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Old and cold. Primitive substances on Phoebe, Saturn's most distant moon, suggest that it arose among the solar system's comets.

Phoebe's Dark, Chilly Birth

Saturn's misfit moon Phoebe may be an intruder from our solar system's outermost frontier. A diverse brew of primitive compounds on Phoebe's surface suggests that the tiny moon was born among a remote reservoir of cometlike bodies and eventually drifted into Saturn's gravitational field, where it has remained ever since.

The most distant of Saturn's 34 known satellites, Phoebe swoops around the planet in a tilted orbit, opposite to the direction of Saturn's spin. That eccentric path made researchers suspect that Phoebe formed elsewhere, before wandering into the influence of Saturn's gravity. Striking images of Phoebe from NASA's Cassini probe, taken last June as the spacecraft first approached Saturn, supported that view by revealing an icy battered world, perhaps rich in the dark organic matter of comets (ScienceNOW, 14 June 2004). Now, detailed analyses from the encounter strengthen the case for Phoebe's chilly birth.

Cassini's spectrometer mapped unusual ingredients on the satellite's surface, according to a team led by planetary scientist Roger Clark of the U.S. Geological Survey in Denver, Colorado. The team found ices and minerals bearing water, iron, complex organic materials, and exotic cyanide compounds not previously seen on other solar-system objects. This mixture resembles the composition of ancient bodies from cold distant orbits--such as Comet Borrelly, which another NASA probe flew past in 2001 (ScienceNOW, 18 December 2001).

In a complementary study, planetary scientists Terrence Johnson of NASA's Jet Propulsion Laboratory in Pasadena, California, and Jonathan Lunine of the University of Arizona, Tucson, used Cassini data to calculate the relative proportions of rock and ice inside Phoebe. The make-up is quite different from that deduced for Saturn's other moons, the researchers conclude. Instead, Phoebe's interior mimics that of Neptune's large moon Triton and Pluto, both at the solar system's outskirts. Both teams report their findings in the 5 May issue of Nature.

Together, the studies finger Phoebe as a likely relic from the distant swarm of frozen cometary bodies called the Kuiper Belt, says planetary scientist J. Brad Dalton of the SETI Institute at NASA's Ames Research Center in Mountain View, California. Researchers who model the early history of the solar system would like to know Phoebe's exact birthplace, but that's unlikely, Dalton notes: The outer planets migrated during that era and quickly scattered Kuiper Belt objects into new orbits.

Related sites
Background on Phoebe
Cassini-Huygens mission
Images and mission diary from Cassini