First Contact (2 page)

This is heady stuff, and it has scientists moving in all directions. If primitive life forms can exist miles below the Earth's surface in the mines of South Africa without contact with the sun or its products, why couldn't the same be true on Mars, or on the moons of Jupiter and Saturn, or on the untold number of rocky planets we now know exist across the universe? The same logic applies to extremophiles in the abyss of the deep sea, in glaciers tens of thousands of years old, in Spain's acidic Rio Tinto or California's alkaline and arsenic-laced Mono Lake. Extremophiles even survive in the upper atmosphere of our planet.

Perhaps you're thinking that the discovery of bacteria deep underground does not seem all that earth-shattering. Perhaps you're thinking, too, that even if Mars or a moon in our solar system turns out to have comparable subterranean life, that would prove little except that primitive life forms come into being and survive in all kinds of places. Astrobiologists see things very differently. As they are quick to point out, life has
existed on Earth for at least 3.8 billion years, and for more than 3 billion of those years single-celled bacteria and related microbes were the only living things around. In other words, butterflies, tree sloths, saber-toothed tigers, and humans all evolved from single-celled organisms too small to see without a microscope. Astrobiologists today have a deep respect for the significance of bacteria and other single-celled creatures—and their ability to evolve into intelligent life.

Searching for and understanding extremophiles is almost universally embraced by the scientific community as essential and revelatory science now, but as late as the mid-1990s it was seen as quixotic and something of a career ender. Tullis Onstott is the man who changed the field by descending into deep gold mines in South Africa and coming back with remains of bacteria that have lived down there in their own peculiar worlds for millions of years. Onstott, a geobiologist at Princeton University, initially couldn't get funding for his research, and his first expeditions were paid for out of his own pocket.

Stories like his are common in astrobiology. The early extrasolar planet hunters were told in the 1980s and early 1990s that they were wasting their time, that there was no way to detect their quarry through the blinding glare of parent stars hundreds or thousands of light-years away. So they developed other techniques based on measuring the slightest movements of those suns, minuscule course corrections caused by the gravity of the orbiting exoplanets. Now, through those methods, planets beyond our solar system are found regularly and the expectation is that billions more remain to be mapped. What we know of them remains limited to their orbits, their mass, and a little about their component parts. The new challenge is to characterize them much better, especially the smaller Earthlike planets expected to be discovered in the years ahead. But these planets are minute at such great distances and are blotted out by the intense light from their parent stars.

So how can astronomers compensate? One proposal, years in the making, involves sending into deep space a football-field-sized sunshade, that
would then work in tandem with an orbiting telescope 35,000 to 50,000 miles away to create an “occulter.” The flower-petal-shaped screen would block light from the star and thereby allow the telescope to see and study orbiting planets, their atmospheres, and any signatures of possible life. The long process of transforming an idea like this into a space-faring reality got a big boost in 2010 when a panel of the National Academy of Sciences gave its highest priority to exploring exoplanets and their atmospheres in the next decade. An occulter system may ultimately not be the technology selected for the job, but it is a serious contender.

The sunshade might sound like a far-fetched project to pursue, but many of the most successful results in astrobiology began as pursuits that sounded impractical or extreme. Take, for instance, the work of Sara Seager, the astrophysicist who first opened my eyes to the breakthroughs and great promise of astrobiology. Her mind easily visits places where few of us can follow. Raised in Toronto, she puzzled her father with an early interest in outer space, which later turned into degrees in math, physics, and astronomy. Her pioneering work on the atmospheres of exoplanets is what persuaded MIT to offer her tenure and an endowed chair in planetary science. She was thirty-four at the time. She is a theorist rather than a hands-on planet hunter: her scientific specialty is to predict and refine ways to identify the elements and compounds in the atmospheres of extrasolar planets, work that she began before the first extrasolar planet was detected. She was told when she started that her ideas were theoretically interesting but couldn't be tested in her lifetime. But little more than a decade later, we know that the gas methane exists on a giant planet orbiting a star 63 light-years away, that sodium exists on a planet orbiting a sunlike star 150 light-years away, and that evidence of both oxygen and carbon has been detected enveloping another planet in that solar system. Now Seager is convinced that extraterrestrial life will be detected within her lifetime, and she wants to be part of that triumph.

