Phoebe Flyby Reveals Comet-like Moon

 

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Among the 30-plus moons of Saturn are a menagerie of surfaces. Scientists consider Saturn, its rings and moons, to be akin to a miniature solar system unto itself. Image Credit: NASA/JPL

Scientists may at last have settled the debate on the origin of Saturn’s moon, Phoebe.

Saturn long ago captured its largest outermost satellite, Phoebe, when the moon wandered in from the frigid region beyond the orbit of Neptune called the Kuiper belt, they conclude.

If as scientists now posit, Saturn’s small moon Phoebe may be a frozen artifact of a bygone era, then it was formed 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.

Scientists have long doubted that Phoebe came from the same disk of material that formed Saturn and most of its moons. Phoebe has an unusual orbit that is inclined to Saturn’s equator, revolves backward with respect to both Saturn’s rotation and orbital motion, and travels in the opposite direction of Saturn’s other satellites.

Phoebe is widely believed to have wandered past Saturn and been captured by that planet’s mighty gravitational field. Where it wandered from was the question.

"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.

It is clear that the materials in Phoebe’s surface bear little resemblance to the predominantly rocky material found in asteroids in the belt between Mars and Jupiter. The materials that make up Phoebe formed farther out in the solar system, where it is cold enough for them to remain stable.

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Close approach image of Phoebe taken June 10 Image Credit: NASA/JPL

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.

Evidence that Phoebe might be chemically kin to comets strengthens the case that it’s similar to Kuiper Belt Objects.

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Cassini will release its Huygens probe in December towards the largest Saturnian moon, Titan Image Credit: NASA/JPL

The VIMS instrument is an imaging spectrometer that produces a special data set called an image cube. It takes an image of an object in many colors simultaneously. An ordinary video camera takes images in three primary colors (red, green, and blue) and combines them to produce images as seen by the human eye. The VIMS instrument takes images in 352 separate colors, spanning a realm of colors far beyond those visible to humans. All materials reflect light in a unique way. So molecules of any element or compound can be identified by the colors they reflect or absorb, their "signature" spectra. The VIMS team knew the basic chemical make-up of Phoebe only a few days after flyby.

That Phoebe likely comes from the Kuiper belt and not from the Mars-Jupiter asteroid belt is another "first" for the Cassini mission, Brown noted. Cassini has become the first spacecraft to flyby a Kuiper belt object, he said.

What’s Next

Cassini will conduct a critical 96-minute main-engine burn before going into orbit around Saturn on June 30 (July 1 Universal Time). During Cassini’s planned four-year tour it will conduct 76 orbits around the Saturn system and execute 52 close encounters with seven of Saturn’s 31 known moons.


The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The VIMS team is based at the University of Arizona in Tucson.

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