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
