Читать книгу The Quest for Mars: NASA scientists and Their Search for Life Beyond Earth - Laurence Bergreen, Laurence Bergreen - Страница 8

Jim Garvin’s collection of Jim Bunning baseball cards got me to thinking about what is perhaps the most famous baseball card of all – the 1909 portrait of Honus Wagner in a Pittsburgh Pirates uniform. When I gaze at the face of this young man, who seems to be staring into space, I find myself asking, “What was it like to be alive in 1909?” I have little idea, although it was just the other day, in geological terms. All I have to go on are artifacts, such as this famous baseball card. I can’t watch Wagner play baseball, and I can’t hear his voice; all I can see is a fuzzy image of the athlete in a uniform, a trick of light and shadow, an impression of life as it once was. To fill in the gaps, I would have to look beyond the card, but if I’m relying on the card, and only the card, I have precious little data.

The card, and its limitations, call to mind the tantalizing images of fossilized bacteria in a 3.9 billion year-old meteorite from Mars – images that may be the first scientific evidence of life beyond Earth. Fossilization occurs when minerals replace organic elements in once living things. The morphology remains, although the chemistry is different. Still, scientists can learn a lot from fossils. They can detect the approximate age, which is crucial, and, by studying fossils in their natural setting – in situ – they can extrapolate a great deal about the geological, chemical, and biological circumstances surrounding them. “Fossils are the autographs of time,” wrote the American astronomer Maria Mitchell. For these reasons, fossilized bacteria from Mars – if that’s what they are – have great appeal; they are our best indication of life beyond Earth. Like the antique baseball card, they offer only a very narrow glimpse into the past. 1909, the year of the Honus Wagner card, wasn’t very long ago, but it’s long enough past to seem quite mysterious. How much more difficult it is, then, to construct a scenario for the existence of life on Mars several billion years ago from the evidence contained in a meteorite.

If the fossilized bacteria are genuine artifacts of Martian life, they raise more questions than they answer. If life started on Mars, how did it begin? Is it still there? If not, when did it end, and why? If it’s on Mars, where else in the universe might it be, and what form does it take? Did it originate on Mars, on Earth, or somewhere else? All these Genesis Questions point up how much scientists have yet to learn about how life began. In the course of asking questions of various scientists who study these problems for a living, almost every reply I received began this way: “No one knows, but …” That’s followed closely by, “There are several possible scenarios,” and “Well, current speculation has it that …” The answers are all variations on the theme that no one knows, yet. But scientists have hypotheses. They have scenarios. The meteorite from Mars has inspired a widely accepted scenario – I’ll call it the Best Guess Scenario – concerning the origins of life on the Red Planet, and how it came to be transported to Earth.

Four and a half billion years ago, the Solar System was in its infancy, and the planets were new. In its first billion years of existence, Mars was a warmer and wetter place than it is now. Water flowed freely over its surface and pooled underground, in reservoirs. The flood channels carved into the surface of Mars, some of them many miles in width, left an eloquent record of catastrophic outpourings of water. In all likelihood, the water on Mars was quite salty. (Fresh water on Earth is due to evaporation and rainfall.) Eventually, the floods subsided, and the water drained into Mars’ vast northern plains, where it might have frozen. In the process, it reshaped the Martian terrain until it resembled a desert that had once been flooded, but became bone dry. Nevertheless, its contours preserved geological memories of rivers and oceans and lakes.

The large Martian pools of standing water were subject to peculiar tides caused by the planet’s two small moons, Phobos and Demos. And they were subject to the Martian winds; when they blew, reaching speeds of hundreds of miles an hour, they generated waves with peculiar shapes, higher and steeper, with more pronounced peaks than exist on Earth. The lower Martian gravity, less than half of Earth’s, allowed the slender waves to tower until they resembled the watery shapes in a drawing by Dr. Seuss; they would flop over and spatter, as if in slow motion. The marine scene on Mars was all oddly familiar, and strangely different.

The Martian sky was blue a few billion years ago, and there were a few clouds, just as Mars has now. It was mostly cold, and extremely cold at the poles, except for the equator, where it was warm. Martian volcanoes erupted with regularity, and in the Red Planet’s low gravity they assumed formations that couldn’t exist on Earth; they were larger and higher. In these ancient Martian conditions of two or three billions years ago, life could have formed and evolved, just as life appeared on Earth within a billion years of this planet’s existence. The volcanoes, especially the ones close to reservoirs of water, or polar ice, created hot spots where life would most likely have formed on Mars. No one knows how far it developed, or if it ever got underway. It might have remained dormant most of the time, for tens of millions of years at a stretch. Or it might have progressed beyond simple bacteria; there might have been Martian insects crawling around, adapted to the Red Planet’s lower gravity, lower density atmosphere, and cooler temperatures. These variations suggested life forms that were spindly, similar to insects. The skeletons might have been external, with many legs to take advantage of the lower gravity. As for the cooler temperatures, life on Earth has shown remarkable adaptive creativity. “Some insects winter-proof themselves with glycerol, a common antifreeze used in automobile radiators,” Carl Sagan theorized. “There is no conclusive reason why Martian organisms should not extend this principle, adding so much antifreeze to their tissues that they can live and reproduce in the extremely cold temperatures occurring on Mars.” The ancient Martian atmosphere would have required similar creativity in the creatures’ breathing apparatus. If they had evolved to the point of multi-cellular differentiation, they might have developed enormous gills or lungs, relative to their size. Even if life never reached this advanced stage of evolution on Mars, it is still possible that tiny organisms formed in the water-drenched Martian rock, and then, for some reason, died off, leaving fossilized remains hidden beneath the surface. It was as though Nature initiated an experiment but abandoned it in the early stages.

