Galileo Sends Detailed Shots of Jupiter Moon

Exultant scientists receiving the first images of Jupiter’s largest moon from the Galileo spacecraft were “dumbfounded” Wednesday by detailed pictures of an ice world where volcanoes belch snow and an underground ocean of water churns up a baby magnetic field.

After a six-year journey through space and a series of mishaps that threatened the mission, Galileo is beaming images of such high resolution that if the moon Ganymede had a Hollywood Bowl on its surface, it would have been visible, said team member James Head at NASA’s Jet Propulsion Laboratory in Pasadena.

Over the past two weeks, Galileo has been steadily replaying data picked up during a close fly-by of Ganymede to waiting receivers in Spain and Australia, and from there to giddy scientists at JPL and all over the world.

“It was well worth the wait,” said Head, a Brown University researcher. “We haven’t slept in 10 days.”

Galileo project scientist Torrence Johnson blushed with excitement as he compared the current images to the much blurrier pictures of Jupiter’s moons sent from the Voyager missions in the late 1970s. “I have one message for Jupiter,” he said. “We’re back!”

The Ganymede fly-by was the first of 10 planned close encounters with Jupiter’s four largest moons over the next 18 months. Since Jupiter is almost big enough to be a sun, and holds a diverse family of satellites in its grasp, researchers hope that a close look at the system will reveal clues about the geological history of Earth.

These first images zoomed in on two patches of Ganymede about the size of the Los Angeles Basin. Like a close-up of an aging Hollywood star, the pictures revealed faults, wrinkles and pockmarks on what scientists had thought was a smooth surface.

Some areas looked like tree bark shot through with bullet holes--ancient impact craters left by asteroids. Others looked like they had been raked smooth, leaving neat rows of steep ridges.

“Imagine skiing down those slopes!” said Head. “And it’s winter all the time!”

Fault lines probably caused by Earth-like geologic activity make parts of Ganymede look like the American Southwest, said Brown University’s Bob Pappalardo. Some markings look uncannily like California’s San Andreas Fault.

Researchers speculate that earthquakes--or rather, ice quakes--were common in Ganymede’s past. At some time, said Johnson, there was probably “a lot of snap, crackle and pop on Ganymede.”

Just when those upheavals took place is a matter of some debate and much speculation. Because no moon rocks from Ganymede have been studied in Earth laboratories, dating is uncertain, at best. Instead, scientists infer the moon’s history and its inner structure from the markings on its surface.

From what they can see, scientists say their thinking about Ganymede’s past has been “turned upside down,” according to Head. For one thing, the moon’s early history seems to be far more active than thought. Moreover, tectonic activity--movement of large areas of the moon’s crust--seems to have had a greater influence on shaping its geography than volcanoes.

Normally, features such as ridges and craters repeatedly get smoothed out on planetary bodies as volcanoes repave surfaces with lava, or other geologic pressures stretch, compress and fold the surface. The first two-thirds of Earth’s geologic history has been “destroyed” in this way, said Head. Studying Ganymede may give us clues to what our own history has erased.

Instead of rock, however, most of Ganymede’s geology is based on ice. Its volcanoes spout water spewed up from its underground oceans, and instead of steam, the tops of the volcanoes are shrouded with snow. “It’s bizarre,” said Head.

Most surprising of all was UCLA physicist Margaret Kivelson’s discovery of “unambiguous evidence” that Ganymede creates its own magnetic sphere of influence within the hugely strong field of Jupiter. Galileo’s sensors picked up loud and clear signals of high energy radiation as they approached Ganymede; the radiation appeared to pivot and point toward Ganymede as the satellite flew by--just as would be expected if the moon had a magnetic field of its own.

No one knows how that could be possible, said Head, because planetary magnetic fields are created by hot fluids carrying electric charge. The sun’s magnetic field is created by hot electrically charged particles. Yet Ganymede is “a giant snowball in space,” said Head.

Still, Kivelson had predicted that Ganymede might have a field about as strong as that measured by instruments on the spacecraft--about one-fiftieth as strong as the magnetic field on Earth’s surface. She suspects that somehow salty ocean currents beneath Ganymede’s icy surface are responsible.

Previously, Kivelson had predicted that Jupiter’s volcanic moon Io might have a magnetic field--a prediction that appears to be confirmed by preliminary data on Jupiter’s magnetic fields.

Now she is predicting that a third moon--Europa--will also have a field. “We’re on a roll,” she said. These findings are particularly exciting for experts in planetary magnetism because no known mechanism can produce the fields they are seeing. “It tells us how little we know,” said Kivelson.

Another surprise discovery was that Ganymede has an ionosphere--a layer of charged particles high above the surface--much like Earth’s. That suggests strongly that the moon, which is the largest in the solar system, has a tenuous atmosphere, possibly of oxygen.

In addition to looking at Jupiter’s moons, Galileo is studying the giant gas planet itself, homing in on particular areas of scientific interest. In several weeks, researchers hope to release new close-up pictures of Jupiter’s great red spot. “We didn’t neglect the big guy,” said project manager William O’Neil.

O’Neil and others were elated and relieved that Galileo was working smoothly and that the images were so clear. The mission, approved by Congress 20 years ago, has been dogged by problems. During its six-year journey from Earth, the main antenna failed to unfurl, forcing Galileo to rely on a painfully slow secondary antenna that dribbles data back to Earth at a rate far below a standard computer modem. And because the mission was delayed so many times, the on-board computer is so out of date, said O’Neil, that “no one would pay for it today.”

The lesson, he said, is that “if you never give up, you can make anything work.”

Of 10 instruments on Galileo, two were temporarily out of service but expected to be up and running again in time for the spacecraft’s next fly-by of Ganymede in September.

“We’re on a marathon,” O’Neil said. “We only played back 10%” of the data from the Ganymede encounter. We’ve got nine more encounters in the next 14 months. We haven’t seen even 1%.”

None of the researchers seemed to mind going days without sleep. “Every day has been like Christmas,” said Head. “We come in the morning and we have to decide, should we get coffee, or should we open Christmas presents?”

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View From a Distant Moon

After a six-year journey, the Galileo spacecraft has begun beaming images from Jupiter’s largest moon, Ganymede. Among the findings:

* Impressive detail: High-resolution images show surface features as small as 300 feet across--craters, basins, grooves and mountains.

* Frozen world: Largest moon in the solar system, bigger than the planet Mercury, has surface of ice. Bulk of moon is half water ice and half rock.

* Geologic activity: Volcanoes expel streams of snow. Surface ice shows signs of tectonic activity.

* Magnetic field: Underground ocean of water apparently creates a baby magnetic field, surprising scientists.

* Quote: “It was well worth the wait.”--James Head, member of the Galileo project team.

Approved by Congress 20 years ago, Galileo was launched aboard the space shuttle Atlantis on Oct. 18, 1989.

Source: Jet Propulsion Laboratory photo; Los Angeles Times staff and wire reports