Europa ocean cruise

Planetary scientist Chris Chyba of the SETI Institute in Palo Alto, California is also anxious to see the search for life on Mars resume. But he’s also focused on another alluring target -- Jupiter's moon Europa. Images from the Galileo spacecraft, with other data, have shown that there is probably an ocean of liquid water beneath Europa’s icy crust. It’s not an easy place to explore: Europa is some 400 million miles from Earth, and it lies deep within Jupiter’s powerful radiation belts, which pose a hazard to any spacecraft.

But planners have envisioned a number of possible missions, including orbiters and landers, to explore Europa. Some have proposed an ambitious mission to melt through the ice and deploy a "cryobot" in the ocean to hunt for life. But Chyba says, "That kind of mission is extremely far in the future. Maybe it will be this century; I hope so. I think it's a very distant mission."

There may be an alternative.

For example, the Galileo orbiter has photographed areas on Europa that resemble smooth, ice-covered ponds on Earth. It may be that in these places, water from the subsurface ocean reached the surface and then froze.

Chyba envisions a Europa lander that would visit one of these places, then melt its way down a couple of feet into the ice, just deep enough to find material that would be shielded from Jupiter's intense radiation. Then, he says, "We'll learn about the composition of the ocean by melting ice and finding out what's in the ice."

Even if there were no living things in the sample, Chyba says, a chemical analysis of the ice should reveal whether life had ever been present. "I think you would be able to tell pretty quickly whether you were looking at a collection of organic molecules that seemed likely to have been biological in origin."

It’s the water, of course, that makes Europa such an alluring place to look for life, as Chyba notes. "The question for a long time has been, Where is there liquid water?" He adds, "What's really exciting about Europa is the idea that there is a second ocean in the solar system."

Life’s rich tapestry

Chyba knows that the discovery of even the simplest life form on Mars, Europa or anywhere in the solar system would be enormously significant, because it would give scientists another point of reference on the nature of living things.

"What's life? What is it?" asks Chyba. "I don't think we're going to get an answer to that question until we have more than one example to work with."

"How many ways is it possible to make something that we would want to call alive? Is there only one way? Is there only the protein/DNA way? Does what we mean by life have to evolve through what we call Darwinian evolution? How diverse might life be? I don't think we're going to be able to answer those questions by looking at life on Earth. We're going to need other examples."

For that reason, Chyba says, "Finding microscopic life on another world that did not share a common ancestor with Earth life would be among the greatest discoveries in science."

And if it did turn out to be similar to terrestrial microbes, "I think it would be less profound, but it would be damned interesting," because it would lend support to the idea that life was transported among the worlds of our solar system by meteorites.

When it comes to life on other solar systems, the statistics seem promising.

Four decades ago, Chyba's colleague Frank Drake authored a now-famous equation that judged the likelihood of extraterrestrial civilizations, or any kind of life, based on the number of habitable planets thought to be orbiting other stars. Back then, that number was pure conjecture. But now, after discovering several dozen extrasolar planets, astronomers are on the verge of learning whether other solar systems like ours are scarce or plentiful.

"My prejudice is that we're going to discover that our solar system is neither rare nor typical," Chyba says. "But it doesn't really matter what I think is going to happen. We're actually going to know. Probably this decade, we're going to know."

That's because of the upcoming Kepler mission, which will search for Earth-like planets around 100,000 stars beginning in 2006. "We're about to have catalogs of other solar systems, the way we have catalogs of stars [today]," Chyba says.

Whether any of these other solar systems are home to living things is something scientists hope to answer without leaving home. With powerful telescopes, perhaps including space-based arrays of instruments, it might be possible to detect the presence of atmospheric gases typical of life.

That’s a shaky bet, though.

"I think it's going to prove very difficult to do in a way that seems convincing," Chyba said. "That doesn't mean we shouldn't be trying to do it."

SETI at the ready

And then there’s the Search for Extraterrestrial Intelligence. SETI has been underway at various telescopes around the globe for more than four decades, listening for signals from extraterrestrial civilizations. Up to now, SETI researchers have heard only the growl of distant pulsars, galaxies, and other celestial objects.

But that may change when the Allen Telescope Array is constructed in northern California. Planned for completion in 2004, the array will be composed of more than 350 radio telescopes, each more than 18 feet in diameter, electronically hooked together to create the equivalent of a single, giant telescope.

The Allen array will "revolutionize" the search for extraterrestrial civilizations, Chyba says. Since SETI began in 1960, he notes, "I think people have this misimpression that we've looked and looked and looked and haven't found anything. But in fact, we've looked at almost nothing with any kind of sensitivity, so far."

During its first ten years of operation the Allen array should allow scientists to look at the 1 million nearest Sun-like stars. With an estimated 100 billion stars in the Milky Way, Chyba notes, "That's still only a small fraction of the galaxy. But it's beginning to knock on the door of the statistics meaning something."

Will that be enough to find ET? Chyba counters with his own question: "Do you believe that one out of 100 thousand stars has a technical civilization orbiting it? I guess my suspicion is that they're more rare than that. But that's just my suspicion. There's only one way to find out the answer to that question. The only way to do this is to look."

Wait a lonely lifetime

Still, the life-hunters are prepared for the possibility that the answers may not come anytime soon.

"I don’t have any real confidence that we would be lucky enough to find life while I was still alive and working," says Ken Nealson.

Chyba is only slightly less guarded in his assessment. "I think it's an open question whether we're going to discover extraterrestrial life in my professional lifetime or not," he says. "But in the meantime a lot of science that's very interesting in and of itself, and that is relevant to that question, is moving forward."

Still, he says, it’s possible that the searches will come to naught, that Earth is alone as an abode of life. Noting that scientists now believe life arose on Earth around 3.8 billion years ago, some 750 million years after the planet formed, Chyba adds "I think given how early life arose on Earth it's likely that there is other life in the galaxy."

Given what's known today, Chyba said it's possible we are alone, but he adds: "If I were placing a bet over beer, no, I wouldn't bet that we're the only life; that would be extraordinary." But as Chyba and Nealson and the other life-hunters know, the only way to find out is to look.