Capture of Irregular Satellites during Planetary Encounters

Abstract

More than 90 irregular moons of the Jovian planets have recently been discovered. These moons are an enigmatic part of the solar system inventory. Their origin, which is intimately linked with the origin of the planets themselves, has yet to be adequately explained. Here we investigate the possibility that the irregular moons were captured from the circumsolar planetesimal disk by three-body gravitational reactions. These reactions may have been a frequent occurrence during the time when the outer planets migrated within the planetesimal disk. We propose a new model for the origin of irregular satellites in which these objects are captured from the planetesimal disk during encounters between the outer planets themselves in the model for outer planet migration advocated by Tsiganis and collaborators. Through a series of numerical simulations we show that nearby planetesimals can be deflected into planet-bound orbits during close encounters between planets, and that the overall efficiency of this capture process is large enough to produce populations of observed irregular satellites at Saturn, Uranus, and Neptune. The orbits of captured objects are broadly similar to those of known distant satellites. Jupiter, which typically does not have close encounters with other planets in the model of Tsiganis and coworkers, must have acquired its irregular satellites by a different mechanism. Alternatively, the migration model should be modified to accommodate Jupiter's encounters. Moreover, we find that the original size-frequency distribution of the irregular moons must have significantly evolved by collisions to produce their present populations. Our new model may also provide a plausible explanation for the origin of Neptune's large moon Triton.

Effects of the planetary migration on some primordial satellites of the outer planets

“…Our code checks this planet-planetesimal distance every 14 years. We define as t = t * the instant when a close approach between Uranus and a planetesimal occurs, such that the separation distance is ≤0.8 AU (Beaugé et al 2002;Nesvorný et al 2007;Nogueira 2008). Starting from t = t * , the planet-planetesimal system is integrated backward until t = t * * , when the separation distance becomes greater than 1.0 AU.…”

“…It is clearly expected that the closer the satellite, the tighter the link to the planet. Tsiganis et al (2005), Nesvorný et al (2007), and others, found that the regular satellites undergo only minor perturbations during the planetary close encounters. The name "regular" satellite, in general, is given to those very close to the planet of small eccentricity and inclination.…”

“…Although the original Nice-I model might not be adequate to reconcile the current status of the terrestrial planets and the LHB history (provoked by this migration theory), it has been tested in several scenarios, reproducing many observational aspects of our solar system Nesvorný et al 2007;Gomes et al 2005). Therefore, we consider only Nice-I in this work, for which the important ingredient is the strong bombardment of the planetesimals on the planets related to some violent close encounters between the planets.…”

“…Moreover, as pointed out in Nesvorný et al (2007) there is no reason to believe that the only massive objects to have been accreted into the disk were the four giant planets.…”

“…Following Fernandez & Ip (1996), many papers about, for instance, Trojans, asteroids, Plutinos, and KBOs, have appeared. However, as mentioned in Nesvorný et al (2007), in our race to understand, at least part of the evolutionary history of our system, the population of the satellites occupies a position of major interest. According to these authors, the study of the satellites can provide information that cannot be obtained from any other source.…”