Uncovering Jupiter's Secrets

Now that Galileo's two-year mission is over, spacecraft will focus on moons.

Lightning 100 times more powerful than any bolt on Earth flashes through continent-size thunderheads. In four months, a volcanic eruption spreads lava across an area the size of Arizona and at searing temperatures not seen on Earth in 3 billion years. A moon's icy sheath bears telltale signs of a warmer water world beneath.

Since Dec. 7, 1995, Jupiter and its major moons have yielded such secrets to a spacecraft named Galileo, rewriting the book on the largest planet in the solar system and its four major satellites.

"We can't just take our previous text and add a few lines for Galileo results. We're having to tear it up and literally rewrite the textbooks," says Torrence Johnson, Galileo project scientist at CalTech's Jet Propulsion Laboratory in Pasadena, Calif.

Now, with the originally planned two-year tour of the Jovian system finished, the hardy orbiter has been given an extension and a new assignment: to focus its attention on two of Jupiter's moons, Europa and Io.

Io is the most volcanically active body in the solar system. And under Galileo's gaze, Europa has come to look increasingly like a potential incubator for primitive life.

Europa's seas

On Tuesday, the orbiter swooped to within 124 miles of Europa to snap its closest portraits yet of the moon's tortured surface. Between now and the end of 1999, when Galileo completes its extended mission, researchers will look for clues to help them determine how recently Europa's internal heat source has reshaped the moon's water-ice crust.

"Europa is the gem of the solar system," says Ronald Greeley, a planetary scientist at the University of Arizona at Tucson and the head of Galileo's imaging team. "We've known for some time that she has an outer, frigid shell of ice and a heart of rock and metal. The big question has been, has that heart ever been warm - warm enough to disrupt the surface?"

The answer has been a resounding "yes," he adds. For example, the final images of Europa from Galileo's primary mission explored a new part of the moon and revealed what Dr. Greeley calls "blister-like" features that rise above the surrounding surface. These icy massifs likely formed as heated material welled up from deep within Europa to deform the crust, he says.

Taken Nov. 6 and released this week, the pictures also show dark-brown regions that Greeley's team interprets as salts either embedded in the icy surface or left on the surface after liquid or slush that welled up from below evaporated.

"The big unknown is the timing of these events," Greeley says. If they occurred within the past 3 million years, the likelihood grows that an ocean of briny water or slush lies below the surface. If the surface age exceeds 3 billion years, the prospects that a liquid ocean lies under the ice crust grow bleak, since the interior would have had plenty of time to cool.

Greeley and his colleagues will try to use impact craters to gauge the surface's age. The fewer the craters, the younger the surface. Although it's a "long shot," he says, the best evidence for a currently changing surface would be images of geysers or pools of material missing from images taken during the Voyager flybys or Galileo's primary mission.

Ganymede and Io

During that mission, Galileo discovered that another major moon, Ganymede, had a magnetic field - the seventh largest known in the solar system. "That was a real surprise," Dr. Johnson says, adding that the information gives clues about the internal structure and the geophysical history of the moon.

Galileo also tracked the eruption of a volcano on Io, giving planetary scientists new insights into the makeup of the moon's crust. Until Galileo, the lavas ejected onto Io's surface were thought to be largely sulfur.

Galileo's data indicate lavas rich in silicates, much like the early Earth's. And the lavas' temperatures are several hundred degrees Celsius hotter than any found on Earth today, testifying to the enormous tidal forces from Jupiter's gravity, as well as to the heat generated by radioactive decay from Io's metallic core.

At the end of its extended mission, Galileo will pass by Io at least once to gather high-resolution close-ups it failed to get when the orbiter first arrived at Jupiter. Flight controllers had shut instruments down out of concern that they might be damaged by the high-radiation fields found at Io's distance from Jupiter.