Renegade moon

It came from outer space. Saturn’s outlier moon Phoebe didn’t coalesce from material near the ringed planet but instead was captured from the distant Kuiper belt, a reservoir of frozen bodies beyond Pluto. The Saturn-orbiting Cassini telescope has found new evidence for that scenario.

Astronomers have long considered Phoebe to be an oddball. Barely held in place by Saturn’s gravity, Phoebe orbits backward with respect to the rotation of the planet (SN: 6/19/04, p. 387: Available to subscribers at Portrait of Phoebe: Cassini images a large Saturn moon).

The moon’s measurements, taken by Cassini in June 2004 just before the probe entered orbit around Saturn, reveal that Phoebe’s density is similar to that of other known escapees of the Kuiper belt. Also, the moon’s relative proportion of rock and ice more closely resembles the composition of Pluto and of Neptune’s moon Triton than it does that of other satellites of Saturn, notes Jonathan Lunine of the University of Arizona in Tucson. Pluto and Triton are considered to have come from the Kuiper belt. Furthermore, spectra indicate that Phoebe’s surface bears material from comets or other denizens of the Kuiper belt.

Before they can be confident that Phoebe formed in the outer reaches of the solar system, however, scientists will have to confirm that the entire moon, not just its surface, carries the signature of Kuiper belt objects. Lunine and other researchers report their findings in the May 5 Nature.