by Marc Kaufman
But McKay kept at it. His initial research had shown that the meteorites contained carbonates, which are minerals formed only in the presence of the liquid water needed for life. That finding faced the same hurdles as the finding of magnetites did: no confirmation of liquid water on or near the surface of Mars and the contention that the carbonates were formed in a superhot environment, where no life could possibly exist. But McKay’s team was again helped by new discoveries on Mars, this time a fast-growing body of evidence that water had indeed once flowed on the Martian surface and that ice can still be found in substantial amounts just beneath the surface. McKay’s colleagues had dated the carbonate globules at about 3.6 billion years old—when Mars was still potentially habitable—and subsequent tests have tended to support that conclusion. Considerable evidence now exists that Mars was much wetter and had a thicker atmosphere at that time. Lakes and even oceans likely existed. The idea that the carbonates were formed in a Martian cauldron is not heard much today.
McKay’s microfossil was also controversial because no bacteria or other life-forms so small had ever been detected. McKay no longer argues as strenuously that the tiny microfossil is an important part of his case, because he is finding what he believes are much larger Martian microbes in other meteorites. Nonetheless, in the time since the paper first came out to such acclaim and criticism, many living organisms as small as the reported Allan Hills microfossil have been identified and categorized on Earth.
McKay’s firm conclusion: The original meteorite did, indeed, show signs of ancient life. But—and potentially far more important—several other Martian meteorites he has since studied contain larger and more readily identified bacterial microfossils, and many are embedded deep in what he considers to be the undeniably Martian core of the rocks. McKay looks back on his time in the scientific trenches and wishes things had been different. “If we knew then what we know now, we would have had a stronger story that was much more resistant to criticism,” he says. “Every bit of new Mars data is ‘pro-life’ in the sense of supporting the hypothesis that life has occurred on Mars. Every bit.”
Where McKay and other students of meteorites are most vulnerable is in defending the rocks from the charge of Earthly contamination. The ALH 84001 and all other meteorites, many scientists argue, are corrupted with bacteria that move in as soon as they hit the planet and possibly even as they pass through the atmosphere. While some meteorites are collected soon after they fall, others remain undiscovered for eons; in the case of the ALH 84001 meteorite, the wait was about thirteen thousand years. That rock fell in the relative cleanliness of Antarctica, but other important samples fall in areas teeming with microbes, which can quickly penetrate the outer crust of the extraterrestrial rocks. Some pieces of the Murchison meteorite—the one that Glavin and Dworkin have used to explore chirality—landed on or near Australian farmland, other samples in the brush. Adding further to the threat of contamination, the Mars meteorites come filled with carbon compounds, which the microbes especially like.
McKay’s answer to this criticism is that it is quite possible to tell the extraterrestrial microfossils from the terrestrial ones. But that view is highly debated, and involves some awkward, even painful history that still hangs over the astrobiology community.
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It was a young but extraordinarily self-confident British microbiologist named Andrew Steele (or “Steelie,” to those who know him) who first made contamination a major issue with Allan Hills 84001. He became involved with the Mars meteorite almost on a lark. Having just earned his doctorate at the University of Portsmouth, in southern England, he contacted McKay’s NASA office soon after announcement of the discovery to say he wanted to get involved and asked for a small sample of the rock. Many others asked for, and received, similar samples, but few with so little experience and in such an informal way. Nonetheless, Steele brought a particular skill and technology to analyzing the sample and helped knock down one of the early criticisms of the McKay paper—that the “microfossil” was simply an artifact of the way McKay’s team had prepared the thin slice of rock for examination. Based on his good work, Steele was invited to a NASA gathering convened in 1997 to discuss future research on Allan Hills 84001 and other Mars meteorites. That Houston meeting instead became an intellectual brawl, with scientists arguing endlessly about the initial article far more than planning any future research. At a key moment, Steele made some levelheaded and insightful comments that organizers of the meeting noticed. Then and there, he was asked to join McKay’s research team. His soon-to-be wife was pregnant at the time in England, but Steele agreed to stay in Texas, and his future brother-in-law was hired as his doctoral student assistant. McKay and his team had already been working on the Martian meteorite for several years and had already published the Science paper before Steele showed up.