Ancient Mars was more turbulent than Mars is now. In the young and volatile solar system, it was constantly bombarded by chunks of asteroids. It is possible that at some point in Martian history, an asteroid of such dimension struck Mars and created cataclysmic changes in the planet’s climate and geography that whatever life forms had managed to take hold were snuffed out, leaving only their skeletons, which became fossilized. Or perhaps the death of Martian organisms came about slowly, as the planet lost its atmosphere a little at a time to space, and its water eventually disappeared below the surface, or vanished with the atmosphere, leaving behind a desiccated, celestial sandbox.

If we had been able to observe the first few billion years in the life Earth and Mars from a vantage point in distant outer space, we might have noticed several common trends. We would have seen watery places on both planets. We would have seen volcanoes on both planets, their plumes of smoke, their pollution of the atmosphere. We would have seen clouds on both planets, and we would have detected seasonal waxing and waning of the polar caps. As the eons passed, subtle differences between the two planets would have become apparent. If we had been looking closely, we might have noticed the atmospheric changes. We might have seen the dramatic increase in oxygen in Earth’s atmosphere, and a corresponding spread of vegetation on its surface; if we were very perceptive, we might have noticed the spread of plant life in its oceans, in the form of algae.

At roughly the same time, we would have seen that Mars was losing its nitrogen-rich atmosphere. It was thinning out, disappearing into the frigid vacuum of space. More obviously, we would have seen the great Martian standing bodies of water recede, exposing a complex erosional system of gullies and playas and rearranged boulders, many of them acting as signposts to the water’s former whereabouts and actions. During the last few hundred million years, if we were sufficiently attentive, we might have noticed the spread and intensification of vegetation on Earth, as the biomass increased and diversified, and various life forms competed for natural resources, or evolved ways to cooperate, or both. At about the same time, we might have watched Mars continue to regress to its early state, with some important differences. It contained geological traces of water, and perhaps traces of biology – clues, ultimately, to its origins, and to ours.

Where did the elusive Martian water and its life-giving properties go? Come to think of it, where did the water come from? Where did water on Earth come from, for that matter? At the Lunar and Planetary Institute in Houston, Steve Clifford has spent years studying water on Mars, and he told me there are several schools of thought concerning the origins of water on this planet and on Mars. “One is that after the Earth was formed, comets bombarded the planet, adding volatiles over perhaps the first billion or half billion years.” A comet is basically a celestial snowball, bearing ice from somewhere – God knows where – to here. “The other school of thought is that much of the water we have on the Earth was contained in the early material that formed the planet. As the Earth started to accrete asteroidal material and dust in the early Solar nebula, it gradually reached a size where the quantity of radioactive material was sufficient to heat up the planet and cause it to differentiate. The heavier stuff sank toward the middle, which is how we got an iron core, while the lighter stuff, which may have contained water, was released during the formation of the crust and atmosphere.” A similar process may have occurred on ancient Mars at about the same time it happened on Earth.

Steve surprised me by suggesting that water on Mars may still linger beneath the surface, more than a little. He was talking about “sizable reservoirs of ground ice and ground water.” The evidence he has for large volumes of water on Mars is mostly indirect. He calculates the amount of “pore space” to be found in Martian rocks and soil; water could be stored there. If it is, some of the water could be in liquid form, especially well below the surface. “Like the Earth, Mars is thought to be radiating internal heat due to the decay of radioactive elements, which means the deeper you go below the surface, the warmer the temperature gets,” he says. “And if you go down several kilometers, you could easily get temperatures that are consistently above freezing,” which means liquid water might exist on Mars today. In fact, he thinks there might be two types of water reservoirs on Mars, a region of permafrost near the surface, as well as larger and warmer reservoirs at greater depths. There you might find liquid water in great quantities, and water, of course, leads to life. This subsurface system would act as a powerful preservationist of life, no matter how harsh conditions on the surface. “If life ever evolved on Mars, and adapted to a subterranean existence, then its survival would be assured for the indefinite future.”

