Three-Body Capture of Jupiter's Irregular Satellites and Resonant History of the Galilean Satellites

Abstract

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.