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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.
Carbonate globules
Another point of contention has centered around the origin of a series of
unusual carbonate globules that appear to be the centers of the possible
biological activity. NASA scientists argued that these carbonates formed
at relatively low temperature, less than 212 degrees Fahrenheit, more than
3.6 billion years ago while the rock was still on Mars.
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.
Magnetite criticism
The original team's report of tiny particles of magnetite that are
strikingly similar in shape and size to those created by terrestrial
bacteria that can sense magnetic fields is another source of criticism.
When John P. Bradley of MVA Inc. put a sample of the meteorite under
his transmission electron microscope he observed magnetite in the form
of rods, ribbons and whiskers, many of which contained a screw-shaped
defect that forms at temperatures between 900 to 1400 degrees
Fahrenheit quite different from what the original team found.
"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."