Subterranean life on Mars could survive the loss of the atmosphere, it could survive intense cold on the surface, it could even survive the largest life extinguisher we know about, the impact of a large asteroid. Imagine what would happen if an asteroid the size of Manhattan collided with Earth, Steve says. It would certainly be calamitous and likely sterilize the surface of the planet down to a depth of several meters or so, but not below that. It would kill life on the surface, but it wouldn’t eliminate all life on Earth, because the impact’s thermal and chemical effects would be limited to the uppermost part of the surface, with the exception of the area where the impact occurred. After the impact, life far below the surface of the Earth would go on. The same holds true on Mars, Steve says: “any microorganisms that might have evolved four billion years ago, when the planet might have been warmer and wetter at the surface, could readily survive to the present day at depths of several kilometers.” If he’s correct, or even partly correct, life could very well exist within the Red Planet’s ancient reservoirs, awaiting discovery.

The Best Guess Scenario assumes that some kind of simple life did exist on Mars a few billion years ago, as Steve described. And it assumes that asteroids have bombarded Mars ever since, pockmarking its surface with craters. Sixteen million years ago, according to the scenario, a refugee from a disorganized asteroid belt struck the surface of Mars with tremendous force. Since Mars has only 38 per cent of Earth’s gravity, the impact was sufficient to drive pieces of rock buried beneath the surface high into the Martian sky and beyond. The pieces shot into space five times faster than a bullet – fast enough to escape the Red Planet’s gravity. Until a few years ago, it was thought the shock of impact would vaporize or severely deform the ejected material – the ejecta – along with everything in it, including any signs of life, but recent computer modeling has shown that the physics involved would allow the ejecta to remain intact. In the model, the asteroid comes in at an angle, strikes, and creates a vapor cloud that sweeps across the surface at an extremely high speed and carries material off Mars into solar orbit. The whole process might take five or ten seconds, long enough for some fragments of Mars to remain intact. “That’s a fairly gentle way to get stuff off the surface,” says Mark Cintala, who studies craters at NASA’s Johnson Space Center – gentle enough to launch a fossil-bearing meteor on a trajectory to Earth.

There’s a variation in the Best Guess Scenario that incorporates another method of ejecting material safely from the surface of Mars: spallation. Much of the work on the spallation model was done by Jay Melosh at the University of Arizona, and it’s extremely simple, in theory. You put a quarter on a tabletop, rap the underside of the table, and bump the quarter into the air; that’s spallation. “You’re sending a stress wave through the table, and that stress wave is transmitted to the quarter. Imagine that happening on Mars,” Mark Cintala says. In that case, the asteroid’s impact would create a compressional wave, as if it were a depth charge below the surface. The resulting shock wave would bounce a rock fast enough to escape the relatively weak Martian gravity and send it on its way. “At least, the calculations say it can.”

In the Best Guess Scenario, one of these dislodged pieces of Martian rock sped off in the direction of Earth, a cosmic message in a bottle floating through an ocean of outer space. The journey lasted millions of years, and the ancient chunk of Mars crashed to the surface of Earth a mere 13,000 years ago. The size and shape of a potato, it buried itself in the Allan Hills of Antarctica. The meteor had become a meteorite.

Meteorites have held a special fascination as relics from the heavens, mute messengers from parts unknown. In the Middle Ages, meteors falling to Earth generated superstition and concern. Where did they come from? What did they mean? The faithful brought them to the authorities, and in time, the Catholic Church acquired a large repository of these curiosities. In 1969, the study of meteorites underwent a quiet revolution when Japanese researchers found high concentrations of them preserved in arctic ice. Since 1977, NASA, a technological Vatican, has been collecting meteorites from Antarctica and housing them at the Johnson Space Center in Houston. Each year, there are hundreds of new arrivals, and when there’s a promising delivery, scientists clamor to get a piece to study. There are now nearly 10,000 rocks under lock and key in Building 31 at Johnson, many of them preserved in nitrogen. By measuring the radiation absorbed by the meteor during its space travels, scientists can determine approximately when the rock arrived on the Earth, and even how long it spent in space before it arrived on our planet.

In December 1984, Roberta Score was hunting for meteorites in Antarctica. At the time, she was employed by Lockheed Martin and working at the Johnson Space Center. Around Johnson, a meteorite collecting mission is not exactly choice duty; join one, and you were said to have become part of the “Houston weight loss program.” Walking across an apparently endless sheet of ice, Robbie Score came across a greenish stone about the size of a potato. Once she removed her sunglasses, she saw the meteorite was not greenish, after all; it was gray and brown, but she knew it looked different from the ordinary meteorites she found in the field. Along with other samples, it was kept in a freezer aboard the ship that brought it from Antarctica to Point Magu, California, where it was packed in dry ice and sent to Johnson, where it was stored in cabinets that once held moon rocks. The meteor curators, including Robbie Score, designated it ALH 84001 – their way of saying this was the most interesting meteorite collected in 1984 in the Allan Hills of Antarctica. But after being delivered to Johnson, ALH 84001 was misidentified as an asteroid fragment, a diogenite, rather than a piece of Mars, and stored in Building 31. It was not ignored, however; small sections were allocated to the scientific community for further study over the years; in all, almost a hundred “investigators” examined it, and everyone continued to misclassify it as a diogenite – with one exception.

