LIFE IN A DEEP FREEZE?

By Jeffrey Kluger

The solar system, to be brutally honest, has turned out to be something of a bust. There was a time when the planets seemed to have a lot of potential, but only if scientists didn't look too closely. Once they did, things got ugly fast. The planets were either flash-frozen or deep-fried, uninhabitable gas giants or uninhabitable rocky pellets, smothered by a toxic atmosphere or almost totally airless--altogether poor company for a glamour world like Earth.

But planets aren't all there is to the sun's family. The solar system is also packed with moons--more than five dozen of them. Increasingly, astronomers are appreciating that these cosmic offspring may be far more remarkable than the parent worlds they orbit. Unlike most of the planets, the moons have oceans, the moons have continents, the moons even have active volcanoes.

And now, it seems, a moon may be the best place yet to look for the most remarkable thing of all: extraterrestrial life. Last week the sturdy Galileo space probe finished beaming back the sharpest images ever taken of Jupiter's ice-covered satellite Europa. The pictures revealed more clearly than ever before that the moon's frosty rind is nothing more than a planet-wide ice cap floating atop a globe-girdling ocean of ordinary water.

What's more, the spacecraft spotted brown stains on the ice that could conceivably be a mix of hydrogen cyanide and other life-related chemicals. "If this is indeed hydrogen cyanide," says Richard Terrile, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, "we have organic chemicals mixed into a bath of water. That's a recipe for life."

Whether there's anything so dramatic waiting on any of the other moons in the solar system is unclear, but NASA wants to find out. The agency is contemplating five or more missions to the planetary satellites in about the next 10 years. Says Terrile: "We're beginning to appreciate that within our solar system, there are all these minisystems worth exploring."

It was in 1979, during the Voyager 1 spacecraft's first encounter with the Jovian moon Io, that astronomers began to suspect there might be more to the moons than met the telescopic eye. While 43 of the 61 satellites measure less than 300 miles or so in diameter, most of the others are more than 1,200 miles across. Bodies with this kind of bulk are capable of supporting an atmosphere--a big plus when you're trying to incubate life. What just about all the moons lacked was the heat needed to get biological chemistry going.

Or so it seemed. In 1979, however, when the first pictures of Io were beamed to Earth, NASA got a shock. Rising from Io's face were the unmistakable plumes of up to 10 erupting volcanoes. "Suddenly," says Torrence Johnson, project scientist for the Galileo mission, "it was clear that other bodies in the solar system could be geologically active."

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."

Jupiter's Io and Neptune's Triton could also prove surprising. Though Io appears largely dehydrated, planetologists don't rule out the possibility of subsurface water, particularly since they think that ordinary steam might provide some of the propulsive muscle behind the moon's volcanoes. Triton presents a greater organic hurdle. At -391[degrees]F, the moon is the coldest known object in the solar system. Nevertheless, it appears heavy with subsurface ice, which seems to have got warm enough, in the past at least, to flow over the landscape in a lava-like slurry. More tantalizing, dark streaks near the poles suggest that occasional geysering on the frozen moon may have spouted carbon or some other organic material. "We don't fully understand what's going on inside Triton," Terrile says, "but something is pumping a lot of energy."

Whether any of the moons will ever be understood fully, of course, is open to question. Before long, however, they will certainly be understood better. Galileo could be functioning until late 1999, with more than 20 passes through the Jovian system still to come. Next fall NASA plans to launch the new Cassini-Huygens spacecraft on a seven-year odyssey to Saturn. In addition to making at least 36 orbital slalom runs through five of Saturn's inner moons, the ship will fire off a probe that will puncture Titan's cloud cover, parachute to its surface and send environmental readings back to Earth.

Even before Cassini's work begins and Galileo's ends, other ships could be on the way to join them in the outer solar system. NASA is tentatively planning several new Europa probes, including one that will photograph its surface and take radar soundings beneath its crust. If the radar picks up the telltale echoes of liquid water, another spacecraft would be sent to land on Europa and release a heated probe designed to melt through the ice layer and look for signs of life in the seas below.

None of these proposed missions will come cheap. Even with NASA's new commitment to building smaller, less expensive spacecraft, interplanetary ships still cost at least $200 million each. Planetologists, however, insist that the potential discoveries could be well worth the money.

"These moons make up one of the most eccentric cosmic families imaginable," says Terrile. "As with any other family, some individuals are underachievers, some are overachievers, and a few may be up to something truly fantastic." It's these last that NASA wants to get to know better.