Perhaps it was his background as a microbiologist, or perhaps he just knew what to look for and how, but relatively soon after joining the team Steele came to McKay with some troubling news: A new sample of the meteorite that McKay had asked him to study certainly housed the remains of Earthly microbes on the outer crust and also in some cracks opening into the rock. As McKay tells it, the news was unsettling not only because it called into question what might have been “eating” the Mars meteorite and leaving those arguably telltale signs behind, but also because numerous teams of researchers had examined the rock earlier and not found the bugs. One of the teams that had searched for contamination but hadn’t found it was McKay’s—which was universally credited with being careful and rigorous in its research, even if many disagreed with its conclusions. The Steele finding led McKay to set up a Red or “Pro-Life” Team that would search for evidence to support their hypothesis and a Blue or “No-Life” Team that would poke holes in the research.
Steele was, not surprisingly, on the Blue Team, where he helped research and write some papers critical of the original McKay research. He spent only fourteen months with McKay before he left NASA for a dream job (for which he got a strong recommendation from McKay) at the renowned Carnegie Institution in Washington. He was on his way to becoming something of a science rock star—his good looks and long ponytail didn’t hurt with that—and was named principal science investigator in 2006 for the long-term NASA-ESA Svalbard astrobiology project that tests robotic equipment designed to identify and assess signs of microbial life from afar. The Svalbard expedition has also played a role in the Mars meteorite debate because Steele and Treiman found carbonates with magnetites in them around an ancient volcano there. The two argued that the Svalbard magnetites were very similar to those found in the Mars meteorite, yet they were clearly not the product of biology. I went to visit Steele in Washington soon after the San Diego “Life in the Cosmos” event.
Steele shook his head and said he respected McKay, but that the samples were too contaminated to be of much use. “The bugs, they get onto and into the rocks in no time,” he said. “And they can slip deep inside much faster than people imagine.” (He once described the Murchison meteorite as contaminated “like a pig’s ass.”) What’s more, Steele said, the microbes can quickly imprint their identities on the rocks through their activities. “You are what you eat,” he said, meaning that if the Earthly microbes feast on the Martian organics in the meteorite, then the microbes will themselves take on the chemical and isotopic characteristics of the Martian rock. Microbes can flash fossilize and presto, you can have a microfossil that registers as extraterrestrial when it was alive on Earth not that long before. “I don’t see any way around it—we can never really know what the meteorites might tell us about extraterrestrial biology because of all that terrestrial interference.”
Steele said his experience led him to embrace a “null hypothesis” when it comes to extraterrestrial life. That means you assume that any apparent biosignatures in meteorites, detected by telescope spectroscopy, identified by missions to planets and moons, or from any other source, are contaminated, the product of n
onbiological processes, or something left behind from the testing process. Only after all the possible nonbiological explanations have been thoroughly knocked down, he said, can biology come into the picture.
With good reason, Steele looks back on his time with McKay and working on the Mars meteorite as scientifically and professionally productive. McKay sees things rather differently. The day after his SPIE talk, I spoke with him about ALH 84001 science, its drama and personalities. McKay is a consummate gentleman, but he couldn’t hold in his feeling about “Steelie.” He volunteered that the young researcher was given way too much credit for work on the meteorite, something that he said “irritated me a bit.” Steele “claims he was part of the team and I guess technically he was in the sense he looked at a few of the chips,” he said. But the Englishman “played almost no part in anything we did, except to come in and find contamination on one chip of Allan Hills….”
