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It was, without a doubt, the biggest party ever thrown in a NASA Ames Research Center aircraft hangar. One evening in April, as electronic band Telefon Tel Aviv twiddled and blurbled away, thousands of beglittered, oddly coiffed people danced to celebrate the launch date of Soviet cosmonaut Yuri Gagarin, the first man in space.

It was hard to tell the NASA employees from the rave kids, since, in an homage to space science, many of the dancers were clad in lab coats and flight jumpsuits. Pete Worden, director of the center, who took the stage around midnight urging the crowd to "party like it's 1961," appeared to be wearing a wizard outfit. And when the band decamped from the cable-strewn stage so that NASA planetary scientist Chris McKay could tell them all how to hunt for life on Mars, everyone stopped dancing long enough to listen.

McKay was a grad student when Viking landed on Mars in 1976 and sent back news that although many of the necessary components for life were there, life itself wasn't. He has devoted his entire career to finding out whether Viking was wrong. What he wants to discover on Mars, McKay told a rapt crowd, is a "second genesis" — proof that life arose independently in more than one place in the universe. "Life," he specified, "but not as we know it."

When McKay started his career, that kind of talk was the province of Trekkies and tinfoil-hat wearers. Now it's a serious science, and you can hardly talk to anyone in the field of what's now called astrobiology without them tipping their hat to McKay's pioneering work.

"For a long time I was sort of a voice in the wilderness," McKay muses a few weeks later from the comparative calm of his office at NASA Ames in Mountain View. But in the mid-'90s, he says, three watershed events hinted that scientists might be on the verge of unpacking some of the universe's more enduring mysteries: The Hubble Space Telescope began beaming back pictures from deep space; researchers identified, for the first time, a planet outside our solar system; and scientists published a paper claiming they'd found traces of life in a Mars meteorite that had smashed into Antarctica. Even though that claim was later debunked, McKay recalls that these events whetted public interest and convinced NASA to fund research into how and where life could begin. "That's when astrobiology became flavor of the month," he says. "That's when I no longer had to start my lectures with explaining why NASA was looking for the origin of life."

In fact, astrobiology has been the flavor of the last decade, particularly here in the Bay Area where UC Berkeley, San Francisco State, and NASA Ames Research Center have led the field in trying to answer the kinds of mind-boggling questions prompted by the search for life in space. Is our planet an aberration, a warm spot in a cold universe — or is life practically inevitable if you throw the right chemicals together? If there's other life, what's it like? Where does it live? Is it related to us? Why doesn't it ever call or write?

• • • •

The simplest answer could be that we are indeed all alone. That's the "Rare Earth" hypothesis, put forth by professors Donald Brownlee and Peter Ward from the University of Washington. Their 2000 book, Rare Earth: Why Complex Life Is Uncommon in the Universe, argues that our planet is the happy beneficiary of an extremely unusual chain of circumstances that made the rise of intelligent life possible, and which is unlikely to be repeated again.

Earth lucked into being born at the right part of the galactic disc in the right orbit around the right kind of star, and just happens to be of the right size and composition to have an atmosphere that can support carbon-based life. It also has many traits that allowed not only life's genesis, but its continued survival: protective magnetic fields, useful plate tectonics, and the stabilizing influence of a large moon, to name a few. While microbial life might be common throughout the universe, the authors argue, animal life isn't.

Some astrobiologists consider this a flawed argument, overly based on a terrestrial understanding of what life requires. When you consider the vastness of space, they say, Earth isn't likely to be that special. But the only way to prove the Rare Earthers wrong is to go out and find some aliens, an enormous task.

Astrobiology is an untidy field as it is, encompassing everything from galactic evolution to the evolution of microbes. By necessity, it is interdisciplinary — it has attacked its core queries with the tools of astrophysics, biology, chemistry, geology, and paleontology. Yet because astrobiologists are in the unique position of theorizing about life on landscapes they will never visit, their explorations must be done by proxy.

Here's the idea: There are places on Earth that make good stand-ins for other parts of the solar system; the Martian permafrost, say, or the icy brine of Jupiter's moon Europa. These places are almost unrelentingly nasty for humans and were once considered lethal to all life. Yet everywhere astrobiologists have looked on our own planet, life has a toehold. In some of these forbidding places, it is thriving.

Work over the last decade has greatly expanded our notion of where organisms might be found, and has fed a growing conviction that life, with its resiliency and adaptability, is probably very common in the universe. Yet the proof, McKay says, will be in the Petri dish: "It's like the Europeans wondering if there was an edge to the Earth or if there were dragons in the Sargasso Sea. The only way to find out was to go look."

