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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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