But the experiments soon went haywire. It was as if the instruments picked up too much life, then none at all: The readings flashed and then died away. The team realized that the rate of responses registered was faster than even the most fertile soils on Earth and that the chicken soup experiment had produced a rapid surge of gas even before the nutrients had been added. They started to wonder if the water introduced in those experiments might be causing a chain of powerful chemical reactions. Perhaps instead of life, there was something chemically corrosive in the soil?
The nail in the coffin was the gas chromatograph-mass spectrometer, the chemistry instrument designed to detect carbon-based organic molecules. The results had been delayed by several days because the scoop had gotten jammed. But when the experiment was finally run, it found no organics whatsoever. Even the lifeless moon contained some organics: simple molecules that had rained down from space on comets and meteorites.
Weeks later, similar results trickled back from Utopia Planitia, the landing site for Viking 2, some five thousand kilometers away. The biology team tried everything they could think of: shorter experiments, longer ones, different combinations. In the end, nearly everyone concluded that those initial detections must have been false positives. How could there be life without any organic molecules, without any of life’s building blocks? Horowitz declared that it was “virtually certain” that Earth was the only life-bearing planet in our region of the galaxy, now that we’d found Mars to be utterly barren: “We have awakened from a dream…” In his view, Viking had found not only no life on Mars but also why there could be no life: The planet was devoid of water and suffused with cosmic galactic rays, both of which were sufficiently sterilizing. He concluded that oxidants like hydrogen peroxide were laced throughout the soils, the result of billions of years of intense radiation. As a result, there were corrosive free radicals everywhere, roving reactive atoms and molecules with unpaired electrons—so many that complex chemistry was constantly under attack.
Predictably, some in the scientific community turned their frustration on Sagan, criticizing his ludicrous optimism, arguing that he had only set the public up for disappointment. Sagan had playfully warned reporters about the possibility that the experiments might come up empty while creatures were “placidly munching on the zirconium paint on the outside of the lander.” He had even suggested that the mission add cameras, bait, and a lighting system to the lander to lure Martian life-forms to the craft, having gone so far as to run tests in the Great Sand Dunes National Monument using a snake, two tortoises, and a chameleon. But of course there were no silicon-based giraffes—and how very irresponsible to pretend there could be. In Sagan they saw a showman, a huckster, a megastar. And it rankled them.
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FOR HIS PART, Sagan still combed through many of the thousands of orbital images that Viking had collected for signs of life. He and his students flipped through image after image, quadrant after quadrant, finding nothing of note. The mission results had sobered him, along with most every other Mars scientist. There was little doubt that “the greatest experiment in the history of science” had failed, and not even Sagan could object when the Viking team stopped the single-slit scanners it was using to detect motion on the surface, one of the instruments he had fought the hardest to include. The Viking data, such as it was, would be all the Mars community would have to work with for the next twenty years, for it would be that long before a spacecraft returned to the planet. Exobiology had flared like a match. And then burned out.
OUT IN THE frozen terrain of Antarctica, where no rain has fallen in two million years—in the land of bleached skies, of no dogs or children—Wolf Vishniac had been attempting to connect what was known about the geology of Mars to what he knew about biology on Earth, to understand whether microbes could survive the harsh conditions. It’s of little consolation, but before Vishniac died, he knew he’d found what he was looking for: life in the “lifeless” soils. The microscope slides he’d pulled from the ground of the Asgard Range showed stunning constellations of growth. When held to the light, they looked like small, shining galaxies.
The cells had taken to the slides, but not to his colleagues’ petri dishes, because Vishniac had basically left the microbes alone, allowing them to grow in their natural environment. After his death, they were sent back to his wife, Helen, who found that they contained hundreds of cells, and not only microbes but also complex eukaryotes.
