Like a woolly mammoth trapped in Arctic ice, Saturn’s
small moon Phoebe may be a frozen artifact of a bygone era,
some four billion years ago. The finding is suggested by new
data from the Cassini spacecraft.

Cassini scientists reviewed data from the spacecraft’s June
11, 2004, flyby of the diminutive moon. They concluded Phoebe
is likely a primordial mixture of ice, rock and carbon-
containing compounds similar in many ways to material seen in
Pluto and Neptune’s moon Triton. 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.

“Phoebe apparently stayed behind, trapped in orbit about the
young Saturn, waiting eons for its secrets to be revealed
during its rendezvous with the Cassini spacecraft,” said Dr.
Torrence Johnson, Cassini imaging team member at NASA’s Jet
Propulsion Laboratory (JPL), Pasadena, Calif.

“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 Dr. 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.”
Cassini’s observations gave scientists the first detailed
look at one of these primitive icy planetesimals.

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.

Spectral measurements, light intensity as a function of color
or wavelength, confirmed the presence of water ice previously
detected by Earth-based telescopes. The measurements provided
evidence for hydrated minerals on Phoebe’s surface, detected
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 Dr. Robert H. Brown, team leader for the
Visible and Infrared Mapping Spectrometer, University of
Arizona, Tucson. Evidence Phoebe might be chemically kin to
comets strengthens the case it is similar to Kuiper Belt
Objects.

Measurements taken by the Composite Infrared Spectrometer
were used to generate temperature maps. The maps show the
surface of Phoebe is very cold, only about 110 degrees above
absolute zero (-163 degrees Celsius, or -261 degrees
Fahrenheit). 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 Dr. 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.

“We have conducted our first analysis of an outer solar
system resident akin to Kuiper Belt Objects,” said Dr. Dennis
Matson, project scientist of the Cassini-Huygens mission at
JPL. “In two short weeks, we have added more to what we know
about Phoebe than we had learned about it since it was
discovered 100 years ago. We did this by having multiple
instruments conducting investigations all at one time during
our flyby,” Matson said.

The Cassini-Huygens mission is a cooperative project of NASA,
the European Space Agency and the Italian Space Agency. JPL
manages the mission for NASA’s Office of Space Science,
Washington. For the latest images and more information about
the mission on the Internet, visit:
http://www.nasa.gov & http://saturn.jpl.nasa.gov.