After 10 years, the hardy little spacecraft Galileo completes its
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.Skip to next paragraph
Subscribe Today to the Monitor
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.