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Saturn's Battered Moon Phoebe Comes Into Focus

New close-ups from the Cassini spacecraft show a heavily cratered and likely ice-rich sphere.

June 15, 2004|Thomas H. Maugh II | Times Staff Writer

New images made by the Cassini spacecraft during its fleeting encounter with Saturn's moon Phoebe show a battered, pockmarked sphere that may be composed primarily of ice-rich material overlaid with a relatively thin layer of darker material.

"A lot has happened to Phoebe," said deputy project scientist Linda Spilker of NASA's Jet Propulsion Laboratory in Pasadena. "There are craters on top of craters, all the way down to the smallest size we can see.... Phoebe certainly looks like it has had a very interesting history."

The dramatic images released Monday from the spacecraft's Phoebe flyby Friday afternoon show bright streaks in the embankments of the largest craters, bright rays emanating from the smaller craters and uninterrupted grooves across the face of the tiny moon. Small, bright features on the surface are probably younger craters. Some of the larger craters have boulders on the bottom ranging in size from 50 yards to 300 yards across.

"The imaging team is in hot debate at the moment on the interpretations of our findings," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colo. "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."

Added Peter Thomas, an imaging team member from Cornell University: "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."

The images were taken at distances ranging from 20,000 miles to 1,285 miles, the craft's closest approach to the moon, which is only 137 miles in diameter. In contrast, the best photos previously available were taken by Voyager in 1981 from a distance of 1.4 million miles. Those images showed a dark fuzzy ball.

The images are probably the last close-ups of Phoebe for decades. Cassini, which was launched in 1997, will fire its engines to go into orbit around Saturn on June 30 and will not travel near Phoebe again.

The cost of this mission -- estimated at $3.3 billion -- combined with President Bush's decision to send manned missions to the moon and Mars, makes another Saturn visit in the next decades unlikely.

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