Volume: 02, Issue: 12 06/16/2004 
Two images of Phoebe obtained during the recent Cassini flyby.  Photo courtesy NASA/JPL/Space Science Institute.
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Images showing bright wispy streaks thought to be ice revealed by subsidence of crater walls are leading to the view that Phoebe is an ice-rich body overlain with a thin layer of dark material. Photo courtesy NASA/JPL/Space Science Institute.
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Other Articles in This Issue:
Planetary Times Summer Hiatus
Opportunity Dips into Endurance
Reagan Remembered Onboard International Space Station
Enhance Your Science Knowledge This Summer

Cassini Flies by Phoebe, Closes in on Saturn

Cassini made great strides in advancing its mission objectives on June 11, 2004, when the spacecraft flew by Phoebe, Saturn's largest outer moon. Images from the flyby reveal a scarred, cratered outpost with a mysterious past. Phoebe displayed an aged surface with a great deal of variation in surface brightness.

"What spectacular images," said Dr. Carolyn Porco, Cassini Imaging Team leader at the Space Science Institute in Boulder, Colo. "So sharp and clear and showing a great many geological features, large and small. It's obvious a lot of new insights into the origin of this strange body will come as a result of all this."

These images have yielded strong evidence that Phoebe may contain ice-rich material, overlain with a thin layer of darker material perhaps 300 to 500 meters (980 to 1,600 feet) thick.

Images reveal bright streaks in the ramparts of the largest craters, bright rays which emanate from smaller craters, and uninterrupted grooves across the face of the body.

"The imaging team is in hot debate at the moment on the interpretations of our findings," said Dr. Porco, "Based on our images, some of us are leaning toward the view that has been promoted recently, that Phoebe is probably ice-rich and may be an object originating in the outer solar system, more related to comets and Kuiper Belt objects than to asteroids."

In ascertaining Phoebe's origin, imaging scientists are noting important differences between the surface of Phoebe and that of rocky asteroids that have been seen at comparable resolution.

"Asteroids seen up close, like Ida, Mathilde, and Eros, and the small Martian satellites do not have the bright 'speckling' associated with the small craters that are seen on Phoebe," said Dr. Peter Thomas, an imaging team member from Cornell University, Ithaca, N.Y.

The landforms observed in the highest resolution images also contain clues to Phoebe’s internal structure.

"Phoebe is a world of dramatic landforms, with craters everywhere, landslides, and linear structures such as grooves, ridges, and chains of pits,” said Dr. Alfred McEwen, an imaging team member from the University of Arizona, Tucson, “These are clues to the internal properties of Phoebe, which we'll be looking at very closely in order to understand Phoebe's origin and evolution."

"I think these images are showing us an ancient remnant of the bodies that formed over four billion years ago in the outer reaches of the solar system," said Dr. Torrence Johnson, an imaging team member from NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Battered and beat-up as it is, it is still giving us clues to its origin and history."

Phoebe may be an icy interloper from the distant outer solar system that found itself captured by giant Saturn in its earliest formative years. There is also suspicion that Phoebe, the largest of Saturn's outer moons, might be parent to the other much smaller retrograde outer moons that orbit Saturn.

"Looking at those big 50 kilometers (31 mile) craters, one has to wonder whether their impact ejecta might be the other tiny moons that orbit Saturn on paths much like Phoebe's," said Dr. Joseph Burns, an imaging team member and professor at Cornell University, Ithaca, N.Y,

All planned 11 instruments operated as expected and all data was acquired. Final conclusions on Phoebe's origins await a combination of the results on Phoebe's surface structures, mass and composition. Scientists plan to use the data to create global maps of the cratered moon and to determine Phoebe's composition, mass, and density.

"This has been an impressive whirlwind flyby, and it's only a curtain raiser on the events about to begin," Porco said.

With Pheobe’s flyby accomplished, the Cassini-Huygens spacecraft is on course for Saturn. Once Cassini reaches Saturn’s orbit on July 1, Cassini will begin a four-year prime mission. It will release its piggybacked Huygens probe about six months later for descent through Titan's thick atmosphere. The probe could impact in what may be a liquid methane ocean.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. JPL, 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 imaging team is based at the Space Science Institute, Boulder, Colo.

For the latest images and information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org. For more information on NASA programs, visit www.nasa.gov.

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