By Jane Sanders
July 6, 1998
In a paper to be published in the July issue of the journal Meteoritics and Planetary Science, the researchers report evidence that crystals found in the meteorite were formed by epitaxial processes at temperatures that were likely too high for biological organisms to exist. The findings cast new doubt on claims by NASA's Johnson Space Center (JSC) researchers (led by David S. McKay) that the so-called "Mars rock" contains forms consistent with nanofossils.
Using transmission electron microscopy, researchers discovered that magnetite crystals in the meteorite, known as ALH84001, were atomically intergrown with the surrounding carbonates by a rigorous form of epitaxy. This process is an ordered growth of one mineral on top of another. The resulting complementary orientation of crystals means the magnetites and carbonates must have grown simultaneously at temperatures much greater than 120 degrees Celsius, researchers said.
Epitaxial formation rules out intracellular precipitation of the magnetites by Martian organisms, a theory hypothesized by NASA scientists who believe the meteorite contains nanofossils, the Tech researchers said. And the implied high-temperature origin virtually eliminates the possibility that fossilized Martian organisms exist in this meteorite, they added.
This article is the third in a series of this research team's technical papers that have disputed claims of biological life in the meteorite. The other papers were published in the journals Geochimica et Cosmochimica Acta (1996) and Nature (1997). NASA has sponsored all of this research, as well as work by JSC scientists who made the nanofossil claims.
"These three papers in combination basically invalidate much of their (JSC's) evidence," said Dr. John Bradley, an adjunct professor in the Georgia Tech School of Materials Science and Engineering and executive director of MVA Inc., a microanalytic company in Norcross, Ga. "Early skepticism has evolved into international consensus among meteoriticists and planetary scientists, with the exception of the JSC team, that this rock does not contain Martian nanofossils. I do not know of a single other individual who believes it at this point."
Bradley conducted the current and previous research with Drs. Hap McSween of the University of Tennessee in Knoxville and Ralph Harvey of Case Western Reserve University in Cleveland, Ohio. In their first paper, the researchers used transmission electron microscopy (TEM) to discover that elongated forms in the meteorite contained crystallographic defects that look like a spiral staircase, Bradley said. These defects, called screw dislocations, typically form during high temperature vapor phase growth.
The JSC team, using field emission scanning electron microscopy, had claimed that these worm-like, elongated forms were nanofossils. If true, they should contain internal "daisy chains" of aligned magnetite crystals called magnetosomes. Bradley's team found elongated, rod-shaped magnetites called "whiskers" instead. But JSC researchers countered that the differences resulted from scientists using different microscopy techniques and thus seeing different objects.
So Bradley's team duplicated the JSC researchers' scanning electron microscopy (SEM) procedures at Georgia Tech using the same metal coatings, gold and palladium, to make the specimen surfaces conductive. With SEM, Bradley's team found the same worm-like objects. Then, however, they rotated and tilted the meteorite specimens to get a different microscopic angle. From that perspective, the worm-like objects appeared to be inorganic mineral lamellae or protruding ledges. Their worm-like segmented surface structures were actually artifacts of the gold and palladium coatings on the specimens. "They looked like the edge of a stack of copy paper in which a few pages are sticking up on edge," Bradley said.
In a rebuttal paper accompanying the Bradley team's 1997 article in Nature, the NASA researchers conceded that these non-biological worm-like structures are present in the meteorite, but that their nanofossils are "different."
"It's like looking for worms in a plate of spaghetti," Bradley said. "If the worms look like spaghetti noodles and they're not wriggling around, how can you be sure they're worms and not noodles?"
In the current paper, researchers focus on epitaxially grown magnetite single crystals. They are key indicators of the geochemical and thermal history of the carbonate-rich fracture zones of the Martian rock, they said. Magnetite crystals, apparently formed by several high temperature growth mechanisms, are found in several distinct mineral settings in this meteorite.
With regard to whiskers, the researchers cite various evidence of epitaxial crystal growth and high temperature origin of magnetites in the meteorite. TEM techniques allowed researchers to view the well-defined spatial orientation relationship between magnetite and carbonate crystals. Epitaxy can occur if two similarly patterned lattice planes of crystal structures are parallel. Previous studies have shown the ideal lattice "misfit" between two crystal structures should not exceed 15 percent. In this case, the lattice "misfit" was only 11-13 percent, which is ideal for epitaxial growth, Bradley said.
