[meteorite-list] Cassini Opens A Cosmic Time Capsule (Phoebe)

[Ron Baalke]

Donald Savage
Headquarters, Washington                June 23, 2004
(Phone: 202/358-1547)

Carolina Martinez
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-9382)

RELEASE: 04-202

CASSINI OPENS A COSMIC TIME CAPSULE

     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.

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