"NEW HINT OF LIFE IN SPACE IS FOUND: Meteorites Yield Fossilized, One-Cell Organisms Unlike Any Known on the Earth," shouted a headline in The New York Times. "Something Out There," chimed in Newsweek. Respected scientists told crowds of reporters that their work, published in a prestigious journal, revealed complex hydrocarbons and what looked like fossilized bacteria buried deep within a meteorite. This, they claimed, provided "the first physical evidence for the existence of forms of life beyond our planet."
That was 1961. And the meteorite in question was not the one from Mars that has made recent headlines but another that had fallen a century earlier in Orgueil, France. Under closer scrutiny, the astonishing evidence was eventually thrown out of the court of scientific opinion. The organic chemicals and "fossils" turned out to be ragweed pollen and furnace ash.
So it is with understandable skepticism that scientists are greeting the bold assertions, made by David S. McKay of the National Aeronautics and Space Administration Johnson Space Center and eight colleagues, that the peculiar features they found in meteorite ALH84001 are best explained by the existence of primitive life on early Mars. Despite public enthusiam about the conclusions, published in Science, many leading researchers who study meteorites and ancient life have weighed the evidence and found it unconvincing. "There are nonbiological interpretations of McKay's data that are much more likely," says Derek Sears, editor of the journal Meteoritics and Planetary Science.
On August 7 the nightly news recounted ALH84001Us impressive resume: born 4.5 billion years ago in Mars's depths; splashed by a huge impact into interplanetary space to drift for 16 million years; captured in Earth's gravity and dragged into Antarctic snow; buried in ice for 10 to 20 millennia until 1984, when meteorite hunters, trudging seven abreast across the Allan Hills ice flow, picked it up and made it famous. That much nearly everyone agrees on; the controversy centers on the rock's less glamorous inside story.
McKay and his collaborators build the case for life on four lines of evidence. The first are orangish blobs, no bigger than periods, that dot the walls of the cracks and crevices perforating the meteorite's shiny crust. These multilayered formations, called carbonate rosettes, tend to have cores rich in manganese, surrounded by alternating iron- and magnesium-enhanced layers and a rind consisting primarily of magnetite. Bacteria in ponds can produce similar rosettes as they metabolize minerals. But "that is a perfectly reasonable sequence to find in a changing chemical environment as well," counters Kenneth Nealson, a biologist at the, who was one of the paper's peer reviewers.
The second line of evidence centers on the discovery of organic compounds called polycyclic aromatic hydrocarbons, or PAHs, in and around the carbonate. Richard N. Zare, a Stanford University chemist and co-author of the Science paper, reports that the rock contains an unusual mixture of certain lightweight PAHs. "In conjunction with all the other data, it seems most likely to me that they all came from the breakdown products of something that was once alive," he says
Critics suggest other possible explanations, however. "Hydrothermal synthesis could take inorganic carbon and water and make aromatic organics; you would get the same ones they report," points out Bernd Simoneit, a chemist at Oregon State University. "And look at the Murchison meteorite, thought to come from the asteroid belt," adds Everett Shock of Washington University. "Hundreds of organic compounds have been identified in it, including amino acids and compounds closer to the things organisms actually use. It has carbonate minerals in it, too--and real solid evidence of water--yet there isn't anybody saying that there is life in the asteroid belt."
Training high-power electron microscopes on ALH84001, McKayUs group found its third and most cogent bit of evidence: tiny, teardrop-shaped crystals of magnetite and iron sulfide are embedded in places where the carbonate is dissolved, presumably by some sort of acid. The authors note that certain bacteria manufacture broadly similar magnetite and iron sulfide crystals. Joseph Kirschvink, a biomineralogist at the California Institute of Technology, agrees that the mineral formations are intriguing. "If it is not biology, I am at a loss to explain what the hell is going on," he says. "I don't know of anything else that can make crystals like that." Shock remains unconvinced. "There are other ways to get those shapes. And in any case," he continues, "shape is one of the worst things you can use in geology to define things."
"The problem," says NASA exobiologist Jack Farmer, "is that at that scale of just tens of nanometers, minerals can grow into shapes that are virtually impossible to distinguish from nanofossils." Nealson moans that "I'd get drummed out of the microbiological society if I showed pictures like that and claimed I had bacteria." But Everett Gibson of the NASA Johnson Space Center, another of McKay's co-authors, responds that "we eliminated that possibility for most of our examples by noting the lack of crystal growth faces" and other mineralogical features. McKay adds that he looked at a different meteorite as a control and saw no similar formations.