Among Vishniac’s personal effects was also a bag of cold desert sandstones. He had written across it “Samples for Imre Friedmann.” Friedmann was a fellow microbiologist, at Florida State University. For ages, he had wanted to see if life could survive inside rocks, but he hadn’t succeeded in securing funding to go to Antarctica. He’d asked Vishniac to pick up some rocks for him, and when he received the bag from Helen, he made a groundbreaking discovery. In 1976, Friedmann published his results in Science: Vibrant unicellular blue-green algae had colonized the air spaces inside the porous sandstone, using tiny rock houses as protection against the elements. Life, in other words, could live not only in freezing-cold deserts but also in freezing-cold rocks.
These discoveries—and others—kicked off a new phase in the search for life after the final, feeble radio wave from Viking 1 oscillated to Earth from the western slope of Chryse Planitia in 1982. Some of the starry-eyed graduate students who wrote their dissertations on Mars in the 1970s—the young scientists who were left with barely any data to work with—turned their attention to our own planet in the 1980s and 1990s. A new field of extremophile biology was born: investigating the crooks and crevices of our planet to better understand the limits of life.
Microbes were soon discovered in brine pools many times the saltiness of seawater, even supersaturated with methane, and in lakes with the pH of Drano. Scientists who began exploring the eternal darkness of the deep sea found that there was not only life but in fact a rich and intricate ecosystem. Despite toxic sulfide gas and temperatures hot enough to melt lead, hydrothermal vents were teeming microbial communities. There were thickets of tube worms, some more than two meters long, waving like human arms. They were tipped with feathery red plumes, seen for the first time as the lights of submersibles cut across the seafloor. They were surviving at pressures that no one imagined life could endure, far beyond the reach of the sun’s photons. A new type of metabolism had to be powering this world, one that wasn’t using photosynthesis as a source of energy.
Microbes were found laced throughout Yellowstone’s evaporites—soft, salt-crusted, sedimentary rocks that formed from the evaporation of waters in strange bubbling pools. In Octopus Spring, there were pink hair-like strands of Thermus aquaticus, tiny organisms that could live and reproduce at extremely high temperatures. Pseudomonas bathycetes was pulled from the crushing pressures of the Mariana Trench, and Deinococcus radiodurans was scraped out of the waste of nuclear reactors. Life, it seemed, was everywhere.
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THE STAGE WAS set for the discovery of a little rock shaped like a potato in the Allan Hills of Antarctica. In 1984, two days after Christmas, a young scientist named Robbie Score spotted a small dark spot on an ice sheet more than two hundred kilometers south of the Asgard Range. She was snowmobiling with a team of researchers from the Antarctic Search for Meteorites. She sped over on her Ski-Doo to have a closer look, then signaled to her colleagues. It looked almost green against the metallic whiteness. The team extensively photographed it, gingerly placed it in a clear plastic bag, and labeled it “ALH84001.” ALH for Allan Hills; 84 for 1984; and 001 because it was the first find of the year.
The Antarctic Search for Meteorites was established because more meteorites are found in Antarctica than anywhere else. It’s not that more fall there, just that they are easier to see. In fact, in certain parts of Antarctica, most of the rocks you find are meteorites. The slowly flowing ice gathers the
m in the immense interior of the continent. Glaciers creep downhill until they fall into the sea or run into a mountain range. If blocked, the trapped ice ablates away, which brings the frozen meteorites back to the surface. On the flanks of the Transantarctic Mountains, which form a spine down the continent, meteorites accumulate at concentrations many times higher than anywhere on Earth. They’re easy to spot, like flecks of pepper on smooth white porcelain.
At the end of the Antarctic summer, ALH84001 traveled back to the United States in a climate-controlled shipping container with all the others found that season. When the rock was brought to a cleanroom in Houston in early 1985, a half-gram chip was cleaved and sent off for classification at the Smithsonian National Museum of Natural History. The young curator who did the analysis classified the meteorite as a diogenite, most likely from Vesta, a large asteroid in the asteroid belt between Mars and Jupiter. There were some strange patches of brown iron-rich carbonate that were unusual for Vesta, but the curator assumed they were due to weathering processes here on Earth.