The way he tells it, McKay found the contamination. “I spotted the chip that was so contaminated with fungal material and asked him to identify it. I didn’t think it was Mars at all, and he confirmed that it wasn’t. But he then used that as a basis to say Allan Hills is entirely contaminated and you can’t trust any identifications in it.” McKay said that Steele did something similar with a sample of another Mars meteorite he was given to examine, one that fell in Nakhla, Egypt, in 1911, and contained a fungus that was clearly from Earth. Again, Steele concluded that the meteorite, which McKay sees as containing important corroborating evidence of Martian life, was too contaminated with Earthly microbes to be of any use. But McKay strongly disagrees. The fungus probably grew in Steele’s lab, he says, and did not render the entire meteorite suspect. “I thought it was not very good science to make that kind of judgment,” McKay volunteered.
So how do McKay and others determine whether a microfossil is from beyond Earth or is simply terrestrial contamination? The chemical composition of the microfossils is important. Newly fossilized bacteria, for instance, still have a lot of nitrogen in them, while ancient ones do not. Microfossils formed on Mars also would have different isotopic forms of carbon and some other elements. Nobody has a foolproof chemical method for telling what microfossils are terrestrial and which are extraterrestrial, so McKay has offered two other defenses of his conclusion: The microfossils in two additional Mars meteorites now being studied are encased in the mineral iddingsite in a way that could have occurred on Mars but not later on Earth. Iddingsite is formed only in water, he said, and water would not reach the inside of the meteorites once they hit Earth. Even more significant, the iddingsite near the outer zone of the meteorite had been altered by the crust that formed there as the red-hot meteorite sped into the earth’s atmosphere. This heating occurred only when the meteorite fell toward Earth, so the iddingsite and whatever it enclosed had to be there prior to the journey. What he says is the inevitable conclusion: The microbes died and became encased within the iddingsite while on Mars.
Also, McKay points out that one of these Mars meteorites fell in Egypt and the other fell in Antarctica. Both have the same or similar microfossils, although the potentially contaminating microbes in the two locales where they fell are obviously very different. But after walking through these efforts to rule out contamination and more, he acknowledged that 100 percent proof will be difficult. His team is using the latest mass spectrometer analysis on the possibly fossilized microbes to try to confirm if they had once lived on Mars and if they contain organic carbon compounds. McKay would much rather work on carefully controlled Mars samples returned by robotic spacecraft missions, but that’s at least ten years away and will cost billions of dollars. Meanwhile, nearly 100 kilograms of Mars meteorites are present in museums on Earth. Why, he asks, aren’t more researchers devoting time and effort to these free samples from Mars while we wait for that illusive robotic sample return?
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Richard Hoover is another astrobiology outlier, and he too took the stage in San Diego before that largely empty room in 2009. He is NASA’s chief astrobiologist at the Marshall Space Flight Center in Huntsville, Alabama, and for more than twelve years he has used the agency’s ultrahigh magnification electron microscopes to study some of the best-preserved and best-known carbonaceous meteorites to have fallen onto the planet. His conclusion is that some are home to large collections of fossilized remains of microbes that once lived on an asteroid, a comet, or some other place that wasn’t Earth. In the mid-1990s, Hoover was the prime mover in setting up an astrobiology program within the SPIE, formerly known as the Society of Photo-Optical Instrumentation Engineers. He argued successfully that the search for extraterrestrial life will be an increasingly important issue for the men and women who make and use telescopes and other optical equipment. The group gave him its annual Gold Medal at the 2009 San Diego meeting in recognition of his work.
The Marshall Space Flight Center is known primarily for building rockets. It’s where Wernher von Braun and his German rocket colleagues landed after World War II and later helped make the rockets that sent Apollo astronauts to the moon. Studying extraterrestrial life is not its strong suit. Yet Hoover, a longtime and respected microbiologist and expert in extremophiles and algae with silica shells called diatoms, was given pretty much free rein at Marshall to search for signs of life in meteorites after McKay’s Mars meteorite was first introduced. Hoover does not study Mars meteorites, but rather some of the best-known rocks that have fallen to Earth from asteroids, comets, or perhaps other planets—legendary finds such as Murchison, Orgueil, Tagish Lake, or Allende. His specialty is carbonaceous meteorites, which are among the most ancient (some from the time the solar system formed), the least typical (less than 5 percent of meteorites), and the most interesting (because they contain the kinds of carbon compounds associated with life on Earth). They are, as a result, among the most precious and widely studied of meteorites.
