PHOEBE: COMPOSITIONAL CONSTRAINTS ON ORIGIN

Cassini’s 2004 fly-by of Phoebe on its way into orbit around Saturn provided the first close-up look at a member of an enigmatic class of outer solar systems bodies – the irregular satellites of the giant planets. Models for the origins of these satellites have ranged from co-formation with the giant planets and their regular satellites from circumplanetary material to capture from other regions of the outer solar system. Capture origins are currently favored by most dynamical studies of their orbital properties. The measured bulk density of Phoebe is 1630 +_33 kg m^-3, from mass determined by radio tracking of the spacecraft and volume measured by the Cassini imaging experiment. This density is about three times the density of regular Saturn satellites of the same size (~100 km radius), and significantly higher than the ~1300 kg/m^3 mean density of the larger regular satellites, suggesting that Phoebe’s composition has a ‘rock’ (silicate plus metals) to water ice ratio significantly higher that of the regular satellites. Johnson and Lunine (Nature, 435, pp 69-71, 2005) argued that Phoebe’s density was consistent with an origin in a CO-rich outer solar nebula rather than a reducing CH4-rich circumplanetary nebula where water ice should be more abundant. Since then ideas concerning the formation and evolution of the irregular satellites during early planetary migration as well as Saturn satellite and ring formation have become more complex. What remains clear however, is that Phoebe’s high density and inferred rock-ice ratio still place it more in family with denizens of the more distant outer solar system, including Pluto, Charon and other relatively high density Kuiper Belt Objects than with the lower density objects formed around Saturn including the Ring system and regular satellites.