Irregular Satellites of the Giant Planets

The irregular satellites of the outer planets, whose population now numbers over 100, are likely to have been captured from heliocentric orbit during the early period of solar system history. They may thus constitute an intact sample of the planetesimals that accreted to form the cores of the jovian planets. Ranging in diameter from ~2 km to over 300 km, these bodies overlap the lower end of the presently known population of transneptunian objects (TNOs). Their size distributions, however, appear to be significantly shallower than that of TNOs of comparable size, suggesting either collisional evolution or a size-dependent capture probability. Several tight orbital groupings at Jupiter, supported by similarities in color, attest to a common origin followed by collisional disruption, akin to that of asteroid families. But with the limited data available to date, this does not appear to be the case at Uranus or Neptune, while the situation at Saturn is unclear. Very limited spectral evidence suggests an origin of the jovian irregulars in the outer asteroid belt, but Saturn’s Phoebe and Neptune’s Nereid have surfaces dominated by water ice, suggesting an outer solar system origin. The short-term dynamics of many of the irregular satellites are dominated by large-amplitude coupled oscillations in eccentricity and inclination and offer several novel features, including secular resonances. Overall, the orbital distributions of the irregulars seem to be controlled by their long-term stability against solar and planetary perturbations. The details of the process(es) whereby the irregular satellites were captured remain enigmatic, despite significant progress in recent years. Earlier ideas of accidental disruptive collisions within Jupiter’s Hill sphere or aerodynamic capture within a circumplanetary nebula have been found wanting and have largely given way to more exotic theories involving planetary migration and/or close encounters between the outer planets. With the Cassini flyby of Phoebe in June 2004, which revealed a complex, volatile-rich surface and a bulk density similar to that of Pluto, we may have had our first closeup look at an averagesized Kuiper belt object.