For seven years, ALH84001 sat in a tightly secured vault at the Johnson Space Center. Then in 1992 a puzzled researcher wandered down the hall of nearby Building 31. He’d been doing a systematic study of fragments thought to have arrived on Earth from Vesta, but there was one rock that he simply couldn’t place: the diogenite with the carbonate. He stopped by the office of David McKay.
McKay was a tall geologist with stooped posture and a hasty gait. He wore wire glasses over his rounded cheeks and had an easygoing demeanor. Originally from Titusville, Pennsylvania, he had moved to Tulsa, Oklahoma, in the sixth grade—his father had an accounting job with the Kewanee Oil Company—and then to Houston, Texas. He rambled for a while, working on remote offshore oil rigs and doing solitary surveying work in the desert, before returning to Rice University in Houston, his alma mater, for a PhD in geology. He’d been sitting in the Rice football stadium in 1962 when John F. Kennedy announced that the United States would go to the moon before the end of the decade. “Why choose this as our goal?” Kennedy asked. “We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win.” Inspired, McKay went on to secure a position at Johnson Space Center, a newly formed complex that was quickly overtaking the salt grass south of Houston. He’d been there ever since.
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AS MCKAY EXAMINED ALH84001 in the early 1990s, expecting it to be from Vesta, he couldn’t help wondering about the rock’s origin, just as centuries of scientists had pondered the origin of meteorites they’d found. The mere idea that rocks could plunge from the sky had once drawn ridicule. A famous eighteenth-century mineralogist remarked that “in our time it would be unpardonable to consider such fairy tales even probable.” Some believed the bizarre objects were volcanic rocks, lofted like small bombs during an eruption, or rocks that had condensed in hail-filled clouds, or rocks that had been hit by lightning, giving rise to the name “thunderstones.” Isaac Newton’s work, which suggested that no small objects would exist in interplanetary space, wasn’t questioned until the turn of the nineteenth century, when a German physicist first suggested, to great mockery, that meteorites from space caused fireballs and might themselves be “world fragments.”
McKay wondered if the rock might in fact be a kind of meteorite called SNC, or “snick”—the shergottite, nakhlites, and chassignites—named for three witnessed falls near the villages of Shergotty in India in 1865, El-Nakhla in Egypt in 1911, and Chassigny in France in 1815. Loud sonic booms accompanied all three. A piece of the first nakhlite was said to have landed on a dog. It was clear that the three rocks from those tiny villages had curious properties, ones that set them apart from all other rocks. But where were they from?
As the group of three SNCs grew over the years, the mystery of their origin intensified, until it was discovered in 1983 that there were gas vesicles in one of the rocks, holding tiny beads of atmosphere. That there was any air at all trapped in the meteorites was remarkable, ruling out all airless worlds, including comets, asteroids, the moon, and Mercury, but leaving open the possibility that a planet with an atmosphere like Mars might be the source. Then everything about the chemical signatures in the atmosphere began to line up. As the vesicles were pierced, the gas matched perfectly with the ratios determined for the Martian atmosphere both by Earth-based spectroscopy as well as the direct measurements made by the Viking landers. In addition, new models emerged to explain how fragments of material, called “spalls,” could be ejected from the surface of Mars without being melted—or, for that matter, completely vaporized.