Furthermore, many of the epitaxially formed magnetite whiskers in the meteorite appear to be free of internal defects, the researchers said. Such is typically the case of crystals formed at elevated temperatures, while those grown at lower temperatures tend to have high densities of internal defects.
Also, researchers found epitaxially formed magnetite crystals in mineral specimens from volcanoes in Indonesia and Alaska. These crystals formed at temperatures in excess of 600 degrees Celsius, researchers said. They compared these to the magnetites in the meteorite because volcanoes also exist on Mars. The comparison provided further evidence of a high temperature origin, Bradley said.
Despite this paper and the other two Bradley team publications, the debate over nanofossils in Martian meteorite ALH84001 will continue, Bradley said.
"Unless the JSC team concedes, the debate will never die," Bradley said. "When this news first became public, the debate was quickly deflected into one about whether life exists or once existed on Mars. But there are really two debates here -- whether there is evidence of life in this meteorite and whether life exists on Mars."
The first question is already answered in Bradley's estimation. The second remains, and Bradley believes it is very unlikely that life exists on the surface of Mars. "It may be down in the depths. We now know that life thrives in very extreme conditions on Earth," he said.
PHOTO CAPTION:
Dr. John Bradley presents electron microscope images of Martian
meteorite crystals that contain"nanofossils," according to some
scientists.
PHOTO COPYRIGHT INFORMATION: Photographs are copyrighted by the Georgia Tech Research Corporation and may be freely used by the news media with credit to the Georgia Institute of Technology. The photographer is Stanley Leary, Georgia Tech Communications Division.
University of California-San Diego
June 5, 1998
In a study reported in the current issue of the journal Science, the UCSD scientists report that the carbonates laced through the potato-sized rock are the product of reactions with atmospheric carbon dioxide.
"This data suggests that the carbonates were made by the interaction with the atmosphere rather than with the water on the surface, as would be required for a biologic process," said Mark Thiemens, professor of chemistry and biochemistry at UCSD and principal investigator of the study. Other co- authors of the study were UCSD chemists James Farquhar and Teresa Jackson.
The new results are based on the first multi-oxygen isotopic examination of carbonate globules in the meteorite, named Allan Hills 84001 (ALH84001). Isotopes are atomic elements that have the same chemical properties, but different weights and slightly different physical properties.
In August 1996, research teams at NASA's Johnson Space Center and Stanford University announced their belief that at least parts of the brown-colored carbonates found in ALH84001 bore a striking resemblance to the earliest microfossils on earth, suggesting past life on Mars.
To see if the origin of the carbonates could be determined, the UCSD chemists measured the proportions of oxygen 16, oxygen 17 and oxygen 18 found in the sample, seeking clues in the form of clear isotopic "signatures." An isotopic signature to achemist is what a fingerprint is to a detective. For the carbonates, the UCSD scientists wanted to find out if the fingerprints matched oxygen from the atmosphere or oxygen from the hydrosphere.
If the signature pointed toward a water origin, that would support that case for life, since on this planet and elsewhere, the genesis for all life is water. If the signature matched the oxygen isotopes from the Martian atmosphere, that would suggest a non-biologic origin.
"So if these things were biogenic, they should have equilibrated with water," said Thiemens. "They didn't. They equilibrated with the atmosphere.
"What it looks like is that the Martian oxygen isotopes came from the Martian carbon dioxide atmosphere. So what we're seeing looks like a garden variety precipitate of carbonates, rather than life."
To continue his search for life's signature, Thiemens and other scientists look forward to the next mission to Mars in 2005 when samples of the atmosphere and rock are expected to be retrieved and returned to earth.
"In the meantime, there's a suite of Martian meteorites that we'll look through and we'll do the same analyses," Thiemens said.
Funding for the UCSD study was provided by NASA.
Arizona State University
May 8, 1998
In an article appearing in the May 8, 1998 issue of Science, a team led by two scientists from Arizona State University reports finding evidence for as many as three different iron sulfide minerals in two different bacteria known for generating magnetic compounds but not other iron sulfides normally found with them.
