In the late winter of 1995, two middle-aged NASA scientists, David McKay and Everett Gibson, happened upon something peculiar in their Houston laboratory. They were using a scanning electron microscope to look at a sample of rock which allowed them to zoom in at over 100,000 times magnification. They were, in effect, flying over an alien landscape at an atomic level, searching out structures which were a hundred times narrower than a human hair.
As a young postdoctoral student, David McKay was in the audience at Rice University in September 1962 when JFK had given his famous "We choose to go to the Moon" valediction. His colleague Everett Gibson later joined NASA to look at the first Moon rocks. By 1995, they had spent the best part of a year examining a rock from the Red Planet that had been blasted into space in the ancient past.
One night, McKay and Gibson alighted on an area that looked a little different. They saw a segmented structure about the size of a single bacterium. "It appeared to be made up of a dozen segments with a head and a tail," Gibson later recalled.
In the quiet of the lab, the two Texans looked at each other. This simply wasn't possible, they told themselves. Our eyes are deceiving us. But they weren't. Close by this feature, which they soon christened "the worm," they happened upon a cascade of objects — later termed "the swimmers" — which looked as though they, too, had come from structures within a cell. But what were they? Some form of contamination? Or could it really be what it looked like: microfossils, the remnants of a once thriving microbiological colony on Mars?
Sometime later, there was a far more telling reaction from Gibson's wife, a bacteriologist. One evening, she happened upon some pictures of the swimmers her husband had left on the kitchen table.
"What are you doing with these pictures of microbes?" she asked.
* * *
So began an extraordinary scientific detective story whose effects are still being debated today. It is the latest in a long line of announcements that life has been found in Mars, which have subsequently become enmired in controversy and have divided the scientific community. "It's a bit like being shown a photograph of an eye," notes one critic who dismisses these findings. "It's quite a stretch to say that it belonged to a Hollywood movie star."
Indeed, everything was unlikely about this story, not least where the evidence had come from. If you want to retrieve a rock from Mars, the obvious place – as the world's space agencies are now planning to do – is to go and fetch them from the Red Planet. Until that can be done, the next best thing is to head to Antarctica.
Meteorites are the flotsam and jetsam resulting from the solar system's messy birth. These free samples of rock are easier to find out on the south polar ice sheets than anywhere else on Earth. More than twenty thousand have been discovered there to date. Of these, fifteen have been determined to be Martian. Air trapped in one particular meteorite – matched with exquisitely precise measurements made on the surface of the Red Planet -- nailed its origin as Martian.
The one McKay and Gibson were examining had been discovered in December 1984 in the Allan Hills, east of the Transantarctic Mountains. Catalogued as ALH84001, it stood apart: it looked green, was larger than the others taken at the time and, when packed for shipping, the words "Yowsa! Yowsa!" were written in the accompanying notes. When it was later examined in Houston, analysis showed it had come from Mars. It had been blasted from the surface of the Red Planet fifteen million years ago. It had landed on Earth 13,000 years previously.
By the start of the nineties, it had become clear it was a very unusual meteorite. Within the rock were unusual patches of carbonates, the result of water flowing through its pores in the presence of carbon dioxide (the prime constituent of the Martian atmosphere). They appeared beautifully yellow with dark edges under scrutiny of the microscope. Even stranger, there were unusually high amounts of carbon in the form of organics, the long chains of molecules which can potentially form the biochemical backbone for life.
The orange globules at their center seemed to contain a mixture of iron, magnesium, and calcium carbonate. There was also something distinctly odd. The amount of organic chemistry within the carbonates was unusually high and could not be explained by terrestrial contamination. That was – throughout this story – their great fear: that what they were seeing had seeped into the rock after it had crashed to Earth. In Houston, McKay and Gibson were determined to unravel the complete chronology of the bizarre carbonates, to get to the bottom of this truly cosmic conundrum.
* * *
"We did not begin trying to find life in this rock," Everett Gibson later explained. "We began to look at the chemical and mineralogical evidence." Within the carbonate globules, they discovered alternating bands of black and white structures at a very fine scale. They were quickly christened "Oreo rims." To examine them, the NASA team used a very powerful scanning electron microscope, normally found in electronics laboratories to look for faults in printed electronic circuits.
"It was just amazing what we could see," McKay says. In this case, their attention was grabbed by signs of magnetite, a magnetic mineral made of iron and oxygen that they knew was normally produced only by bacteria.
The Texans were fascinated by the Oreo rims. Some sort of complex chemistry had given them their distinctive coloration. The white parts were primarily composed of carbonates, while the black contained an abundance of iron minerals and hints of sulfur. "When we find some of these phases together," Gibson has explained, "it strongly suggests biological activity."
At the greatest magnifications, McKay and Gibson found within the globules some strange, oval-shaped structures that appeared akin to curiously elongated swimmers. They looked vaguely biological. McKay, in particular, had never come across anything remotely similar after looking at lunar samples for twenty years.
