DPS 2001 meeting, November 2001
Session 49. Outer Planet Satellites
Oral, Chairs: R. Johnson, C. Dumas, Friday, November 30, 2001, 4:30-5:50pm, Regency GH

## [49.01] On the Orbital Distribution of Irregular Satellites

J.A. Burns, V. Carruba, P.D. Nicholson (Cornell University), B.J. Gladman (Obs. de la Cote d'Azur), M.J. Holman (SAO)

Clusters of irregular satellites, moons that occupy large orbits of significant eccentricity {\em e} and/or inclination {\em I}, circle Jupiter, Saturn and Uranus. The irregulars often extend close to the orbital stability limit, about 1/3-1/2 of the way to the edge of their planet's Hill sphere. These distant, elongate and inclined orbits suggest capture, perhaps by energy loss (gas drag by a circumplanetary nebula or collisions) or by rapid planetary growth. Capture, however, should provide random orbits and yet no known irregulars have inclinations (relative to the ecliptic) between 47\circ and 141\circ. Secular (Kozai) perturbations allow two kinds of high inclination ({\em I} >39.2\circ) orbits: circulating and librating. In circulating orbits, the argument of pericenter {\em \omega} sweeps through all possible values (0-2\pi). In contrast, for libration, {\em \omega} oscillates around 90\circ (or 270\circ).

Circulating objects suffer appreciable periodic changes in eccentricity, and reach a maximum value of their eccentricity when \omega=90\circ). As a result, particles of high inclination (~70\circ<{\em I} < 110\circ) are found to penetrate the realm of the regular satellites where collisions and scatterings are likely to remove them from planetocentric orbits. In addition, long-term (109~yr) orbital integrations of hypothetical satellites show that solar and planetary perturbations considerably broaden this zone of avoidance to ~55\circ<{\em I} < 130\circ. Such objects either escape, or collide with their parent planet.

Librating bodies can in theory be stable for larger values of {\em I}. However, our integrations show that tidal perturbations from the Sun and the other jovian planets also restrict the region of phase space accessible to these bodies. Interactions with the Galilean satellites will reduce it further. Several N-body simulations have allowed us to estimate the size of the region where librating objects can be stable for periods of ~107-108 years.