Using transmission electron microscope observations, the team, which includes ASU Geology and Chemistry/Biochemistry faculty members Mihaly Posfai and Peter R. Buseck, Iowa State University microbiologist Dennis A. Bazylinski, and California Polytechnic physicist Richard B. Frankel, found clear evidence of mackinawite and greigite and, possibly, cubic iron sulfide.
One of the iron sulfides they did not detect is pyrrhotite, a mineral that has been found in the now-famous Martian meteorite ALH84001 and that frequently occurs as a breakdown product of the other sulfides. Though pyrrhotite's presence in the meteorite has been cited as possible evidence of past Martian bacterial life, the study's evidence suggests that the bacteria may actually prevent its formation.
The study found evidence that the bacteria first produce mackinawite, a nonmagnetic iron sulfide, which then naturally converts to the magnetic greigite. It also suggests that this process may actually begin with cubic iron sulfide, which is unstable and rapidly becomes mackinawite. In the geological environment the bacteria are found in, the reaction sequence would also eventually lead to greigite breaking down into pyrite and pyrrhotite, but that reaction does not occur when the bacteria are present.
The team's research finding contradicts an earlier study that found pyrite and pyrrhotite present in the bacteria. As no subsequent study has been able to duplicate this result, the current team posits that earlier researchers may have confused cubic iron sulfide with these minerals, which give similar selected area electron diffraction patterns.
Stanford University
Januray 21, 1998
Two studies published last week find that much of the organic material in the meteorite appears to be terrestrial, rather than extraterrestrial, in origin. Richard Zare, the Marguerite Blake Wilbur Professor of Chemistry at Stanford who headed the team that discovered organic material of possible Martian origin in the potato-sized rock, says that the new findings do not directly refute the original research. One of the analyses, however, does suggest that the meteorite contains considerably more terrestrial contamination than he had thought, Zare acknowledges.
ALH84001 was thrust into the limelight in August 1996 when a team of scientists published a controversial analysis in the journal Science. They argued that they had discovered organic material, unusual mineralogical features and electron microscope images showing tiny oval and worm- shaped features that, when taken together, provided compelling circumstantial evidence that the meteorite was inhabited by Martian microorganisms more than three billion years ago.
Scientists at NASA's Johnson Space Center in Houston provided the electron microscope images of the putative nanofossils. Researchers from the University of Georgia and McGill University contributed the mineralogical evidence. Zare's research group produced data showing that the meteorite contained a family of organic compounds called polycyclic aromatic hydrocarbons (PAHs) that could have been produced by the decomposition of alien microorganisms.
In the 17 months since the research was announced, other scientists have published dozens of independent analyses that have both supported and attacked the Martian microbe hypothesis. In the last month, however, the weight of new research appears to be stacking up against the pro-life position.
In December, John Bradley of MVA Inc. and Ralph Harvey of Case Western Reserve University published a paper in the journal Nature that attacked the NASA group's interpretation that the oval and worm-like shapes that it reported could be the fossils of microorganisms. Duplicating the NASA researchers' methods, Bradley and Harvey reported that all the shapes that they could find in the meteorite are non-biological in nature and consist of the fractured surfaces of common crystals. In the same issue of the journal, the NASA team strongly contested this interpretation.
The two analyses of the organic material in the meteorite appeared in the Jan. 16 issue of the journal Science.
A team from Scripps Institution of Oceanography, headed by Jeffrey Bada, analyzed the meteorite for amino acids, the building blocks of life. They found amino acids present in very low concentrations (between 7 parts per million and 100 parts per billion). "What we found was that, yes, there are amino acids in the meteorite at very low levels, but they are clearly terrestrial and they look similar to amino acids we see in the surrounding Antarctic ice," Bada said in a Scripps news release. (The meteorite spent an estimated 13,000 years in the Antarctic ice before it was discovered.)
Bada based his conclusion that the amino acids were due to terrestrial contamination on the results of an analytic technique called liquid chromatography. The method determines the "handedness" of the amino acids. Terrestrial organisms produce only left-handed amino acids, where non-biological processes produce a mixture of left- and right-handed molecules. Bada found that the amino acids in the meteorite were left- handed and so concludes that they must be terrestrial. There is at least a 50 percent chance, however, that Martian life (if it exists) would also favor left-handed molecules. So the experiment is by no means conclusive, Zare said.