There were clusters of objects that startled them further: the worm-like features at a tiny scale. It looked as though they had crawled over the microscopic cliffs within the rock's fractures. Other features appeared as though they had undergone division, like cells do, with telltale clover-leaf structures. "It was just incredible and exciting," McKay recalls. "I mean, we really wanted to jump up and down when we saw those."
McKay and Gibson knew that they had merely scratched the surface. They needed to probe deeper and make more sense of the minerals in the meteorite, and for that, they literally walked down the hallway.
* * *
These guys are nuts. That was Kathie Thomas-Keprta's first reaction when her ostensible boss, David McKay, let her into the secret of what he and Gibson had discovered. Thomas-Keprta was a contractor to NASA — she worked for Lockheed Martin — and it seemed impolitic to argue. After showing their findings, she was stunned. They were either playing an elaborate practical joke or had gone bonkers. That night she went home and said as much to her husband.
In early 1995, Thomas-Keprta was in her late thirties with twelve years' expertise with a different sort of microscope that had a much higher resolution. Transmission electron microscopy is the high-tech equivalent of holding something up to sunlight to see what is inside. In this case, it involves illumination at the elemental level as electrons are passed through a sample. When she, too, looked at the features within the globules for herself, Thomas-Keprta was gradually won over. The amazing colors of the carbonate globules is an experience that few have experienced firsthand.
For her it was a revelation.
"As time went on, I became more and more convinced that the chemistry and the structure of the magnetic minerals found in the carbonate globules provide good evidence for ancient life on Mars," she later recalled.
Close to the center of the globules, she saw curious pancake shapes in which she found tiny grains of magnetite, plus grains formed by pyrite, made from iron and sulfur. Magnetite is a common enough mineral, but the grains inside the globules were very unusual. Not only were they extremely small, they were nearly pure, with no structural defects. The only known way of producing such pristine crystals was by the action of bacteria. The flawlessness of the crystals, with virtually no visible imperfections, also implied they had been formed within the bodies of bacteria.
Even so, they had to be sure what they were seeing was real. So the Houston team enlisted the help of Hojatollah Vali, an Iranian-born mineralogist at McGill University in Montreal. He was given another sample and used a completely different technique to examine the grains which allowed him to expose new surfaces and scan them afresh. He too found similar microscopic structures that appeared curved and fragmented.
"I think we then began to have some real confidence that what we were seeing was real and in the sample," David McKay said. "The relationship of all these things in terms of location, found within a few hundred thousandths of an inch of one another, is the most compelling evidence."
As in any investigation, they needed to make sure their crime scene – in this case, a rock from the ancient past – had not been unduly contaminated. So three further samples — given the names Mickey, Minnie, and Goofy — were sent to Professor Richard Zare at Stanford. The Zarelab, as it is known, had a laser absorption mass spectrometer capable of detecting one part per trillion (or one part per billion of a part per billion), almost down to the level of individual molecules. In particular, they could observe Polycyclic Aromatic Hydrocarbons (PAHs), little hexagons of carbon around which various hydrogen atoms were dotted. Often produced by decomposition of biological matter, they could also result from terrestrial contamination.
"The PAHs belong to the rock," Zare concluded. "The question is, where did they come from?" The presence within the carbonates implied they were formed at lower temperatures, and if that was the case, "then it's hard to imagine that they weren't produced biologically."
It was the equivalent of issuing an arrest warrant.
The concentration of the PAHs within the globules, the magnetite —produced when bacteria consume sulfur — and the presence of the microfossils suggested something stronger than unusual forms of carbon. "The coincidence of all these data suggest that there's a possibility that it could have been produced by primitive life on Mars," Zare says of his reaction at the time.
So the Houston researchers decided to test the water. In March 1995, Kathie Thomas-Keprta felt confident enough to present these preliminary findings on the PAHs at an annual meeting in Houston. She was astonished to be asked point-blank by a reporter from the Houston Chronicle if what she meant was that she'd found life. "Absolutely not," she replied.
It was a close call. Thereafter they spent another eighteen months trying to eliminate any other rational explanations. The extended team submitted their findings to the leading American research journal, Science, in the spring of 1996. After a few revisions, McKay and Gibson were told their paper, entitled "Possible Biogenic Relic Fossils in a Martian Meteorite," would appear in the August 15 issue.
* * *
Or at least that was the plan. Nothing was ever clearcut in this extraordinary story, which soon became an object lesson in how not to reveal that life has been found on another world. Indeed, the actual timing of the announcement had more to do with farce than anything else. There was certainly a leak. An industry journal, Space News, ran a small item about the possibility of life having been found. According to some, that tip-off came from the vice president's office. Others have suggested a truly mind-boggling scenario: that it came from the bedroom whisperings of a Clinton aide, who had been ensconced inside a Washington hotel room with an expensive escort who telephoned a supermarket tabloid.
The leading player himself was blissfully unaware.
In early August 1996, David McKay took his annual summer vacation with his family. They made their way to central Texas, where they camped along the Frio River. Just in case, he took a beeper. At the end of the first week, he decided to call into the Johnson Space Center. He was told that publication of the Science paper was being brought forward, and that NASA had called a press conference for the next day.
