Galileo, the robot Jupiter explorer, ends its spectacular mission this month. But don't suggest it should retire. Project scientist Torrence Johnson will have none of that.
Against all reasonable expectation, the hardy spacecraft survived what could have been its final suicide assignment. On Oct. 11 and again on Nov. 25, it braved punishing radiation around the moon Io to get up close and personal with the most volcanically active body in the solar system. Galileo took more radiation dosage than it was designed to withstand. Yet it's still working. Dr. Johnson and his colleagues at the California Institute of Technology's Jet Propulsion Laboratory in Pasadena are lobbying NASA to keep the spacecraft in service.
Whether or not NASA grants their wish, Johnson says planetary scientists are grateful for what he calls "a fantastic feat" that has enabled them "to explore the whole Jupiter system in ways we could only dream about before." This has made the faces of the four large moons that Italian astronomer Galileo discovered in 1610 as familiar as a National Geographic wall map. It has given scientists new insights into how planets function. It has left all of us with the intriguing question: Is there life on Europa, a satellite the size of Earth's moon?
Galileo researcher Richard Greenberg with the University of Arizona at Tucson has little doubt about the answer. He told a press conference during a meeting of the American Geophysical Union in San Francisco last month: "There's a probability, I think, of life on Europa. It's a great environment [for hardy organisms] to live in."
His expectation is supported by discoveries of life's adaptability on Earth. Bacteria live in boiling hot springs, deep down in the continental crust, and inside rocks in desolate Antarctica. Whole communities of organisms thrive on the deep sea floor around nutrient-rich hot water vents. These discoveries have given many scientists the intuition - if not the proof - that, where there's liquid water, there can be life.
With an iron core, rocky mantle, and outer water layer, Europa teases scientists with what seems to be evidence of a liquid ocean underlying its icy crust. Massaged by Jupiter's tidal forces, that crust is scored and broken in suggestive ways. Planetologist James Head of Brown University in Providence, R.I. has explained that scientists "see evidence of a cold brittle material on the surface of Europa." They also "see evidence of movement underneath." The issue, he says, is whether it's liquid water underneath or ductile ice.
Again, Dr. Greenberg has little doubt. He sees evidence of giant cracks that last for 10,000 years. He says tidal action brings warm water up daily along these cracks. He believes this "really is an environment conducive to life." He speculates that brownish reddish deposits seen on the surface are organic material. "We don't know what it is," he admits, "But it sure looks delicious to me."
Sounding a note of caution, project scientist Johnson warns against eating up such speculation. He thinks the dark material may be inorganic. He explains that the most scientists can confidently conclude is that Europa very likely has had a liquid water ocean in the past and may well have one today. Speculating about prospects for life is fun but very iffy.
However, there is no doubt about the importance of Jupiter's tidal forces. For Io, Europa, and Ganymede, they are a more powerful source of stress, strain, and heating than any radioactive material or residual heat left from the bodies' formation. Only the farthest of the four Galilean moons, Callisto, is too distant from Jupiter for tidal forces to dominate.
The power of these pulsing forces is dramatically evident on Io. About the size of our moon, Io has 100 times more volcanic eruptions than does Earth. Johnson says the lava they spew is "probably hotter than anything that has erupted on Earth in the last 2 billion years." A quarter of Io's volcanic heat comes from a single giant volcano called Loki. On Oct. 11, Galileo measured temperatures over 1,000 degrees F. near Loki's eruption.
On Nov. 25, it fortuitously caught a previously unknown volcano, quickly named Trashtar, in a spectacular eruption. Lava at 1,700 degrees F. or more was shooting up to a mile high along a 16-mile fissure. Galileo team member Laszlo Keszthelyi with the University of Arizona, Tucson says that, to him, "this is the most amazing discovery Galileo has made."
Galileo scientists don't think that Io has explosive volcanism. They see mountains towering to 52,000 feet. But these don't appear to be volcanic. Scientists don't know how they form. They eventually collapse under gravity, generating huge mudslides.
Dr. Keszthelyi's Arizona colleague Alfred McEwen sums up the situation by saying, "Io makes Dante's Inferno seem like another day in paradise." Less dramatically, Galileo science operations manager Duane Bindschadler calls Io "a natural laboratory for volcanoes ... [which] may even help us predict the behavior of volcanoes on Earth."
Galileo's reports on Ganymede and Callisto have captured less attention. Ganymede is the largest moon in the solar system - larger than Mercury and a third as large as Earth. It's a cold place where ice behaves like rock. The images show an icy surface scored by ridges and troughs reminiscent of California's San Andreas Fault. Johnson says that "you would have a lot of snap, crackle, and pop" as ice quakes rock the surface. The moon appears to have a three-layer structure with an iron-rich core, a silicate mantle, and an outer ice shell.
Callisto, on the other hand, appears to be an undifferentiated mix of metallic rock and ice with no known central core. Nearly the size of Mercury, its heavily cratered surface suggests that liquid water or slushy ice wells up.
Meanwhile, recent analysis of the data from the atmospheric probe Galileo sent into Jupiter is rattling planetologists' notions of how solar systems form. Jupiter has three times too much Argon, Krypton, Xenon, and Nitrogen to have formed at its present distance from the sun. Either it formed in a much more distant colder place and then moved in, or the planetismals that coalesced to make the planet did that. Current theory doesn't allow such migration. However, if it is possible, it may help explain the equally puzzling fact that many giant alien planets lie very close to their stars.
The Jet Propulsion Laboratory, which manages Galileo for NASA, also managed the failed Mars missions. These are a disappointing embarrassment. In contrast, Galileo's mission has unfolded in the JPL tradition of turning failure into success. Failure to fully deploy its main antenna turned Galileo into what Johnson calls a "communications challenged spacecraft." Beefing up earth-based receivers and signal processing and reconfiguring Galileo's data management overcame the handicap. This enabled Galileo to carry out its planned $1.5 billion two-year main mission and the $30 million two-year extension. Along the way, the spacecraft also scrutinized asteroids Gaspra and Ida.
"After orbiting Jupiter for nearly four years, the spacecraft has more than fulfilled its mission objectives," says deputy project manager Wayne Sible
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