Even if the amino acids in the meteorite come from terrestrial contamination, Zare says this does not prove that the PAHs which his group found are also terrestrial in origin. "Amino acids are soluble in water. So water provides a mechanism for carrying them into the interior of the meteorite. But PAHs are highly insoluble and I don't know of any mechanism that would transport them into the rock's interior where we found them," he said.
Zare finds the second analysis, performed by a University of Arizona research team headed by A. J. Timothy Jull, much more interesting and compelling. "It is state-of-the-art and an extremely valuable study of the degree of contamination in the meteorite," he said.
Jull's group burned samples of the meteorite at two different temperatures to separate the organic carbon from the carbon contained in inorganic minerals, which burn off at higher temperatures. They then analyzed the isotopic ratios of the carbon from the two sources.
In previous work, Jull had determined that the carbonates in ALH84001 are substantially enriched in the isotope carbon-13 compared to those on Earth. He and his colleagues interpret this as an indication that the carbon dioxide in the early Martian atmosphere was also enriched in carbon-13. If that is the case, then the tissue of Martian organisms would also have elevated levels of carbon-13. When the team analyzed the ratio of carbon isotopes in the organic carbon, however, it found that fully four-fifths of the material had the same isotopic signature as terrestrial carbon. The other 20 percent appears to have a preterrestrial origin, they found.
"It looks like regular terrestrial organic material, with the exception of one small component in ALH84001," Jull said in a University of Arizona news release.
The analysis "indicates a much greater degree of terrestrial contamination in the meteorite than I suspected was present two years ago," Zare said. "In that sense, Jull's study does cast new doubt on our hypothesis that the meteorite contains evidence of past Martian life."
On the other hand, the Stanford chemist does not believe that the study completely rules out an extraterrestrial origin for the PAHs. "Jull's work is for the whole rock. As in real estate, location is everything. His study does not give any indication of the locations from which these different carbon isotope fractions are coming. So I cannot tell where the PAHs, which are concentrated around carbonate spheroids in the meteorite's interior, fall in the terrestrial or preterrestrial fraction."
The saga of the provocative rock is far from over. Last summer NASA and the National Science Foundation awarded grants for 23 new investigations of AHL84001 as part of a coordinated program designed to determine whether it contains traces of alien life. These studies will be producing results in the next two to three years.
Although it may be decades before the significance of the meteorite is determined conclusively, Zare sees several beneficial effects that are independent of the debate's ultimate outcome. These include a revitalization of research on meteorites, increased efforts to extend the boundaries of the scientific ability to measure trace quantities of chemical compounds in materials, and its illustration of the critical importance of multidisciplinary research.
Most important, he says, it has given a major new impetus to research that addresses the closely related questions of "How did life begin on Earth?" and "Is there life beyond Earth?" A concrete example of this is NASA's decision to found a new $7 million to $10 million-per-year Astrobiology Institute specifically for this purpose. Zare is serving as chair of the search committee for the institute's first director.
Case Western Reserve University, Cleveland, OH
December 2, 1997
The famous Martian meteorite, ALH84001, contains no biological life forms, according to a Case Western Reserve University researcher and colleagues.
The team issues this report in the December 4 issue of Nature, duplicating the methods of a team of scientists from the Johnson Space Center and Stanford University. In rare counterpoint writings in the "Scientific Correspondence" section, Nature allowed the Johnson Space Center team to respond to the group's findings. This paper also appears in the December 4 issue.
CWRU's Ralph Harvey, senior research associate in the Department of Geological Sciences, was on the research team. The lead researcher on the paper was John Bradley from MVA Inc. and the School of Material Science and Engineering at Georgia Institute of Technology. The third researcher is Hap McSween from the University of Tennessee.
The trio reports that most of the purported nanofossils or "worm-like images" are nothing more than lamellae, or fractured surfaces of pyroxene and carbonate crystals.
Last year, the Johnson-Stanford team announced it found evidence of nanofossils in the meteorite. Reports of life on Mars spurred the July 4 mission to Mars to look for further evidence of life.
Allan Hills 84001 -- a meteorite the size of a potato -- remains in the center of a spirited controversy about the possibility of life on Mars. The meteorite was found in the 1980s in Antarctica by the National Science Foundation's Antarctic Search for Meteorite Program (ANSMET), headed by Harvey with headquarters at CWRU.