McKay left his family on vacation and rushed to San Antonio, where he jumped on the first available flight to Washington, DC. In the capital, he was joined by Gibson and their co-authors. The Clinton White House had already been alerted. President Bill Clinton soon hailed the discovery as "surely [one] of the most stunning insights into our world that science has ever uncovered."
At a packed press conference held at NASA Headquarters on August 7, 1996, McKay and Gibson emerged blinking into what was later called their "Pearl Harbor moment" — or at least, a day of infamy. Each member of the team described their painstaking part in this extraordinary detective story. It was always the images of the "worm" and the "swimmers" that garnered the greatest attention. As lead author, David McKay, normally reserved and hesitant at the best of times, appeared nervous.
"None of these observations is in itself conclusive for the existence of past life," McKay emphasized. "When they are considered collectively, we conclude that they are evidence of primitive life on early Mars." As McKay was at pains to point out, they were bound to be contentious, yet throughout, his team were as open as they could be. "If we're wrong, we'll admit that we're wrong," McKay said a few days later. "I think our data will hold up."
More than 1.2 million people clicked onto Science's website on that first day. By the time McKay returned home to Houston, he was welcomed by a barrage of vitriolic emails: "Shame, shame, shame! This is cocaine and Crayola science!" read one. "A bunch of catchphrases and suppositions purporting to be fact," ran another. Battle lines were being drawn, the skirmishes continuing until today.
* * *
Since 1996, that weird rock has gotten even weirder. The more ALH84001 is examined, the less clear everything becomes. To some extent, the original findings remain inconclusive. Experts in many fields have refuted a number of the arguments that supported the original conclusion that life had been discovered. Yet nobody has come up with a single fact devastating or conclusive enough to demolish or prove the central hypothesis. As one scientific paper notes, there are still "animated discussions" about what it all means.
"ALH84001 is really a test for all the scientific community," Everett Gibson has stated in public on several occasions. "Can we decode the information in this sample, which is of an unknown locale on Mars; can we interpret the record it has brought to us?"
Today, the remaining central argument — concerning the shape of the magnetite crystals — rests on whether they are biological or not. Was it really bacteria that caused their unusual shapes? Everything hangs on the word of experts who enjoy arguing with each other. Others have found that some of the structures of the crystalline shape suggest that they have been deposited at a very high temperature, which would have killed off any biological activity. "We certainly have not convinced the community," David McKay lamented on the tenth anniversary of the announcement in August 2006, "and that's been a little disappointing."
Nevertheless, Kathie Thomas-Keprta has reported that a quarter of the grains are the exact same size, shape, and structure as magnetite grains made by bacteria. In 2000, she found six hundred magnetite crystals that looked identical to the ones produced by microbes. "If you did not know this rock was from Mars," she said, "you would conclude that it contains evidence for past life."
Until his passing in February 2013, David McKay still believed the magnetite findings were significant. "The shape of the magnetite grains is still rather distinctive," he said in 2006. "If it were found on Earth, it would be a very strong biosignature."
Ironically, a second team of NASA researchers have created a perfect alignment of magnetite from nonbiological processes. That one member was McKay's brother gave the subsequent debate a delicious twist. "He got a little testy about the results we were getting," Gordon McKay said of his sibling. Yet David McKay insisted they were not the same shape. His brother's samples had been synthesized from ridiculously pure ingredients.
Today, the debate concerns the coincidence of the carbonates and magnetites — and which came first. Some authorities believe the small magnetite crystals formed from the partial thermal decomposition of the host carbonate. "Their origins may be unrelated," Kathie Thomas-Keprta says. "From the perspective of the carbonate, the magnetite may have formed somewhere else."
Yet her most recent work suggests that the magnetite could have formed by neither thermal decomposition nor shock waves. The magnetite crystals could have been "brought in from somewhere else, suspended in the ground water, and then were added to the carbonates as they crystallized," Thomas-Keprta says.
While it doesn't prove the biological argument beyond doubt, it probably represents their last word on the subject. To others, it is akin to a kind of scientific Rorschach test. Someone with a biological sciences background will give undue weight to shapes and sizes. "This also showed early on in the incorrect identification of the carbonate 'worms' as microbes," says one critic.
* * *
To some, the arguments over "that damned meteorite" represent diminishing returns, a reductio ad absurdum with no way out of a particularly complicated labyrinth. Yet one tangible benefit has been the weaving together of several different approaches, the working together to find a way out of these kinds of scientific riddles in the future. Certainly, the missions to Mars due for launch in July 2020 will use a similar philosophy and, for the first time in forty years, to look for organic material and signatures for ancient life directly. Twenty years after the original announcement, Kathie Thomas-Keprta provided a summary that could act as a leitmotif for the final answering of the question that has endured: What exactly constitutes life?
"At the most fundamental level," she says by way of conclusion, "we still don't know whether the difference between animate and inanimate is simply a difference in kind or degree. In absence of such a definition, the search for evidence for life on Mars is plagued by ambiguities."