In late 1993, David Mittlefehldt, a veteran Lockheed Martin scientist also working at Johnson, reexamined ALH 84001. Mittlefehldt was an expert on diogenites, and this particular rock didn’t look like one to him. It seemed to have more oxidized minerals in it than your normal diogenite, for one thing. Using new technology in the form of high-resolution laser spectrometry, two other scientists, Donald Bogard and Pratt Johnson, extracted gases trapped inside the strange meteorite and discovered that their very idiosyncratic characteristics exactly matched gases on Mars as measured by the Viking spacecraft in 1976. Mittlefehldt published his findings in 1994 in a scientific journal, and attracted the notice of the science community. Although this wasn’t the first meteorite from Mars to have been discovered, the reclassification created a stir. Of the thousands of meteorites that have been cataloged, only fourteen are believed to have come from Mars; the overwhelming majority come from asteroids, and a few from the moon. Meteorites are named for the places where they have fallen to Earth, so the Martian meteorites have some fairly exotic names – Shergotty (India), Nakhla (Egypt), and Chassigny (France), among them – and are known collectively as SNC or “snick” meteorites. “SNC meteorites” is an elaborate way of saying “meteorites from Mars.”

Carefully considering his find, Mittlefehldt noticed minuscule reddish-brownish areas deep within ALH 84001; they looked a lot like carbonates, and on Earth, carbonates such as limestone tend to form close to water. What made this all so curious was that no one had detected carbonates – and their suggestion of water – in the other Martian meteorites. They were billions of years newer; they probably came from a more recent era in the geologic history of Mars, after the water that once flowed freely across its surface had disappeared. This one, however, apparently harkened back to that warm and wet golden age on Mars. Dating confirmed that the meteorite was indeed very old: 4.5 billion years old, much older than other known Martian meteorites, and it contained carbonates that were 3.9 billion years old. Mittlefehldt wanted to get some idea of the temperature range in which the carbonates had formed billions of years ago, so he went to yet another NASA scientist, Everett Gibson, who examined the very curious meteorite with Chris Romanek; they published a paper in the December issue of Nature in which they said the carbonates had formed at temperatures below 100° C, in other words, at moderate, Earthlike temperatures – “well in the range for life processes to operate,” as Gibson puts it.

By now a line of reasoning was beginning to take shape. The team had their meteorite; it was from Mars. Almost no one disputed that singular fact. And it was very old, when water was thought to exist on the Red Planet. And it had carbonates, suggestive of water, formed at moderate, Earthlike temperatures. With each new discovery, the stakes became exponentially higher. ALH 84001 had gone from being a curiosity to an interesting and instructive case study to a potential harbinger of a scientific revolution. Each new link had been more difficult to fashion than those that had preceded it, and the final link – to life on Mars – would be the most difficult of all to fashion.

Other scientists soon began angling for a piece of the curious, potato-shaped Martian meteorite, among them David McKay. Over the years at Johnson, he’d become known as a solid and reliable scientist, not the type to go out on a limb. Carl Sagan he was not. If you asked around about McKay, you often heard words like “cautious” and “self-effacing,” yet he had a distinct air of authority; he’d published hundreds of scientific papers, and he knew his way around Johnson and around NASA. Over the decades, he’d learned about science and about maneuvering in the world of scientists. He knew about the pitfalls, how quick others were to leap on “discoveries” and tear them to bits. Yet with all his experience, he seemed destined to retire in honorable obscurity, until ALH 84001 came to his attention.

“I’m going to get a piece of that meteorite and look for signs of life in it,” he told his wife.

“Sure you are,” she said.

McKay had a vast storehouse of information and impressions about rocks on which to draw. In his long career, he had looked at perhaps 50,000 of them, and he spent many hours studying the most intriguing he’d ever seen, ALH 84001, with a scanning electron microscope capable of magnifying objects 30,000 times. With this instrument, McKay identified a bunch of – well, they looked a little like miniature subterranean carrots, or worms, or tubes. Whatever they were, they didn’t look like something you’d expect to find in a meteorite.

Again, he turned to another scientist for assistance. Kathie Thomas-Keprta was a biologist who had spent almost a decade studying extraterrestrial particles – space dust – before she focused her attention on the meteorite from Mars. She was accustomed to making do with very little. A specially modified B-57, flying at high altitude for an hour, might collect just one extraterrestrial particle from an asteroid, a particle too small to see but big enough for her to examine under a powerful microscope. When McKay invited her to study a Martian meteorite, she was delighted to have something as big as one millimeter by one millimeter to work on after all those years of studying specks. Even better, she was an expert with a new type of electron microscope that could reveal the mineral composition of the carbonates locked in the meteorite. McKay and Gibson showed her the photos taken by the scanning electron microscope, and they proposed that she examine those peculiar, worm-like structures to see if they were fossils. She listened respectfully to their proposal, and when she got home that night, Kathie told her husband, “These guys are nuts!”