A Web page offers details on ANSMET.
Harvey, who is currently on his annual expedition to Antarctica to collect meteorites, commented before leaving November 21 that the Johnson- Stanford team has always argued that they had used different techniques to study the meteorite.
Bradley, Harvey, and McSween published a paper last year in Geochimica et Cosmochimica Acta (GCA), announcing that what the other researchers observed was formed geologically, not biologically. The Johnson-Stanford group also announced that these nanofossils were lying on the surface of the meteorite.
In the first GCA study, which used transmission electron microscope imagining (TEM), the researchers found non-biological magnetite whiskers on or near the surfaces of the carbonates. Superficially the whiskers look like worms, but in fact they have nothing to do with biological processes, according to Harvey and colleagues.
The latest study took place over the past six months as the researchers re-examined the meteorite using the new techniques. This time they found yet another population of worm-like forms that are actually mineral lamellae formed by non-biological, geological processes. The lamellae look like worms or nanofossils, but when the specimen is tilted and viewed from another angle, it clearly shows that the lamellae are attached and part of the mineral surfaces.
"The surface topography is highly irregular on a nanometre scale, with emergent lamellae following the major cleavage direction of the substrate," Bradley writes in the paper. The researchers have published pictures of the TEM images to support their findings.
"Peculiar surface structures or segmentation on the worm-like forms are artifacts from conductive metal coatings applied to the samples for imaging in the electron microscope. This is not the first time metal coating artifacts have lead to misidentification of nanofossils in rocks," Bradley said.
"We have now found two different types of mineral forms in ALH84001 that look just like nanofossils, but they are strictly non-biological origins. Sometimes even nature has a perverse sense of humor," he added.
Harvey stressed that during this latest study, the team was careful to use exactly the same methods as the Johnson-Stanford group to lay to rest any arguments that the research methods had affected the findings.
The worm-like mineral lamellae are commonly found at the fractured surfaces of planar crystals. Harvey noted that lunar rocks -- in which there has been no evidence of life found -- contain these same formations.
Does this put an end to the life on Mars debate? "We haven't driven the final nail in the coffin yet about organisms in this Martian rock, but our latest article offers a lot of insight that shows these fractures zones in the rock are incredibly complex," Harvey said, "and that it is very dangerous to try to draw any hypothesis from a few pictures from here or there."
National Science Foundation
July 17, 1997
After the announcement last August that the meteorite may harbor fossils of ancient Martian life, NSF and NASA called for further research into the evidence. The agencies set up a coordinated, interdisciplinary program which included joint review of research proposals. NASA announced on June 19 that it had awarded 16 individual grants under the program.
NSF's seven new grants, totaling nearly $800,000 for projects over two or three years, will use advanced instrumentation to further analyze the provocative rock. Some projects will study ALH84001 itself. Others will investigate analogous features in terrestrial rocks from environments that may resemble those of ancient Mars -- hot springs and other extreme habitats of earthbound microbes -- to provide a better context for understanding the tiny structures in the Martian rock.
Meteorite ALH84001 is one of about 8,000 meteorites collected in Antarctica by U.S. researchers. NSF is the lead agency for managing the collection and distribution of Antarctic meteorites, done in collaboration with NASA and the Smithsonian Institution. Samples of ALH84001 are being sent to the researchers from the Antarctic Meteorite Laboratory at NASA's Johnson Space Center in Houston. The samples, typically only a few grams apiece, are handled similarly to the lunar samples collected during the Apollo program.
The new research will include scanning the meteorite for extremely fine-scale alteration of the mineral interface by microbes. Other studies will focus on the meteorite's carbon isotopes to see if they reflect a ratio typical of microbial life, and develop a chemical method to fingerprint biological activity in meteorites using different isotopes of iron, some of which may be taken up preferentially by living organisms.
Still other projects will look at mineral particles -- oxides and sulfides of iron -- with potential as "biomarkers" (signs of past life) both in the Martian meteorite and in bacteria on Earth. Some researchers will attempt to: fix the temperature and fluid composition under which the meteorite's minerals formed, presently an area of controversy; develop thermodynamic models for mineral alteration in hydrothermal environments; and delineate the rock's temperature history and its past infiltration by fluids.
