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A Glimpse of Ganymede Reveals the Face of a Planet

Jupiter's largest moon has ice quakes, magnetic field

In a looping, seven-year journey across the solar system, the Galileo spacecraft has arrived at a point 480 million miles from the sun to discover ... California?

Images transmitted by Galileo show that Jupiter's largest moon has planet-like features, including an unexpected magnetic field and an icy surface scored by ridges and troughs that remind scientists of California's San Andreas fault.

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Ganymede is a place so cold that ice behaves like rock. Its features in rock-hard ice reflect processes that scientists think are similar to those that shape the crust and cause earthquakes on our own planet.

Ice quakes on Ganymede should be "very similar to what we see in southern California," says Galileo imaging team geologist James Head of Brown University. "You would have a lot of snap, crackle, pop right across Ganymede" when these features form, adds Torrence Johnson, Galileo project scientist at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena. JPL manages the Galileo project for the National Aeronautics and Space Administration.

On Wednesday, JPL released the first images taken when Galileo zipped by Ganymede on June 27, coming within 516 miles of the moon's surface. Dr. Head explains that the close-ups of faults, fractures, and impact craters are the kind of evidence geologists need to understand Ganymede's geological history. This gives scientists another place to study what makes planets tick.

"We really get a new perspective on how our [own] planet works," Head says.

Dr. Johnson adds that this typifies the bottom-line payoff of planetary exploration. He explains that geophysicists can form theories of what shapes Earth. But projects such as this one allow them to test their theories on other planetary formations.

On Ganymede, he says, nature has provided just such an experiment. Forces similar to those shaping Earth are at work on the moon. But rock is replaced by ice, and Earth's warmth is replaced by cold. By understanding what goes on in a place like Ganymede, scientists say they will better understand what happens on Earth.

The photographs sent back by Galileo are mosaics made up of four shots. The spacecraft's camera focused on a ridged area called Uruk Sulcus and another region called Galileo Regio. One image of an area about the size of Los Angeles shows "the surface is tremendously cut up by faults," Johnson says.

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Galileo Regio, considered one of the older areas on Ganymede, was expected to have bowl-shaped craters like those of Earth's moon. "It doesn't look like that at all," Johnson says.

Although from Jupiter's perspective Ganymede is a satellite, it can be considered a planet in its own right. It's the largest moon in our solar system, larger than Mercury and a third as large as Earth. Orbiting three times as far from Jupiter as does our own moon from Earth, Ganymede also has its own distinctive space environment. This has produced a major surprise: The moon has a significant magnetic field.

That field appears to be about 1/40th as strong at Ganymede's surface as is the magnetic field on Earth's surface. That's strong enough for Ganymede to have its own magnetosphere. This is a region of space, normally around a planet, that's controlled by that planet's magnetic field. Jupiter has an extensive magnetosphere in which Ganymede is embedded. The moon has created its own small magnetosphere within Jupiter's sphere of influence.

Now, Johnson says, scientists have a new puzzle. They believe a planet's magnetic field is created by electric currents flowing through its interior. Such currents flow through the molten or partly molten core on Earth. But Ganymede is so cold it's hard to explain where such currents could arise.

The spacecraft now has embarked on what JPL's Galileo project manager William O'Neil calls "a marathon" of activity. It will make nine more close passes of Jupiter's major moons between now and November 1997. Its next encounter will be another pass by Ganymede Sept. 6.

The spacecraft is performing "just beautifully," Mr. O'Neil says. Glitches with two instruments should be corrected shortly, he adds.

Meanwhile, Johnson says the Galileo team has a new message for Jupiter: "Earth to Jupiter - we're back."

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