We are now tantalizingly close to getting that look. UC Berkeley researchers have built a Mars life-detection chip they believe is up to a million times more sensitive than Viking's instruments. Researchers at Cal and NASA Ames who study life in extreme environments have provided a game plan for where to look for life in our solar system. And the Bay Area's famed planet-hunting team led by Cal astronomer Geoff Marcy has trained its telescopes on nearby stars and found increasing evidence that small, rocky planets like ours are common. The team recently announced that it had even found a planet with water, the number one prerequisite for life — as we know it.

Yet despite the field's banner decade, NASA's recent and highly controversial U-turn in mission priorities, which emphasizes a return to the moon instead of exploring deeper space, has sucked the wind from astrobiology's sails. NASA funding for its astrobiology program was cut in half this year, with no prospect of reinstatement any time soon. At the same time, several long-anticipated space missions to seek evidence of life in our solar system, and habitable planets outside of it, have been scrapped. It's an agonizing time for some of the field's top researchers. They've come so close to finding answers, yet remain so far.

• • • •

The Case for Mars

NASA's plan to return to the moon went over like a lead brick with astrobiologists, many of whom deride it as having little scientific value — we've already got plenty of moon rocks. The agency's next priority, a manned expedition to Mars, has a few more supporters, among them Chris McKay.

His top three reasons we might find life on Mars: It used to have liquid water, it has the chemicals necessary for life, and its cold, dry, low-pressure atmosphere is essentially a giant freezer vacuum that would have preserved samples nicely. "I really want to find aliens," he says, "even if they are little, green, microscopic, and dead."

That may well be the best-case scenario for Mars. If life started there, it didn't evolve very far. Anything still living would likely be buried in the polar permafrost or deep under the surface, where it's protected from the arid climate and surface radiation. More likely, McKay says, it's long dead. But that's okay, since death proves life: Dead tissue is different than organic matter that never lived. Better yet, it leaves behind evidence.

That's why McKay found himself in Chile's Atacama Desert with UC Berkeley chemist Richard Mathies two summers ago. Mathies has designed a life-detection chip he believes can pick up where Viking left off, even if Mars is pretty much a ghost planet.
The Atacama is the driest, and therefore most Martian, place on Earth. It gets one millimeter of rain a year, and even that, Mathies says, is "one night when the fog came in." It is so dry that McKay once thought his instruments were broken because they hadn't recorded any moisture in two years.

The aridity, and the soil's chemical reactivity, which is similar to that observed by Viking on Mars, makes the Atacama incredibly popular with astrobio types wanting to test-drive prototypes. If your equipment can't find life in the Atacama, good luck finding it anywhere.

To get there, Mathies says, you drive toward the Andes until the terrain completely drains of life. "There's no twigs, no little bugs, no ants crawling, nothing," he says. "It's just nothing." Any microbial life that's managed to survive there is hiding out in the soil, beneath a gypsum crust that has likely not been disturbed in a hundred thousand years. Hack into it, and see if anyone's home
.
The hacking is important — Mathies believes one of the flaws of the Viking landers, as well as the Mars Rovers that were able to churn up a few centimeters of soil by grinding their wheels, was that they looked only near the surface. His chip, the Mars Organic Analyzer, is scheduled to fly on the 2013 ExoMars mission sponsored by the European Space Agency. The mission should be able to drill two meters below the Martian surface, a depth at which Mathies believes life would have been sheltered from the climate and radiation. His 2005 trip to the Atacama showed that although surface life was barely detectable, it was another story once they dug under the crust. "Go to a shielded environment, bam!" he says. "You get a whole bunch more."

Mathies' microfluidic chip is a thing of beauty, a transparent glass and plastic disc ten centimeters across, woven with a delicate labyrinth of channels. Soil samples are propelled through the channels using a system of water and tiny pneumatic pumps and valves. A linked-in computer analyzes the samples and would transmit results back to Earth.

But how, exactly, will Mathies know if he's found alien life, especially if it's really bizarre? And what is life, anyway?
You'd think there'd be a concise, universally accepted answer to the latter question, but there isn't. After all, we've only seen one sprawling example of life. Even though Earth is teeming, its organisms are all thought to be descended from the same source, all part of the same tree of life. At best, astrobiologists usually agree on some working guidelines — that life reproduces, consumes energy, and evolves — but even these criteria, they point out, describe what life does, not what it is.

This has produced a sort of "We'll know it when we see it" attitude in the field, and even that is iffy. Selective pressures on other planets could have pushed evolution in directions we can't even imagine, and scientists have to struggle against their Earthcentric biases. For example, Mathies says, you wouldn't want to go to Mars and sample for DNA, because what if Mars life doesn't have DNA? "One of the hard things about structuring an experiment that looks for life is that if you make your hypothesis too specific you may not find it," he says. "And if you make it too general, you may not learn anything."