In some ways ALH84001 looked to Dave McKay like the other Mars meteorites, but at the same time it was clearly different. For one, it was three times older. When one of McKay’s colleagues ran an analysis to determine how long the rock had been exposed to cosmic rays—the radiation that is constantly bombarding the surface of the planets from space—he got an astonishing number: At the time, it appeared to have formed only fifty million years after the birth of our solar system, making ALH84001 the oldest rock from any planet ever discovered, including Earth. An impact sixteen million years ago likely cleaved it from the subsurface of Mars, where it had been protected from the harsh surface environment, then flung it into space on a vector aimed at Earth. A set of exposure age calculations put the date of ALH84001’s arrival in Antarctica at about thirteen thousand years ago—before the beginning of agriculture, before the rise of civilization. ALH84001 landed just as the last ice age was ending, as glaciers began pulling back from their vast hold on our planet. The fistful of rock was frozen beneath the ground for all those years, cocooned from wind, storms, and sunlight.
McKay liked to listen to Enya, an Irish singer with an otherworldly voice, while he worked in the lab, often late into the night. As he peered into the rock, he began noticing orangey knobs of carbonate, the first of several strange discoveries. The knobs were extremely bizarre: ringed like the eyes of owls. The proportion of carbonates in the meteorite, about 1 percent, was far higher than anyone would ever expect to find in a rock that cooled from a hot volcanic mass. Carbonates on Earth, like the vast sheets of limestone that cover North America, almost all form in the presence of water and in the temperature range where that water is in a liquid form. The presence of carbonates in the rock suggested Mars had been awash in water, that the rock had formed in a habitable environment.
Soon, delicate magnetite crystals laced throughout the carbonates like strings of beads were spotted by McKay’s colleague Kathie Thomas-Keprta. This was another unexpected finding. On Earth, microbes produce these crystalline arrangements, which serve as tiny compasses for the microbes as they glide around. The magnetite crystals in ALH84001 were also extraordinarily pure. Magnetite formed by indiscriminate geologic processes typically contains magnesium, calcium, and iron, whereas microbes tend to select only for magnetite with iron, which has the best magnetic properties. Under natural conditions, magnetic minerals form under different pH conditions from carbonates, so the presence of the carbonates and magnetite crystals together was unusual and potentially indicative of life. Billions of years ago, did microbes float through the Martian seas, guided by the tug of an ancient magnetic field?
If so, McKay and his colleagues reasoned, there should be remnants of organic material, the building blocks of life, along with the magnetic structures. To test this idea, McKay sent samples of the meteorite to a widely respected laser chemist at Stanford University. In a matter of weeks, ringed clusters of carbon and hydrogen atoms called polycyclic aromatic hydrocarbons (PAHs) were detected. PAHs are found in oil, coal, and tar, in the charred remains of burned forests, and in the black residue of steaks grilled on an open flame.
McKay and his team still wavered. The PAHs weren’t a smoking gun—while PAHs are commonly formed as a by-product of cellular decay on Earth, they are also formed by the creation of new stars. Even so, McKay seemed to have stumbled upon the first true evidence of organics on Mars. At the very least, the PAHs signified that a kind of chemistry conducive to life was at one point present on Mars. The PAHs were much more plentiful in the core of the rock, arguing against contamination, and not only that, the PAHs in ALH84001 appeared to cluster precisely where the carbonates and magnetic crystals were concentrated.
McKay continued to probe ALH84001 back at Johnson Space Center, and in a stroke of fortune, he was given access to a sophisticated new scanning electron microscope, one NASA had recently purchased to inspect space hardware for tiny cracks and flaws. McKay knew it would allow him to peer into the mineral structures of the meteorite at unprecedented resolution.
One day in January 1996, he and a colleague carefully placed a tiny speck of the meteorite into the machine and turned on the beam. As they looked down on what appeared to be entire continents of unfamiliar terrain, something bizarre suddenly came into view. Both of them sat completely still. There on the edge of one of the orange carbonate blobs was something that looked like a worm ascending a hill, suspended mid-crawl as if captured in some small Martian Pompeii. It was shaped like a rope, just fifty or a hundred nanometers across, and, wildly, it seemed to be segmented like a primitive microorganism. It appeared for all the world to be a nanobacteria fossil, an actual fossil of life.
The Sirens of Mars Page 9