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Ordinarily, a body the size of Io should have cooled off long ago, making volcanoes impossible. But for every pass the moon makes around Jupiter, it makes several passes by its large, slower-orbiting sister moons: Europa, Ganymede and Callisto. Every time Io does that, the gravitational tug of these nearby satellites gives it a twang. On Earth, the gravity of just one moon is sufficient to cause the oceans to rise and fall in great crashing tides. On Io, the gravitational influence of three nearby moons is enough to distort the shape of the world itself, causing it to pulse with a heartbeat-like lub-dub. This rhythmic motion churns up internal heat, which in turn stirs up moonwide volcanoes.
Though all such otherworldly erupting is dramatic, it amounts to little more than geological pyrotechnics. On Europa, however, tidal heating may have produced something truly remarkable. The formations Galileo spotted last week are definitely icebergs, though less jagged-looking than those found on Earth. Astronomers don't know why Europan ice and terrestrial ice would not fracture the same way, but they admit they have no experience with the kinds of cracks that are produced when an entire world is frozen over. More to the point, the bergs are small, rising just 300 to 600 ft. above the surrounding ice. Since only 10% of an iceberg shows above the water, that means these measure a mile or so from top to bottom--and so, therefore, does the planet-wide ice crust from which they came. On the scale of a 2,000-mile-wide moon, that's not much of a crust at all.
No matter how thick the ice is, the waters beneath it must still be liquid, thanks to tidal heating. This is good news for biology. Scientists don't pretend to know how warm a Europan ocean might be, but even waters that are just a degree above freezing would feel downright balmy to organisms that evolved in it.
While Europa may be the solar system's most promising Petri dish, it is by no means the only one. Saturn's Titan, larger than both Mercury and Pluto, has an atmosphere fully 60% denser than Earth's, forming a sort of photochemical haze that appears to be full of the stuff of prebiology. The problem is that Titan is cold. With temperatures hovering near -290[degrees]F and no signs yet of significant heat to drive chemical reactions, the moon could be awash in organics that are nevertheless unable to combine in biologically useful ways.
"I expect fantastic chemistry on Titan," says astronomer Steven Squyres of Cornell University. "I don't expect a trace of life." Others aren't so sure; if there's lightning in the Titanian atmosphere, it could energize organic molecules in a hurry. "I would be surprised if there is life on Titan," says astronomer Toby Owen of the University of Hawaii, "but we've been surprised by the solar system before."