Yet scientists do believe there are some themes the cosmic creation process is likely to play repeatedly. Earth life is based on the most common chemical elements in the universe — so other life is likely to use them too. Liquid water seems to be, as one scientist puts it, the "the cosmic cocktail mixer" that allows biochemistry to occur, and water also is commonly available. Amino acids are the building blocks of Earth life, and they form readily in nature, even through nonbiological processes. They have been found in meteorites, showing that they don't have to originate on Earth.

Inspired by science Mathies developed while working on the Human Genome Project, his chip exploits an amino acid quirk particular to Earth life. In nature, amino acids have two forms that are mirror images of each other: They can be either "right-handed" or "left-handed." Nonbiological processes produce an equal mix of these two forms. By contrast, Earth life uses only left-handed amino acids. As McKay puts it, "Biology selects; chemistry doesn't."

So if an extraterrestrial specimen has an equal mix of the two forms, it's not biological. But if its amino acids are uniformly left- or right-handed, it's nearly a sure sign of life. If they all happen to be right-handed, well, that's even more exciting — it indicates not only life, but life different from that of Earth, compelling evidence of an independent genesis.

This is the sort of scenario that makes McKay light up with glee. He's already got plans for what we should do if we discover we have neighbors: unfreeze them by creating global warming on the Red Planet, a sci-fi notion if ever there was one. "If I found alien life on Mars, a second genesis, my vote would be to bring it back to life and restore the planet to habitable conditions — habitable for that life form," he says. "My slogan would be 'Mars for Martians.'"

Even if that meant a planet covered in bacteria?

"Yep," he says. "As long as they're alien bacteria."

But wouldn't that set in motion an evolutionary chain whose endpoint we can't possibly predict?

"Exactly," McKay responds. "What if three billion years ago somebody came to Earth and said, 'Oh, nothing here but microbes, hit the erase button, alt-shift-delete'? We'd be unhappy with that."

Fine, but imagine the equally likely scenario in which the amino acids are left-handed, like ours. "Then the simplest conclusion is that they're our cousins, and that Earth and Mars exchanged material and share a common origin, and the planets Earth and Mars are not any more isolated from each other biologically than the continents on Earth than Australia is from North America," McKay says.

After all, we know that meteors can criss-cross between the two planets — some theories even speculate that Earth life originated on Mars, then flourished here, meaning that we have been the Martians all along. More tests would be needed to determine whether — and how — we map onto one another's trees of life. To McKay, that would be disappointing. "I would really much prefer we found a second genesis and we just smashed the Rare Earth hypothesis to smithereens," he says.

Another plot twist to consider if Mars life looks like ours: that we've contaminated the planet by repeatedly landing on it. Although great pains are taken to sterilize spacecraft, and Mars' harsh UV radiation is likely to have killed off any Earth organisms that could have hitched a ride, it's still a possibility. That's why some people think that if you want conclusive proof of a second genesis in our own solar system, and a chance of finding life totally unrelated to ours, you have to look further out than Mars. Someplace colder, darker, and altogether weirder.

• • • •

The Case for Europa

UC Berkeley paleontologist and geologist Jere Lipps spent a good deal of his career planning expeditions to Antarctica to search for life under the ice, which more or less qualifies him for planning expeditions to Jupiter's icy moon Europa.

After Mars, Europa is the number two pick of astrobiologists for places in our solar system that could harbor life. Two of Saturn's satellites are runners-up: Titan, because of its carbon-rich atmosphere; and icy Enceladus, where cryovolcanoes shoot geysers of water vapor into space. But Europa, an iceball slightly smaller than our Moon, is the clear favorite — astrobiologists believe it contains a deep, salty ocean below its frozen crust. Both the ice and the ocean can make fine habitats, Lipps believes.

You might think ice is dead. Au contraire. "Life loves ice," Lipps explains. So long as they don't freeze and can find food, organisms can flourish, as Lipps observed during his years studying the habitats of the microorganisms, crustaceans, fish, and sea sponges that make their home around Antarctica's Ross Ice Shelf.

But, he cautions, there are differences between our own ice-covered oceans and Europa's. For example, Europa doesn't get much sunlight, so there's probably not much photosynthesis. Even creatures that live in Earth's deep seas often depend on a food chain that begins with photosynthetic creatures. Lipps believes that Europa may have other nutrient sources, such as chemicals that well up from the ocean floor.

Europa's surface certainly is more hostile than Earth's — it bathes in Jovian radiation, and its surface temperature is estimated at minus 260 degrees Fahrenheit. But the outer few meters of the ice likely shelter everything below, Lipps says, and while the surface is frigid, the liquid water below can't get any colder than it gets on our planet. The tides may generate enough heat to keep Europa's oceans from freezing solid, and scientists have posited other heat sources that could help sustain life, such as radiation and thermal vents in the ocean floor.