Some critics also find the small size of the "fossils" hard to square with the other evidence. "These structures contain one one-thousandth the volume of the smallest terrestrial bacteria," points out Carl R. Woese of the University of Illinois, who studies the biochemistry of ancient life. "They really press the lower limit," he says, of how tiny a living unit can be. (But Stanley Miller of the University of California at San Diego, himself a doubter, asks, "Who knows how much space you really need?") Moreover, the putative Martian bacteria are hardly larger than the mineral crystals they are supposed to have produced. "Magnetic bacteria are mobile; they swim," Kirschvink adds. "That's why magnetic orientation is selective--it aids navigation. But these nannobacteria are too small to contain enough magnetite to make them align properly to the magnetic field.
If not life, then what can account for this odd collection of features? One possibility is a hydrothermal process. "Imagine hot fluids flowing through the crust," suggests John F. Kerridge, a planetary scientist at the University of California in San Diego. "The crystallization of magnetite, iron sulfides and carbonate with a change in the chemistry over time is perfectly reasonable. If anywhere in the subsurface of Mars there are PAHs, then they would be carried by this fluid and deposited where the fluids crystallize. I think the nanostructures are most likely an unusual surface texture resulting from the way in which the carbonate crystallized."
Then there is the specter of contamination during the meteorite's long residence in the Antarctic. Jeffrey Bada of the Scripps Institution of Oceanography in La Jolla, Calif., notes that PAHs have been found in glacial ice, albeit at very low concentrations; when he analyzed a different Martian meteorite, he found that terrestrial amino acids had worked their way into the rock. McKay and his colleagues tried to avoid being fooled by contaminants by running the same tests on several Antarctic meteorites. They showed, among other things, that nothing was living inside ALH84001 at the time it was analyzed, that most (but not all) of the carbonates harbored isotopes associated with Mars and that PAHs were more concentrated inside the rock than on its surface. "These arguments are flaky and simplistic," Sears rebuts. "Weathering is a sloppy process. Things leach in, then leach out; they do not do the obvious. The chemistry of the water can change."
A number of scientists also fret that the attention given to the McKay paper could ultimately prove counterproductive. "I'm really somewhat taken aback by the hullabaloo," Bada says. "If it turns out to be wrong, a lot of people are going to be very disappointed. And it will take an ongoing effort for a decade or more before we sort it out."
That endeavor is already under way. Researchers in many disciplines are scrambling to obtain pieces of ALH84001 and the 11 other meteorites identified as being of Martian origin in order to run further tests. Zare says he wants to search for amino acids and to compare the carbon 13 in the PAHs with that of Mars--work that some feel he should have done before going public with his results. McKay has talked about obtaining electron micrographs of thin sections of the nanofossils to look for cell walls and internal structure, but such efforts will push the limits of present technology.
Nealson and others insist that the only way to prove beyond doubt that something once lived on Mars is to send a robotic probe to Mars to deliver rocks back to Earth--another daunting technical challenge. NASA plans to launch two probes later this year, but the agency did not envision a sample-return mission until 2005, although NASA administrator Daniel S. Goldin recently spoke of accelerating the schedule. Also, a committee chaired by Ronald Greeley of Arizona State University has warned that tight budgets are slowing the development of the rovers and drilling equipment needed to search for Martian fossils or subsurface life.
Beneath all these thoughts lies a nagging question: Will we even know life if we see it? Some unconventional thinkers such as Thomas Gold of Cornell University and English astronomer Fred Hoyle have suggested that life may exist in unlikely environments, such as cometary nuclei or the crust of the moon. Even identifying carbon-based life is ferociously difficult--what if some life follows unconventional chemistries? "It boggles the mind to think of these questions," reflects Miller. "You have to stick with what you know; the alternative is sitting around, looking at your navel.
If the results reported by McKay's group hold up, Miller suspects it will be just the tip of the iceberg. "My impression is that bacterial life exists on planets around one in 10 stars, maybe more," he speculates. "I would view life on Mars not as a surprise but as a new frontier."