A team of researchers based at Stanford subjected chips of the meteorite to further laser tests, which yielded polycylic aromatic hydrocarbons – PAHs, for short – which are often associated with life. That finding raised more questions than it answered, for PAHs are also associated with inorganic material such as pollution and exhaust. If that were the case, the carbon in the meteorite could be the result of very recent contamination on Earth, not evidence of ancient life on Mars. Additional tests showed that the PAHs were buried inside the meteorite and probably quite old, lessening the likelihood that they were the result of exhaust. It looked like the PAHs came from Mars, after all.

The team felt confident enough to announce some initial findings at the 1995 Lunar and Planetary Science Conference, held at the Johnson Space Center. In the planetary science community, the LPSC is a very big deal, a sort of scientific Super Bowl. If you don’t show up for this event, scientists say, everyone assumes you’ve died, and when you do show up, you come to make news, if you can. On behalf of the meteorite team, Kathie Thomas-Keprta presented a paper about the unusual and provocative features of ALH 84001 observed by the team. The paper stopped short of declaring they had found evidence of life on Mars, even very ancient, very tiny life. In fact, she adamantly denied it to a reporter from the Houston Chronicle who suspected she was hinting at it.

She knew other scientists would soon challenge her findings, no matter how cautiously expressed. Faulty science or clumsy handling of the situation could mar several carefully-tended careers. So McKay and his colleagues ran still more tests on the meteorite with an even more powerful scanning electron microscope designed to inspect rockets for minuscule fissures; this instrument was capable of magnifying objects up to 150,000 times. McKay put a four billion year-old piece of Mars under the microscope, and on his monitor there appeared a bunch of worm-like forms. He printed an image, and gave it to his teenage daughter.

“What does it look like to you?” she asked her father.

“Bacteria,” he answered.

Kathie Thomas-Keprta eventually decided the guys on her team weren’t nuts, after all. Her conversion occurred in Building 31 at the Johnson Space Center one night when she was working late. As she examined the shapes of the nano-fossils in the meteorite, she knew from experience they were of biological origin. “It was gregite, an iron sulfite present in the carbonate. It had a certain morphology known to be produced by bacteria. It was actually a biomarker, a thumbprint left by biological activity. I thought, ‘That’s it. There’s life on Mars.’

“I walked out the door to the parking lot, half-expecting to see flags waving and bands playing, but there was nothing at all out there, just a dark, empty parking lot at night.”

The chain of reasoning was more or less complete. The meteorite was old enough to contain of a record of Mars’ early days, when water was plentiful. It had carbonaceous material; it probably had Martian rather than terrestrial PAHs; and it had gregite, a universally accepted sign of biological activity. Although each distinct link could not be taken as proof, they all added up to a fairly strong argument for ancient life on Mars.

The team, now grown to nine, approached Science magazine. They realized that getting the prestigious journal to accept their paper would be difficult and delicate; they might have to withstand as many as four or five anonymous critiques of their work. Science was tempted by the paper but reluctant to support invalid conclusions, so the publication sent out the manuscript to nine readers. The resulting article relied solely on sober observations and rigorous science, and its title reeked of compromise: “Search for Past Life on Mars: Possible Relic of Biogenic Activity in Martian Meteorite ALH 84001.” The most important sentence was the summary: “Although there are alternative explanations for each of these phenomena when taken individually, when they are considered collectively … we conclude that they are evidence of primitive life on Mars.” In other words, the meteorite offered the first scientific evidence that ours is not the only planet in the Solar System where life emerged. Publication of the issue of Science containing the article was set for August 16, 1996.

When Jim Garvin heard about the impending Science paper, he felt the skin on the back of his neck prickle. “I was dumbstruck,” he said. In 1990, he had looked at another meteorite, Shergotty. At the time, no one realized that particular rock had come from Mars. He borrowed a piece of it from the Smithsonian, where it is stored. “I took it up to our lab and made the measurements I’d wanted to make for impact metamorphism” – looking for evidence of shock waves, that is. “This was a passive measurement, by the way, like bouncing a laser pointer off a rock; we weren’t destroying it.” There he was, examining a piece of Mars without realizing it.

The force of the new paradigm – that life on other planets was probably tiny – spun Jim’s thinking in a new direction. “We were still a few months before the launch of Pathfinder and Mars Global Surveyor, and the question was asked, ‘What could be done with these ready-to-go spacecraft to look for more signs of life?’ ” Suddenly, Jim’s Mars mission had a new reason for being. He had always believed it presumptuous to assume that life existed only on Earth, and he was sympathetic to the meteorite team’s conclusions about ALH 84001. Their research science was rigorous, it was cautious, and it was consistent with the latest findings concerning extremophile life. There was something in that meteorite that could not be explained away by conventional arguments. Jim agreed with the team that the burden of proof had now shifted to those who insisted there was no life on Mars. If that was the case, he said, “an interesting explanation as to why life failed to make at least a tenuous foothold would have to be crafted.”