If there's any life on Europa, it's trapped beneath a giant slab of ice, and it can't come to the surface without being frozen or irradiated. So how to detect it? Lipps explains that, because ice is constantly on the move, you can do paleontology with it just as easily as you can with rock. On Europa, ice sheets thrust above or below one another, smash together, and crack apart, pushing things once buried to the surface. The tidal action of Jupiter also opens rifts in the ice, which let materials well up from underneath and then freeze along the edges of the cracks. That these cracks have a distinct reddish-brown discoloration delights astrobiologists — they wonder if this is residue formed by layers of dead bacteria.

The discolorations also are the sort of thing cameras can spot. For the last several years Lipps has been working with Lockheed Martin on a concept for a telescope called MIDAS (Multiple Instrument Distributed Aperture Sensor) that could fly by Europa and take pictures with a resolution of up to two centimeters from a distance of one hundred kilometers. Lipps' job is to tell them where to point it. "My search strategy for Europa is exactly what I would do if I was going out into the Grand Canyon," he says — to look for broken, exposed areas and places where chaos in the terrain might have mixed up the layers and tossed up a residue of biological material. This includes not only the discolored ridges, but impact craters from meteorites, cliffs formed by clashing ice sheets, and places where warmer water from below rose to, and then flattened, the surface ice.

MIDAS is still in the planning stage, and no one knows when, or if, it might fly. Until recently, most scientists interested in Europa had pinned their hopes on NASA's planned Jupiter Icy Moons Orbiter, but the project was scrapped in 2005. Other probes that could penetrate Europa's ice or chip off bits of it to sample are still in the theoretical stage.

That hasn't dampened anyone's enthusiasm for Europa. "The chemicals are there," Lipps says. "There are energy sources there. There's water there, and those three things are what you need for life generally as we know it on Earth. So that's a good sign, that's why we want to go there."

• • • •

Of course, there's a difference between having the right conditions for life and actually creating it, cautions NASA Ames astrobiologist Lynn Rothschild. She's an expert in what kind of conditions organisms can put up with: She has explored Yellowstone hot springs and Kenyan lakes, finding creatures that live in hot acid baths. She's been to the Bolivian Altiplano to learn about critters that can withstand intense UV exposure. Recently she and her research associate Dana Rogov have been studying salinity-loving microbes that live in the South Bay's Cargill salt flats. These extremophiles have taught her that the kinds of habitats that can support life are much more diverse than we once thought. "The chemical and physical conditions for life, we're increasingly realizing, are not even uncommon in our own solar system," she says. "One can't help but believe that there are literally millions of places where life could form."

That said, she points out, "We have yet to make life. We can't even take a cell that's dead and bring it back to life, and we've got all the building blocks right there." There's a weird gap in human knowledge — we know what conditions life needs, and once it exists, Rothschild says, "we have very well-described evolutionary theory to understand how to go from microbes all the way to an elephant or a human or a sequoia." But the crucial step in between — the animation of life's raw materials — eludes us.

"Maybe we just haven't added the secret ingredient yet or we don't have the right proportions," she continues. "But say that I'm wrong, that there is some incredibly difficult step between having the right chemistry and physics and actually setting something that's alive in motion — then it may not be common at all."
She weighs her expectations for what we might find in our solar system with similar caution. Unlike McKay, Rothschild would be delighted to find any life at all, even if it's our evolutionary cousin. (After billions of years of separation, it has probably developed some interesting kinks.) And if it's an independent genesis that hasn't, as the Rare Earthers predict, proceeded much further than pond scum, that's okay, too. "I think if people are sitting around waiting for Europa to come out and shake hands, that's really quite silly," she says. "To me, life is what's exciting. The rest is just details."

In fact, the most exciting part of finding life in our own solar system may boil down to sheer math. Our star is one of two hundred billion stars in our galaxy, the Milky Way, which is one of hundreds of billions of galaxies in the universe. If life originated twice around a single, unremarkable star, then really, the sky's the limit. "The difference between one data point and two data points is more than just double," McKay says. "If we found that life started twice right here in our own solar system, then that to me would be scientifically convincing evidence that life is widespread throughout the universe."

If we struggle to investigate even the next planet over, how are we going to search for life farther away? Answer: with really big telescopes. Next week, we'll introduce you to the other half of the scientific tag-team searching for life in the universe. If astrobiologists are figuring out what kind of neighborhoods life prefers, these planet hunters are looking for likely street addresses. As they're discovering, there may be many other places like home.