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J.A. Burns (Cornell U.), B.J. Gladman (Observatoire de la Cote d'Azur), P.D. Nicholson, R.A. Jacobson, V. Carruba (Cornell U.), M.J. Holman (SAO), JJ Kavelaars (McMaster U.)
Each of the giant planets has irregular satellites, moons that occupy large orbits of significant eccentricity e and/or inclination i. Since 1997, when only 10 irregular satellites were known, 29 new ones have been discovered, allowing the dynamical structure of these systems to be discerned. The irregulars often lie close to the orbital stability limit, about 1/2-1/3 of the way to the edge of their planet's Hill sphere. The distant, elongate and inclined orbits suggest capture, perhaps by energy loss (gas drag by a circumplanetary nebula or collisions) or by rapid planetary growth. No known irregulars have inclinations (relative to the ecliptic) between 47 and 140 deg. Due to the Kozai mechanism (secular solar perturbations), high inclination orbits (~70 deg < i < 110 deg) suffer appreciable periodic changes in eccentricity, driving particles deep into the realm of the regular satellites where collisions and scatterings are likely to remove them from planetocentric orbits; times of 107-109 yrs are required. Timescales today for mutual collisions and for cometary impacts are much greater than the solar system's age. By carrying out long-term integrations of the orbits for a variety of hypothetical satellites, we demonstrate that periodic solar perturbations and resonant effects will considerably broaden this zone of avoidance. The orbits of many irregulars fall into tight clusters, reminiscent of asteroid families, suggesting that the objects originate from fragmentation of larger bodies. The size distributions also favor collisional break-up. Earlier studies have shown some Jovian irregulars are trapped in secular resonances, into which they were presumably captured during slow orbital evolution. However, other properties of the irregular distribution are inconsistent with much orbital evolution.
Supported by NASA.