The midsummer Martian madness started in earnest a couple of weeks before publication of the article, when Dan Goldin, the mercurial, publicity-loving head of NASA, heard that Science had accepted the article for publication. Next, the White House wanted to make a grand occasion out of the discovery of possible life on Mars. In preparation for the announcement, Goldin summoned David McKay and Everett Gibson to Washington. “We had thirty minutes scheduled with Dan to talk about the meteorite,” Everett told me. “After an hour and a half, Dan said, ‘You guys take a break, I’ve got some things to do, and then we’ll continue.’ During the break, he dictated a commencement address he was going to deliver at UCLA and handled a few other things, and then we continued for another hour and a half. I felt like I was giving an oral defense of a Ph.D. thesis. I mean, Dan went back to first principles, and he took twenty-eight pages of notes.” At the end of the ordeal, Dan Goldin had one last question for the two scientists: “Can I give you a hug?” The gesture was pure Goldin. In general, NASA is not a touchy-feely place – but Goldin is a man of enthusiasms.

After that, the story began to leak everywhere. Space News, a weekly trade journal, hinted at the forthcoming Science paper about ALH 84001, and the buzz preceding an important Washington story started; then things suddenly went awry. At the stylish Jefferson Hotel in Washington, Dick Morris, an advisor to President Clinton, told a prostitute named Sherry Rowlands about the discovery, in the vain hope of impressing her. “Is it a bean?” she asked. Well, no, not really, he replied. It was, uh, more like … a “vegetable in a rock.” When Rowlands got home and opened her diary to write about her day with Dick, she noted, “He said they found proof of life on Pluto.” Scientists dread being misunderstood by the public, but who could have imagined the magnitude of misunderstanding generated by this discovery? The situation deteriorated even further when the befuddled hooker tried to peddle the story to the tabloids, which turned out to be more interested in extraterrestrial life in the form of little green men than vegetables in rocks, thank you. And her inability to recall just what planet Dick said they’d found life on – Saturn, maybe? – didn’t help her credibility, either. There was no sale.

The life-on-Mars story quickly took on a life of its own. The CBS Evening News was making disturbing noises that it might break the news even before confirmation, according to an account that appeared in Texas Monthly. Other networks sensed news in the making and assigned reporters. Science tried to halt misunderstandings by posting the article on the Internet shortly before publication. On the first day alone, the website received over a million hits. Giving substance to an age-old dream, and terror, the article’s findings excited worldwide attention. The announcement gave new impetus to America’s expensive, beleaguered space program, especially its investigations of Mars. Goldin was delighted to confront a challenge of this magnitude, and the mood surrounding it recalled the great days of the space race, when Americans had an emotional investment in NASA and the nation’s fortunes seemed to rise and fall with the agency. But the issue of life on Mars was more complicated to explain to the public and sell to Congress than sending people to the moon had been. There was no life-on-Mars race for politicians to exploit. National security and national pride were not at stake. Only the science really mattered. The discovery involved concepts difficult for most people, even scientists, to understand, including a meteor of unimaginable age that had traveled to Earth from an unimaginable distance, containing evidence of life that was unimaginably tiny.

NASA finally made the announcement at a flashy press conference, at which an exuberant Dan Goldin proclaimed, “What a time to be alive!” (And the head of NASA, he might have added.) Bill Clinton, campaigning for reelection, appeared on the South Lawn of the White House to hail the discovery as if it were another triumph for his administration, but he actually sounded a note of caution that went largely ignored: “If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered.” That was still a big if. And his declaration that the American space program would now “put its full intellectual power and technological prowess behind the search for further evidence of life on Mars” did not necessarily mean additional money for a beleaguered NASA. His words amounted to a mere presidential pat on the back.

The summer of Mars was underway. For a while, the names of the several NASA scientists on the meteorite team – McKay, in particular – were known to journalists and the general public. The sudden popularity threw the scientists for a loop. They naturally desired professional recognition, but not celebrity. In their line of work, being famous meant being considered suspect, a semi-charlatan, a talking head rather than a working research scientist. None of them aspired to become the next Carl Sagan, bridging the gaps among the media, the scientific community, and the public. Although their thinking was revolutionary, they weren’t visionaries; they just wanted their funding, and they wanted to pursue their scientific interests. The announcement concerning ALH 84001 made it harder for them do that, as publicity insinuated itself into the normally orderly process of disseminating scientific information. Instead of addressing specialists at conferences and publishing in specialized journals, science teams proclaimed their findings in press releases, in advance of publication. Freed of the constraints imposed in a refereed publication such as Science, the releases tended to make larger claims than the articles that inspired them. Conducting science by press release troubled many, including those engaged in the practice.

