or a long time,” said Chris Chyba, the last Frontiers of Science speaker, “the difficulty with looking for life on other planets was finding water.” The concept of the “habitable zone,” developed by Stephen Dole of the Rand Corporation and Michael Hart of NASA’s Goddard Space Center and further elaborated by Penn State’s Kasting, along with Ray Reynolds of NASA Ames and Dan Whitmore of the University of Southwest Louisiana, put this dilemma in black and white. Of the planets in our Solar System, Earth, Kasting and his colleagues calculated, is the only one close enough to the Sun to be warm enough for liquid water, yet not so close that the water boils away. Actually, Mars is in the ballpark too, except that present-day Mars has too little atmosphere to retain the necessary heat — at the surface. But what about down below?

Recent research has heightened interest in “worlds that may be rich in liquid water below the surface,” said Chyba, associate professor of geological and environmental sciences at Stanford University and director of the Center for the Study of Life in the Universe at the SETI Institute. Mars is one such world. Another, in some ways even more tantalizing, is Europa.

ourth largest of the 16 known satellites of Jupiter, Europa is a chunk of rock and metal about as big as Earth’s moon, sheathed in ice. Voyager photographs taken 20 years ago show a smooth surface scored heavily with cracks, like a favorite skating pond in late winter. The absence of craters, Chyba said, shows that unlike our moon, Europa is geologically active. “Its surface is being reworked every 10, or 20, or 30 million years,” by new material churned up from below.

The reason for this activity, he said, is the strong tidal pull exerted by Europa’s giant parent, which causes bulging and shrinking of the satellite’s crust as Europa moves through its orbit. All that movement creates friction — and heat. “And we can calculate how much,” Chyba said. Doing so, he added, “enabled one of the few important successful predictions in the history of planetary science”: that Io, Jupiter’s closest satellite, was so heated by friction it would be “the most volcanically active world in the Solar System. And it in fact is — Voyager has taken pictures of its volcanoes erupting.”

The pull on Europa, farther out, is less than that on Io. But there’s still enough friction to heat Europa’s core substantially — enough to melt away most of its icy layer from the inside, Chyba said. So, although the surface, which has no atmosphere, remains a rock-solid –170 degrees C (–274 degrees F), beneath Europa’s ice in all probability lies a vast body of water.

The evidence of resurfacing seems to corroborate this, Chyba said, with smooth areas suggesting water flowing out from the interior only to be quickly re-frozen, like the contents of a bucket spilled across a frigid sidewalk. The wealth of cracks, he added, “seem to be related to stretching ice as it rides up on top of an ocean deforming underneath.” But in a way the most compelling argument for an ice-bound sea is the magnetometer data.

Jupiter has the strongest magnetic field of any planet in the Solar System. That field sweeps past Europa every ten hours, as the giant planet spins on its axis. “If there were a conductor on Europa — salty water, for example — the changing magnetic field would set up a current in that conductor,” Chyba explained, and that current would create Europa’s own magnetic field. Such a field has now been measured — its strength consistent with an ocean 100 kilometers deep with a salt content about equal to that of the ocean on Earth.

“It’s hard to avoid the conclusion that there’s a salty conducting ocean on Europa,” Chyba concluded. “But we’re not completely certain. And we would like to be, because if there is a second ocean in the Solar System, we’re going to go back and have a program of exploration on Europa that rivals the Mars program. I would go so far as to say that if there is an ocean on Europa, it is the most exobiologically interesting place in the Solar System. That is to say, there might be life there.”

hat do we mean by life? That’s the first thing that needs to be agreed on. “There have been many attempted definitions — thermodynamic, metabolic, biochemical — but all of them seem to either leave something out that we know is life, or let something in that we know isn’t,” Chyba said. “So we have to fall back on a simpler idea, that of life ‘as we know it,’” made of liquid water, organic molecules, and an energy source. On Europa, “there is almost certainly liquid water present. There are hints that there are organic molecules present.” What about an energy source?

“It’s hard to say anything at all about this,” Chyba admitted. “You can’t have photosynthesis. Light couldn’t penetrate that surface ice.” Might there be hydrothermal vents at the bottom of that ocean? “We have no idea.”

A look at life on Earth, he continued, shows that higher life forms — eukaryotes — require something beyond the three basics: they need oxygen, too, to help metabolize energy. “Even tubeworms and clams at hydrothermal vents need oxygen; it’s produced at the surface and finds its way down. If not for photosynthesis these organisms would die.” Oxygen, whether in Europa’s atmosphere or in its ice-covered ocean, is likely to be scarce, Chyba said. So, “as much as I would like to see giant squid swimming in Europa’s ocean, we probably have to content ourselves with one-celled organisms analogous to bacteria or archaea.”

On the other hand, he noted, there are some creatures on Earth that get along fine with no oxygen at all. Methanogens, for example, are a class of bacteria that digest hydrogen and carbon dioxide to produce methane. “And they probably get that hydrogen from rocks. If Earth froze over tomorrow and became a world that looked like Europa, we would probably continue to have an ecosystem living underground for billions of years.”

Conceivable, too, are energy sources on Europa that we simply don’t know about: Chyba offered a suggestion based in photochemistry. Jupiter’s strong magnetic field, he said, acts like a particle accelerator, shooting charged particles — radiation — into Europa’s ice. “We know from Galileo’s observations that there are carbon dioxide molecules mixed in with that ice. Once you irradiate carbon-dioxide-bearing ice, you make simple organic molecules, like formaldehyde. And you can make oxidants from the ice itself. These molecules are frozen together, and at melt-through events they could get mixed into the ocean.” Using Earth analogies, Chyba said, “We can estimate that Europa’s ocean, in this way, could support a bacterial ecosystem.” Not a very robust one — “only about 1/10,000 as dense as that in Earth’s ocean” — but, hey, it’s a start.

he only way to know whether such an ecosystem is out there,” he said, “is to go look.” That’s the rationale behind NASA’s Europa Orbiter, planned for launch in 2006. The Orbiter’s primary objectives, which Chyba helped to draft, are to verify the presence of an ocean, measure how thick the ice is, and spot evidence of organics. “After the Orbiter, there is planned a Lander. And after that, maybe a series of missions, to get beneath the ice.”

All of which is a bit more involved than missions to Mars. “It takes three years just to get there,” Chyba said, “and another to get into orbit. And once you’re there, you have that punishing radiation,” conditions so harsh that the Orbiter is expected to survive for only a month. Then, too, there’s the possibility of contamination, “both forward and back, but it’s the forward contamination I’m worried about. We need to be extremely careful that we don’t introduce organisms that would interfere with Europa’s possible ecosystem. And if the ocean is sterile, we don’t want to introduce any false positives.”

Securing answers from far-off Europa will be an extraordinarily complicated endeavor, as difficult, perhaps, as humans have ever attempted. To Chyba, however, the effort required will be well worth it.

“My suspicion is that if we find an ecosystem on Mars, it’s quite possible that it will share a common ancestor with life on Earth,” he explained. “Whichever world evolved life first will have inoculated the other,” through asteroids or other space-borne debris. “But I think that if we find life on Europa, it’s probably an altogether different form of life.” Something beyond even our current power to imagine.