Institutions receiving the grants are the University of Wisconsin-Madison, the University of Wisconsin-Milwaukee, California Polytechnic State University-San Luis Obispo, Iowa State University, Arizona State University, University of Minnesota, University of California-Santa Cruz, University of Hawaii, Washington University in St. Louis, and the California Institute of Technology.
Stanford University
May 29, 1997
What Zare didn't expect was the course that the scientific debate has taken. He thought that the resulting discourse would be skeptical and opinionated, but also highly reasoned and dispassionate. But because of the high stakes - nothing less than the first discovery of alien life - and the intensity of the media spotlight, the scientific interchange has proven to be highly emotional and highly disruptive, he said.
"Sometimes it is all too easy to forget that the object of this effort is not to win a debate, but really to understand as much as we can about this fascinating meteorite," said Zare, the Marguerite Blake Wilbur Professor of Chemistry.
Since the initial study was published in Science magazine last summer, a dozen or so scientific papers have been presented at meetings, circulated to the media and published in scientific journals. A number have attacked various aspects of the original study, but more have claimed to provide additional support.
When they released their findings, Zare and his co-authors expected this sort of give and take. They did not have direct evidence for Martian life, so they wanted other scientists to examine their work and come to a collective judgment about whether they were right or wrong. The quality of much of the criticism and support, however, has not been terribly impressive, according to Zare and graduate student Simon Clemett, who took part in the study and has stayed on at Stanford to do additional research in this area.
"I've become less sanguine about the possibility of a quick answer," Zare said. "I would have thought by now that our hypothesis would have been either shot down or accepted. But the situation has proven to be much more complicated than I first realized."
That said, the two scientists have not seen anything yet that makes them want to retract their hypothesis. In fact, they argue that the additional supporting evidence that has been produced slightly outweighs the criticisms. At the same time, they don't know of any research going on that is likely to resolve the issue definitively.
The original paper by Zare and his co-authors - NASA scientists David McKay and Everett Gibson Jr., Kathie Thomas-Keprta of Lockheed Martin, Christopher Romanek from the University of Georgia and Hojatollah Vali from McGill University plus Clemett and fellow graduate student Claude R. Maechling with postdoctoral student Xavier Chillier -- contained four basic lines of argument to support its pro-life hypothesis: (1) The meteorite, designated ALH84001, that they examined came from Mars; (2) it contains a unique pattern of organic compounds that could have been produced by the fossilization of microorganisms; (3) it contains several unusual mineral phases commonly produced by microbes; and (4) when examined by an electron microscope, it reveals textures that appear to be the images of tiny microfossils. In addition, the features suggestive of biological activity all appear in the same locations with the rock.
One immediate criticism was that these objects were probably just artifacts of the electron microscopy process. To get the surface features of a rock to show up in an electron microscope, they must be coated with a thin layer of gold. If this coating is not uniform, it can obscure the underlying sample's face and create features that don't exist. Andrew Steele at the University of Portsmouth has ruled out that possibility by examining an untreated sample of the meteorite with an atomic force microscope (AFM). The AFM creates images by scanning the surface with a very fine tip. The fact that he imaged these features means that they are real, not artifactual.
The critique that Zare considers most serious has been advanced by William Schopf from UCLA. Schopf, who is a leading authority on the detection of terrestrial microfossils, has dismissed all the evidence for Martian life except the fossil-like shapes. If the shapes are not fossils, then it is possible to explain away the other evidence by various physical processes. And the only convincing way to determine if these shapes are fossils, rather than solid splatters of some mineral, is to cut some open and see if they contain internal structures, he argues.
"The NASA people are trying to do this, but they are finding it very difficult. The tiny shapes shatter very easily," Clemett said. Furthermore, Schopf casts doubt on the biological origin of these shapes by arguing that they are simply too small. The smallest shapes in the meteorite contain about one-thousandth the volume of the smallest known living microorganism on Earth. That is not enough space to perform the basic chemical operations necessary for life, he claims.
Initially Zare and his colleagues did not worry overly about the small size of these objects because there had been several published reports claiming that nanobacteria of comparable size had been discovered on Earth. Clemett has researched the matter since then, only to find a lot of skepticism about such reports. "People have seen these very small objects and have claimed to grow them in the lab, but don't seem to know exactly what they are," he said.