The announcement concerning ALH 84001 transformed NASA. For the first time, many people realized that NASA supports scientists, not just astronauts and engineers and the crews that send them into space. In its youth, NASA had accomplished one spectacular engineering feat after another: putting an astronaut in orbit, sending astronauts to the moon, keeping astronauts in orbit for months on end. These missions included science, but science was rarely the point. Flags and footprints on the moon were the point. Astronauts did collect a few hundred pounds of moon rocks for scientists to analyze, but the public had scant interest in lunar geology. Now, with the announcement of possible nanofossils in ALH 84001, NASA scientists were no longer overlooked. And with the end of the cold war, they could participate in missions that were primarily scientific rather than political, missions that might become more significant than sending people to the moon. They suddenly had an opportunity to devise experiments exploring fundamental questions about the nature of the universe and the origins of life. Their results of their search, a NASA report concluded, “may become a turning point in the history of civilization.”

The message in a bottle had arrived, but who would decipher it correctly?

Throughout the summer of 1996, David McKay expected a backlash concerning his discovery, but it was slow in coming. At first, members of the public, some of them deeply suspicious of all federal agencies, NASA included, sent him angry e-mails, most of which echoed the theme, “What kind of fools do you take us for?” One said, “Your life on Mars story is a good example of your mistaken belief that the general public is comprised of a bunch of total idiots.”

Eventually, scientists joined the clamor. Some insisted that ALH 84001 proved absolutely nothing. The wormlike structures, said critics, were far too small to be bacteria; in fact, they were many times smaller than the smallest bacteria ever seen on Earth. Others insisted that if the meteorite contained evidence of biological activity, it was the result of contamination. Still others challenged the team’s analysis of the PAHs. Some scientists stated flatly that McKay and his team had unfairly manipulated the evidence to support a flawed hypothesis. Everett Shock at Washington University invoked the Murchison meteorite, believed to have come from the asteroid belt, to invalidate the discovery. “It has carbonate minerals in it,” he said, “and real solid evidence of water – yet there isn’t anybody saying that there is life in the asteroid belt.” True, no one was saying it at the time, but that situation is beginning to change as scientists have come to think of life as widely distributed throughout the Solar System. Finally, the scientists attacked the reputations of McKay and his team, a tactic that took cooler heads by surprise. “It’s kind of strange when scientists, who are thought to be rational, become emotional,” said Marilyn Lindstrom, a curator of meteorites at the Johnson Space Center. “What bothers me most is that so many people have made up their minds before the data come in. I mean, sometimes I’m amazed by McKay and Gibson’s almost true-believer attitude.”

Carl Sagan was seriously ill at the time of the announcement, with only a few months to live. During his decades with NASA, he had become familiar with both the science and the passions involved in the search for life beyond Earth, and his pronouncement on the subject was enlightening yet equivocal. “For years I’ve been stressing with regard to UFOs that extraordinary claims require extraordinary evidence. The evidence for life on Mars is not yet extraordinary enough. But it’s a start.” Although he was deeply intrigued by the meteorite team’s findings, Sagan insisted that more study was required. Yet other scientists were convinced by McKay’s rigorous approach. “If this is not biology,” said Joseph Kirschvink of Caltech, “I am at a loss to explain what the hell is going on. I don’t know of anything else that can make crystals like that.”

Because McKay, Gibson, and company were cautious, even cunning, in the way they stated their findings, they made it difficult for their critics to disprove their argument. The meteorite team held that the fossils were merely possible evidence of relic life; they were not the only explanation for what they’d found, merely the best explanation. To disprove or dismiss these findings, their critics would have to understand ALH 84001 even better than the original investigators did. They would have to refute four separate, interrelated lines of argument. They would have to be familiar with geochemistry and physics and geology and of course biology. No one person knew enough about all these fields as they applied to the meteorite; it would take a team, a bigger and better team, to show McKay and his colleagues the error of their ways.

The significance of the debate transcended the meteorite itself. Even if it contained crystals that mimicked biological morphology, or contamination, the search for extraterrestrial life had undergone a sea change. Even scientists who thought ALH 84001 contained no life signs at all now found themselves thinking that if we were going to find evidence of extraterrestrial life, it would probably be tiny and ancient and carried throughout the Solar System in a meteorite. McKay, Gibson, and Thomas-Keprta’s real discovery was a new paradigm. Even if their conclusions turned out to be incorrect, their thinking was too sophisticated to dismiss. From now on, they would define the terms in the search for extraterrestrial life. Their credibility rested not so much on what they found as on how they found it: their precise, rigorous methodology.

Two years after the announcement, I found Kathie Thomas-Keprta in the featureless Building 31 at the Johnson Space Center, where many of the crucial discoveries concerning the meteorite had occurred. She is tall and slender, with long blond hair swept up in back. Despite the intense debate concerning her work, she didn’t look embattled; she was poised, with a certain swagger and the smooth delivery of a television talk show host, at least in one-on-one conversation. We were standing beside another Martian meteorite, EETA 79001, a cousin of the more famous ALH 84001. EETA 79001 resembles a black ice cube, about two inches by two inches. I peered carefully at this Martian specimen. There wasn’t much to see except for a little hole in one side drilled by a laser to extract gases trapped within.