Given the size of the challenges taken on by astrobiology, however, the big break won't come from the inspired minds of one or two great thinkers,
as they did with Galileo, Copernicus, Newton, Darwin, Einstein, and Watson and Crick. This search is more of a broad-based, inexorable, but oddly unheralded Apollo program, an undertaking that requires thousands of researchers with very different backgrounds, technical skills, and obsessions. The enterprise is playing out in plain view, yet is so big it is almost invisible.

Some astrobiologists (and astrobiology fans) no doubt dream of a “Eureka!” moment when life is discovered beyond Earth or synthesized here—an equivalent to Neil Armstrong's giant step on the moon, or the unraveling of the structure of DNA. Someday that may come, but science generally works incrementally, and takes much smaller bites. Even the biggest, hottest research questions in astrobiology involve work akin to crime-scene forensics, often drawing on small left-behind clues to help put together pieces of the larger puzzle. These are the kinds of questions absorbing, inspiring, and at times dividing the inherently fractious tribe that constitutes the field of astrobiology.

That tribe is fractious because it's attempting to answer a set of unavoidable and obnoxious questions—obnoxious because they appear so simple, yet actually are so complex: What, when all is said and done, is life? Could we encounter it elsewhere and simply pass it by? Are we blinded to extraterrestrial life by our Earth-based assumptions of what life must be? We have a substance on Earth—a blackish rock coating called desert varnish found in many arid places and often used as a background for Native American petroglyphs. Experts in the field going back to Charles Darwin have studied it, and they still sharply disagree about where it comes from: whether it is a product of microbial biology or of geology and chemistry. Getting a better sense of what is living and what is not on Earth seems pretty essential to the quest for life beyond Earth, and so these borderland cases attract lots of attention. Desert varnish is especially intriguing because something that looks similar to it has been seen during several Mars missions, or so some scientists contend.

Nobody knows how or why, but virtually all the amino acids—molecules that make up essential building blocks of proteins (and therefore
of life as we know it)—share a necessary quality that is otherwise seldom seen on Earth: Their molecules are all organized in a formation that scientists call “left-handed,” enabling them to interact with uniformly “right-handed” sugars. Because virtually nothing else on Earth is structured like this—all “left-handed” or all “right-handed”—some scientists suspect the initial overabundance of left-handed amino acids arrived here by way of meteorites or comet dust. Evidence from one large and quickly recovered meteorite that fell in Australia in 1969 lends some support to that conclusion, with potentially major implications about how life began and evolved here, and the possible makings of life elsewhere.

Did life on Earth start in scalding, sulfurous hydrothermal vents on the deep ocean floor, or perhaps at the less intense side vents that tend to spring up in the same regions? Did it start in subterranean rock fractures where it could be protected from the heavy meteor bombardment of early Earth? Did it begin around the plumes of erupting volcanoes, where intense lightning activity (the kind of energy needed to start the chemistry needed to support life) is now known to be common? Or did it begin in the “little warm ponds” put forward by Charles Darwin, or perhaps via those meteorites? All of these possibilities have their advocates. If scientists can get a clear sense of how nonliving chemicals were transformed into the self-replicating, energy-consuming, evolving entities that ultimately produced us here, they'll have the beginning of a road map for what might be happening out there.

While this search is under way, another hunt for signs of extraterrestrial life—a broad range of potential “biosignatures” ranging from the presence of liquid water to the organic molecules associated with life on Earth—is also moving quickly ahead. For instance, the methane gas recently detected in the atmosphere of Mars is released in plumes at specific sites and at predictable times, suggesting previously undetected, even unimagined Martian geology and biology. On Earth, about 90 percent of methane is a by-product of biological processes released by living, or once-living, things ranging from bacteria to rotting trees to flatulent cows. That isn't necessarily the case on Mars, but it certainly is a real possibility.