To make his case, he puts on a slide show: First he shows images of a living cyanobacterium (a microbe sometimes called blue-green algae) and highlights some characteristics—long filaments made up of cells with small indentations where they meet. He also puts up an image of a cyanobacterium that appears to house a stack of dimes all just faintly connected. Then he puts up an image of a microfossil from the Murchison meteorite and it looks quite like the first, including the presence of trichomes (or strands of cells) in sheaths, and later he shows images from Murchison very similar to those stacks of dimes. It is quite a wild microbial zoo: Some of the microfossils are rod-shaped, some spherical, some sluglike, some almost slithery. The filaments often appear to be collections of once-alive cells and some clearly contain those telltale small indentations where the “cells” met. Hoover believes his menagerie constitutes “the unambiguous remains of extraterrestrial organisms.”
At its most basic, identification of an extraterrestrial microfossil requires two findings: that the forms really represent the remains of a living organism rather than a never-alive but interesting formation in the rock, and that it comes in with the rock rather than infiltrating it once on Earth. To establish the first, Hoover showed samples from meteorites of forms that appeared to have cells and cell walls, that were in the process of splitting or otherwise reproducing, that were attached to the rock with what he identified as a primitive stalk called a basal heterocyst, and that had lived in what appeared to be colonies of microbes known to coexist on Earth. Hoover can identify sheaths of fossilized carbon filled in by the minerals of the extraterrestrial rock, and show with graphs that the presumed microfossils contain higher levels of the carbon required for life than the surrounding rocks. At the San Diego conference, he put up a hard-to-read image of hypermagnified rock with small rod-shaped grains and small filaments. “Now, these structures don’t have very much morphology—it’s hard to tell if it’s really evidence of biological forms. But this isn’t,” he said with some delight as a much more clearly defined image appeared. “Here we see a sheath, and on the inside can see a chain of cells, ro
d-shaped cells, that make up the trichome of a cyanobacterium. We can see definite cross-wall constrictions.” It was, he said, an obvious microfossil. Then came images with groups of the same. “We’re not looking at single organisms, we’re looking at an entire assemblage of organisms. There was a water body in the parent body of the meteorite, and huge masses of biology were growing at one point in time.”
Looking at the bizarre elegance of the many forms, it was easy to imagine them once living on a large and watery meteorite, or hunkered down and barely surviving in the icy center of a careening comet, or perhaps on a distant moon. They didn’t look like they could have been formed through crystallization, mineralization, or the other nonbiological forces that shape the interior of rocks. But as I knew from an earlier visit to his office in Huntsville, those images are maybe .004 inches long or less. They take on their life-size incarnation through the intervention of a very high-power electron microscope. The instrument magnifies so powerfully that it’s sometimes impossible to go back and locate a previously discovered sample: The magnification is just too great, the size of the field too vast, and sometimes the geometric distortion is too confounding. Hoover says he routinely goes back to re-study individual microfossils; others say it is often impossible.
But Hoover had also measured the ratio of carbon to nitrogen in the microfossils and found that it was extremely high. Both elements are essential for life, and both are known to exist at particular levels in living organisms. When organisms die, their carbon is gradually transformed but remains identifiable while the nitrogen—needed to build life-critical amino acids, proteins, and DNA—is depleted slowly over millions of years. So as the age of the fossil increases, the ratio of carbon to nitrogen would also increase. His microfossils, Hoover has argued, have carbon-to-nitrogen ratios consistent with ancient life and inconsistent with modern microbial contamination.