Her team expected a lot of debate after their discovery, she told me, although the vehemence came as a surprise. “Still, all the criticism and attacks on our findings don’t bother me because I’m from Green Bay Wisconsin, and I’ve been a Packers fan for thirty years, and I know what it’s like to hang in there from one losing season to the next.” She thought it would take five to ten years for their findings to be fully vindicated, and she couldn’t wait for that day. Her case now was stronger than ever, she said. The recent discovery of microorganisms far below the Columbia River, in Washington State, gave her a lot of corroborating evidence for nano-life on Mars. No one expected to find nanobacteria a mile or more below the surface of the Earth, and no one knows how they started growing. Like their ancient Martian cousins, they live in basalt. More important, they are almost as small as the Martian nanofossils. Critics of the meteorite team insisted that the presumed nanofossils in ALH 84001 were much smaller than any organisms found on Earth – too small to be considered micro-organisms. Since the Columbia River discovery, that objection lost much of its force.

I wondered what kind of energy source for life could be found in rock a mile or two underground, where there is no sunlight, no lightning, no real heat from the Earth’s core. Some scientists think the source could be as simple as water passing over the basalt, which might cause a chemical reaction. If this is the case, the answer to the Genesis Question becomes simpler all the time; it appears that the rock bottom (so it might be said) requirements for life are even more minimal than scientists believed only a few years earlier. All you need is water and an energy source for life to emerge. Water might be running through subsurface basalt everywhere; the same thing might have happened on other planets, or even on asteroids; it might be happening now. There might be more ways for life to emerge than we now imagine – enough to suggest that life really is an inevitable outcome of chemistry and an inevitable part of the universe, predestined, as it were, but so simple that we hardly acknowledge the phenomenon for what it is.

David McKay is tall, slender, silver-haired, professorial, imposing. As the leader of the meteorite team, he is suspicious of outsiders and chooses his words with care. His office, where we met, is capacious, even by the standards of the sprawling Johnson Space Center, and the walls are lined to the ceiling with plaques, awards, degrees, citations, and a child’s squiggly drawing of a small Martian meteorite beside a large man labeled, “Dad.”

“We are still getting new data,” he said, as he snacked on a small bag of pretzels, eying me warily. He wasn’t exactly thrilled that I’d appeared in his lair; he was sensitive to criticism and assumed I was about to add my voice to the chorus of those who angrily criticized his findings. He was about to dismiss me – or so it seemed – but he thought again, and decided to test his case with me. “We are very excited about the data from the meteorite called Nakhla that fell in Egypt in 1971,” he said. “The British Museum had a piece the size of a potato, covered with fusion crust, which protects it from contamination. The problem with the Allan Hills meteorite, ALH 84001, is that it may have been contaminated with carbon or terrestrial bacteria. A chunk of the Nakhla meteorite came in here, to our lab, and we had permission to break it up and pass it out to various investigators. We requested six grams. We think it’s likely to have the least contamination of any Martian meteorite.” I sensed he knew more, but this partial revelation was all he would risk revealing at the time.

He also wanted me to know he hadn’t given up on ALH 84001 as the prime suspect in the search for life on Mars. He didn’t want me to think for one second that Nakhla was a substitute for ALH 84001; rather, it offered supporting evidence. As he talked, it became apparent that he felt that all the criticisms leveled at his findings, and there had been a lot of them, more than most scientists encounter in a lifetime, had only strengthened the arguments he originally advanced. To illustrate what he meant, he invited me to sit with him before a large monitor. “Here’s a new picture from the Allan Hills meteorite. We really suspect these are fossilized bacteria. They have better characteristics than what we have already seen; they are curved, segmented. If you gave this to a biologist, he’d say, ‘Of course it’s bacteria,’ but we have to prove beyond a shadow of a doubt it’s of Martian origin and fossilized. Fossilization is very common with bacteria; the organic components are replaced by mineral components such as iron oxide or silica. This can happen quickly, in a couple of weeks, and it happens when you bury the material in water. They are one hundred to two hundred nanometers long and forty to fifty nanometers wide, smaller than the big worms in the published pictures, which were five hundred nanometers long. My guess is that life is still on Mars, but it’s underground, in the water system. That’s where the underground organisms are living, a couple of kilometers underground. On Earth,” he reminded me, “there are microbes growing four kilometers underground.”

As we parted, David McKay insisted, “Our critics have proved nothing. Our research has defeated each and every one of their arguments, and the case for ancient life on Mars is now stronger than ever.”

Nine months after our meeting, McKay made his latest findings public at the 1999 Lunar and Planetary Science Conference in Houston; his announcement added to the controversy and ensured that the debate surrounding fossilized Martian bacteria would continue for years. To his way of thinking, there were now two meteorites from Mars bearing evidence of fossilized bacteria, ALH 84001 and the newcomer, from Nakhla, Egypt. His detractors claimed his analysis of the newer meteorite, Nakhla, compounded the errors he had made in his analysis of the first, but his supporters insisted it offered compelling confirmation of extraterrestrial life.