SAN FRANCISCO—The Galileo spacecraft, flying in orbit around the giant planet Jupiter, sped past the violently ruptured surface of the planet's icy moon Europa Tuesday, as scientists began a new two-year mission to explore that most fascinating of all the many Jovian satellites.
With the remote possibility of life existing in a warm global ocean deep beneath Europa's ice, and mysterious eruptions of dark salty matter blistering its surface, the elated scientists said they already have achieved a "magnificent mission over two fabulous years," and expect still more discoveries as the spacecraft's flight around the planet continues.
From their findings so far, Europa must be unique among all the planetary moons in the solar system. The moon is an object whose nature exemplifies the infinite variety of the heavenly bodies that populate Earth's neighborhood, the scientists say.
Crammed with instruments and cameras, Galileo skimmed along at exactly 124 miles above Europa's surface at 4:49 a.m. Earth time Tuesday. Scientists expect that images transmitted from the extraordinarily precise flyby will help answer some of the major mysteries that make the moon and its history so intriguing.
During a briefing at the Jet Propulsion Laboratory in Pasadena, where Galileo's flight is controlled, the principal scientists and engineers on the team reported new details of Europa revealed from images taken by Galileo one month ago.
The evidence so far, said Torrance Johnson, the project's chief scientist, indicates that beneath Europa's surface of deeply fractured ice lies a layer of relatively warm slush—and deeper still, perhaps, a warm, salty ocean of water that covers the entire moon and could be 200 miles deep or more.
Wherever there is heat and water, the scientists already have speculated, conditions could be ripe for life to emerge. Many members of the Galileo team have considered the warm ocean on Europa to be at least a remote candidate for the existence of some form of rudimentary living organisms.
The recent images of Europa show giant rugged "blisters" and streaks of dark brown material bulging upward through Europa's ice like huge islands. Some of those formations rise as bulky mountains or "massifs," the scientists said.
The dark material streaking the surface appears composed of salts of magnesium and sulfur, and Europa's deep ocean probably is laden with those salts leached from the moon's rocky mantle below, Johnson said.
"Europa is really the gem of the solar system," said Ronald Greeley, an Arizona State University astronomer on the Galileo imaging team. "With its coating of frigid ice and its heart of molten metal, the intense heat flow from deep in the interior is sufficient to rip its entire surface apart."
That heat flow comes from radioactive elements in the rock surrounding Europa's central core and from deep volcanic activity, according to Johnson. More heat also is generated by violent tides disrupting the surface ice so that it stretches and contracts, cleaving it with huge fissures in some places while crunching long ridges upward in other areas.
The tides, Johnson said, are generated by the tug of Jupiter's gravity as Europa rotates on its axis and flies in an elliptical orbit around the planet.
Close examination of the fractured ice on the surface shows that Europa has been peppered with dozens of impacts from wandering meteorites, Greeley said. But it has been impossible to count the impacts because so much of the surface is riddled with slushy material constantly welling up from below through through wide cracks in the ice.
As a result, Greeley said, scientists have estimated that Europa's surface could have been formed at any time between 3 million and 3 billion years ago. It's a mystery, he said, that might yet be solved as Galileo continues orbiting Jupiter and flying past Europa eight more times during the next year and a half.
The Galileo flight around Jupiter was designed as a two-year mission. But its discoveries have proven so spectacular that it has been assigned to continue for two more years, with eight more consecutive flights around Europa between now and February of 1999, followed by four past the moon Callisto and one or two around the smallest moon, Io, at the end of that year.
The primary mission, launched six years ago with a budget of $1.3 billion, began when Galileo reached Jupiter two years ago. And
Although its main antenna was hobbled en route to the planet, the spacecraft's secondary antenna still has been able to transmit more than a billion computer bits of data and 1,800 images.
They have revealed a host of discoveries about the giant storm-battered gas planet, as well as about the four major moons that the Italian astronomer Galileo discovered through his primitive telescope in 1610.
Among the key findings cited by the Galileo team members Tuesday:
—The observation of giant lighting bolts, a hundred times more powerful than those on Earth, streaking across the stormy face of Jupiter itself.
—The existence of a magnetic field around Jupiter's largest moon, Ganymede.
—The volcanic ice flows on Europa with their implication of a vast ocean beneath.
—The discovery of an atmosphere of hydrogen and carbon dioxide on the moon Callisto.
—Evidence of constant volcanic eruptions on Io, with lava far hotter than on Earth.
—The presence of metallic cores inside Io, Ganymede and Europa, and the curious lack of a similar core inside Callisto.
With the primary mission now ended, the two-year extended mission has been budgeted at a mere $30 million—another example of what NASA administrator Daniel Golden has proclaimed as a "faster, better, cheaper" way to explore the planets and their neighborhoods. Publication Date : 1997-12-18
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