of Io and Europa. We conclude that the moons never captured into resonance, and that their resonant angles librate because of long-range resonant forcing.