Scientists believe bodies like Phoebe were plentiful in the outer reaches of the solar system about four and half billion years ago.
These icy planetesimals (small bodies) formed the building blocks of the outer solar system and some were incorporated into the giant planets Jupiter, Saturn, Uranus and Neptune. During this process, gravitational interactions ejected much of this material to distant orbits, joining a native population of similar bodies to form the Kuiper Belt.
It has been a longstanding question whether Phoebe is a captured Kuiper Belt Object. It's backward orbit and small size suggest so. The Cassini observations are the best look ever at such an object.
"All our evidence leads us to conclude, Phoebe's surface is made of water ice, water-bearing minerals, carbon dioxide, possible clays and primitive organic chemicals in patches at different locations on the surface," said Roger N. Clark, team member for the Visual and Infrared Mapping Spectrometer, U.S. Geological Survey in Denver. "We also see spectral signatures of materials we have not yet identified."
Phoebe's mass was determined from precise tracking of the spacecraft and optical navigation, combined with an accurate volume estimate from images. The measurements yield a density of about 1.6 grams per cubic centimeter (100 pounds per cubic foot), much lighter than most rocks, but heavier than pure ice at approximately 0.93 grams per cubic centimeter (58 pounds per cubic foot).
This suggests a composition of ice and rock similar to Pluto and Triton.
Other observations confirmed the presence of water ice previously detected by Earth-based telescopes. Also detected were carbon dioxide and solid hydrocarbons similar to those found in primitive meteorites.
"One intriguing result is the discovery of possible chemical similarities between the materials on Phoebe and those seen on comets," said Robert H. Brown, team leader for the Visible and Infrared Mapping Spectrometer, University of Arizona, Tucson. Evidence that Phoebe might be chemically kin to comets strengthens the case it is similar to Kuiper Belt Objects.
Phoebe is very cold, only about 110 degrees above absolute zero (-163 degrees Celsius, or -261 degrees Fahrenheit), the observations show. Even colder nighttime temperatures suggest a fluffy, porous surface layer.
"One of the first results from this map is the surface of Phoebe has been badly chewed up, probably by meteorite impacts," said John Pearl, a Cassini co-investigator for the Composite Infrared Spectrometer, at NASA's Goddard Space Flight Center, Greenbelt, Md. "We are discovering Phoebe is a very complex object, with large variations in topography."
Cassini also made radar observations of Phoebe's enigmatic surface, making it the first spacecraft radar observations of an outer-planet moon. The results are consistent with the dirty, rocky, icy surface suggested by other observations.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The probe is set to go into orbit around Saturn at the end of this month.
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