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New data from NASA's Galileo spacecraft have prompted scientists to modify their concept of the interior structure of Jupiter's moon, Callisto, and suggest that Callisto has evolved differently than the other largest Jovian moons -- Io, Ganymede and Europa. The new findings, to be published in the journal Science on Friday, June 5, will be presented Monday, June 8 at the American Astronomical Society meeting in San Diego, CA.

"Previous Galileo data had indicated that Callisto's interior was totally undifferentiated," said Dr. John Anderson, planetary scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA. "But new information suggests Callisto has a strange interior--it's not completely uniform nor does it vary dramatically. There are signs that interior materials, most likely compressed ice and rock, have settled partially, with the percentage of rock increasing toward the center of Callisto."

The new information was collected during Galileo's third Callisto encounter in September 1997. Anderson reported on the findings, along with UCLA geophysics and planetary physics professor Gerald Schubert, a Galileo gravity investigator, and Dr. William B. Moore, also of UCLA; and Dr. Robert A. Jacobson, Eunice L. Lau, and William L. Sjogren of JPL.

Scientists now believe Callisto is different from Io, Ganymede and Europa, which have differentiated structures with separated layers. There is strong evidence that Ganymede is separated into a metallic core, rock mantle, and ice-rich outer shell, while Io has a metallic core and a rock mantle but no ice.

"The fact that Callisto is the only one of the four large Jovian moons that is not completely differentiated raises an intriguing possibility," said Schubert. "Because Io, Ganymede and Europa are closer to Jupiter, they have been more affected by gravitational squeezing and subsequent heating. Over time, the forces exerted on the three inner moons have caused different constituents such as water ice, rock, and metal to separate into different layers. However, because Callisto is farther from Jupiter, it is "half-baked" compared to the other moons, with its ingredients somewhat separated but still largely mixed together," he said.

"Learning about the structure of these celestial bodies enhances our knowledge of how all planets and moons form and evolve, including our own Earth and Moon," Schubert added.

Scientists had previously reported a differentiated interior for Europa, consisting of a metallic core surrounded by a rock mantle and a water ice-liquid outer shell. They are now refining the model by studying the newest Galileo data, including that gathered during the closest-ever Europa flyby in December 1997, at an altitude of 205 kilometers (127 miles). Europa's metallic core could be up to half the size of the moon's radius, with the water ice-liquid shell estimated to be between 80 to 170 kilometers thick (50 to 106 miles), with 100 km (62 miles) considered the most likely thickness. As more data become available from additional flybys, scientists hope to learn more about Europa's structure. Europa is of particular interest because of the prospect that liquid oceans may lie beneath its icy crust.

Information about the interior structure of Jupiter's moons is obtained by studying radio Doppler data that is gathered when the Galileo spacecraft flies by the satellites. Each moon exerts a gravitational tug, and the strength of that tug is affected by the distribution of rock inside. The tug, in turn, changes the spacecraft's speed and the radio frequency of its signals. By studying those changes, scientists can characterize the rock content and structure of the body.

The Galileo spacecraft entered orbit around Jupiter on December 7, 1995, and spent two years studying Jupiter, its four largest moons and its magnetosphere during its primary mission. The spacecraft is now in the midst of a two-year extension, known as the Galileo Europa Mission. JPL manages the Galileo mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena, CA.

Additional information about the Galileo mission and images sent back by the spacecraft is available on the Internet at: Images are also available at: .

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