One of the strongest attacks on the low temperature origin of the carbonate minerals was issued by Harry P. McSween Jr. at the University of Tennessee and Ralph Harvey of Case Western Reserve University. They argued that the carbonates were formed by a sudden reaction between rock and boiling, carbon-dioxide-rich water during an impact, probably the same one that launched the rock from Mars and ultimately caused it to crash to Earth some 13,000 years ago.
Two other papers, however, have produced results consistent with a low-temperature origin. John W. Valley and co-workers at the University of Wisconsin-Madison measured oxygen isotope ratios at a number of different places within the carbonates and found fluctuations that suggest a low-temperature formation. Additionally, Joseph L. Kirschvink and co- workers at the California Institute of Technology found a dramatic difference in the response between adjacent fragments of the meteorite when they applied a magnetic field, which Kirschvink said indicates that it has not been heated above the boiling point of water for at least 4 billion years.
"Our problem with Bradley's work is that we don't know where in the meteorite he is looking," said Clemett. His images appear to be taken of another part of the meteorite so their relevance to the basic question is unclear, he added.
"There seems to be a strong element of the blind men and the elephant at work here. Different researchers are looking at different parts of the meteorite and coming up with wildly different reports," Zare said.
Another example of this problem is a paper published in the journal Nature just last week. In it, Edward Scott of Hawaii's Institute of Geophysics and Planetology announced that his analysis of a sliver of ALH84001 supported a high-temperature origin for the carbonates. However, his sample did not include the carbonate globules.
Stanford's contribution to the original study was the discovery of significant amounts of organic material in the meteorite. These compounds, called polycyclic aromatic hydrocarbons (PAHs), can be created either by biological or physical processes. But their presence substantially bolstered the other evidence and played a critical role in getting the paper through the peer review process.
The Stanford scientists took painstaking efforts to ensure that the PAHs did not originate from laboratory contamination. But a group of scientists at Scripps Oceanographic Institute, headed by Jeffrey L. Bada, suggested that the PAH's were caused by terrestrial contamination during the 13,000 years that ALH84001 sat in Antarctica. According to these researchers, the same kinds of PAHs turn up in Antarctic ice samples. To explain the higher concentrations of PAHs measured within the meteorite, the Scripps scientists conducted an overnight lab experiment that they said proved that carbonates scavenge PAHs from water and that this can explain how the PAH levels in the meteorite could build up until they are a million times higher than those found in the Antarctic environment.
Clemett, together with Chillier and graduate student Seb Gillette, has spent much of the last nine months looking into the Scripps team's contention. They discount it entirely, saying that the La Jolla researchers made a fundamental mistake by not distinguishing between soluble and insoluble PAHs. Most of the compounds that the Stanford team identified in the meteorite are highly insoluble. They do not mix well with water, which makes it extremely difficult for melt water to be the carrier of PAH contamination. The Stanford team also determined that carbonates do not scavenge PAHs. When they duplicated the Scripps experiment they found that most of the PAHs ended up with the carbonate because they were insoluble, not because the carbonate removed them from the water, as Bada and co-workers thought.
The Stanford chemists took the matter a step further. They examined several other Antarctic meteorites and micrometeorites for PAHs. They found virtually none in some cases. In others, the PAH distributions varied considerably among samples. If Antarctic melt water were a general mechanism for PAH contamination, then all the meteorites should be heavily contaminated, and contaminated in the same way, Clemett argues.
Other than the Scripps team's critique, the news has generally been positive on the organics front. Colin Pillinger and Ian Wright at the Open University, working with Monica Grady at the British Natural History Museum in London, reported detecting an abundance of organic carbon in ALH84001. They also measured the carbon isotopic ratio of the material and found that it matched biominerals produced by terrestrial bacteria. Two other papers also have confirmed the existence of organic material in the meteorite using different techniques.
Zare confessed that he feels badly about one consequence of the whole matter: "The need to respond to criticisms has set progress in our lab back considerably. I never imagined how disruptive this could be." Last August, he and Clemett had expected to alter their instrument to hunt for amino acids in the meteorite but have not made much progress.
"Nevertheless, this is a real science story unfolding, warts and all. It shows that the course of true science, like that of true love, seldom runs smoothly."
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