Scientists are also probing whether Mars was more hospitable to life at its inception than was Earth, which after all did take quite a hit when a Mars-sized body crashed into it and ejected the material that most planetary scientists believe became the moon. And if life did start on Mars, could it have traveled via ejected rock-turned-asteroid to Earth? Bacteria in Antarctica and other glaciers frozen for hundreds of thousands of years come back to discernible life when brought to higher temperatures, and researchers contend they could last in a suspended state (or maybe even carrying on life functions) for millions of years more. Other microbes have shown a previously unimaginable ability to withstand the cosmic radiation of space. Put all this together and the unavoidable question becomes whether, at bottom, we're all Martians—quite literally descendants of life from Mars. If methane can ultimately be traced to a biological source on Mars, astrobiology will enter an entirely new phase and the quest to find extraterrestrial life will become something more like a race.

Have we actually already found extraterrestrial life on previous Mars missions and in meteorites found on Earth? This is one of the most contentious issues in astrobiology—and in science as a whole—and many highly qualified scientists on opposing sides of the issue are 100 percent convinced they're right. Feelings are especially high because as astrobiology's patron saint, Carl Sagan, once said, “Extraordinary claims require extraordinary proof.” But extraordinary proof is very hard to come by, and tantalizing findings are hard to keep under wraps. The result has been a number of long-running scientific grudge matches—intellectual blood sport at the highest of levels, with seemingly many rounds to go. Interdisciplinary cooperation is the mantra of astrobiology but it has yet to repeal the laws of human nature.

The most significant dispute is no doubt over the contested discovery that gave birth to the new era of astrobiology: the 1995 announcement that NASA scientists had discovered a meteorite from Mars that contained numerous features consistent with extraterrestrial life. Critics quickly tore into the report and left it seriously wounded. But the authors have continued
their work and say they are more convinced than ever that many Martian meteorites show signs of long-ago extraterrestrial life.

Just as those immersed in astrobiology now theorize that extraterrestrial life does—perhaps even must—exist, astronomers long theorized that planets circled stars in other solar systems. It wasn't until the mid-1990s, however, that the first definitive detections were made. Now, more than five hundred exoplanets have been identified, seven hundred more are awaiting confirmation, and billions more are believed to exist throughout the universe. As much as any other discoveries, the peek into the world of exoplanets has supercharged astrobiology and encouraged scientists to substantially increase their bets on the existence of extraterrestrial life. But the discoveries have come with big surprises. Most of the extrasolar planets found so far are large gas giants like Jupiter, orbiting close to their suns with smaller but also giant planets farther out—a kind of solar system that virtually nobody predicted. That so many of the planets discovered are in this category is, to a substantial extent, a function of how astronomers are looking for them—bigger and closer to the central star is what we have the technology to detect. But the notion that any Jupiter-sized planets would be orbiting their suns in four or five days was, until recently, unthinkable. Equally unexpected was the discovery that many solar systems consist of planets that travel in wildly eccentric orbits, not the circular or near-circular ones we're accustomed to. The fact that solar systems come in such peculiar arrangements has both promising implications for astrobiology—with solar systems so varied, the probability is that some others are “just right”—and some negative because planets in those wildly eccentric orbits would probably make their solar systems unstable and uninhabitable.

So the big question for planet hunters is no longer simply how to find planets, but rather how to find more of the smaller, rocky, Earth-sized planets the right distance from their suns to be potentially habitable, and to find solar systems structured in ways that could allow these cousins of the Earth to become nurseries for life. NASA's Kepler spacecraft was launched in 2009 to make a broad search for Earth-sized planets, and it's expected to
begin delivering substantial results in 2011. But much of the serious planet hunting is being done using Earth-based telescopes, and the ingenuity of the scientists operating them is the stuff of legend. Anyone betting against them finding habitable planets and solar systems has not been following their fevered discoveries.