When, in 1996, a group of NASA researchers presented several lines
of evidence for fossil bacteria in a Martian meteorite, a wave of
excitement passed through the public and the scientific community
alike. Of course, that wave was followed by a storm of controversy.

Five years of scrutiny and debate over the NASA group’s claims
have since brought all but one of their arguments unceremoniously
back to Earth. Non-biological processes and contamination could
explain the "bacterium-shaped objects" and organic chemicals
found in the meteorite, other scientists have argued.

Only one line of evidence for bacterial life in the meteorite still
stands: Microscopic crystals of a mineral called magnetite. According
to the NASA scientists, the magnetite crystals found in the meteorite
are so structurally perfect, chemically pure, and have such unique,
distinctive three-dimensional shapes that only bacteria could have
produced them, not any inorganic process. This claim, too, is now
being assailed by new data and criticisms from an Arizona State
University research team and their collaborators.

Peter Buseck, Regent’s Professor of geological sciences and professor
of chemistry and biochemistry at ASU, and Martha McCartney, a research
scientist at the ASU Center for Solid State Science, argue that
the match between the meteoritic crystals and those in bacteria
is at best ambiguous. At worst, they say, the data used in the NASA
group’s analysis is mistaken.

In their paper, "Magnetite Morphology and Life on Mars,"
published November 20, 2001, in the Proceedings of the National
Academy of Sciences, Buseck and his co-authors assert that the evidence
for bacterial magnetite crystals on the Martian meteorite is inadequate.
In doing so, they may have cut the Martian meteorite’s last tenuous
hold on life.

The magnetite crystals in the meteorite are tiny, even by an electron
microscopist’s standards, at only 40 to 100 billionths of a meter
wide. And there’s the rub. The technology necessary to accurately
describe the three-dimensional shape of such small crystals has
become available only in the last few years, and has not yet been
used to study the magnetite grains in the meteorite. Therefore,
says Buseck, it is too early to say for sure what the exact shapes
of the meteoritic crystals are, let alone whether they provide identical
matches to those in bacteria.

The only kind of microscope powerful enough to produce clear images
of such small crystals is a transmission electron microscope, or
TEM. By using a beam of electrons rather than a beam of light to
view the sample, the TEM allows researchers to see objects smaller
than one billionth of a meter wide. But a TEM sees only in two dimensions.
It generates a spectacular silhouette image of the sample, but conveys
little about its thickness.

An accurate description of the crystals’ complex three-dimensional
shapes requires that they be examined from a variety of perspectives.
Discriminating between their flat facets and tapered edges is a
particular challenge – when viewed in profile, the two are indistinguishable
straight edges. Only by tilting each crystal at dozens of angles
can scientists unequivocally identify their three-dimensional shapes,
says Buseck.

At the time of the NASA group’s study, the tilting experiments
could be done only by hand, with great technical difficulty. "It’s
a lot of work and it’s not very precise," says McCartney. The
NASA group used this approach to create images of the magnetite
crystals from both the meteorite and from one strain of bacteria.

Since then, scientists studying the three-dimensional shapes of
crystals have upgraded TEM technology and merged it with computer
technology. "The microscope stages and beam shifts and focuses
have come under computer control, which makes the experiments much
more doable" and more precise, says McCartney.

Only two laboratories, Buseck and McCartney’s and that of their
co-authors in Cambridge, have applied the new technology to study
magnetite crystal shapes. Using these new developments, they have
reexamined the evidence described in the NASA team’s study.

"The shape [the NASA group] came up with disagreed with what
we thought the shape was," says McCartney. This difference
calls into question whether the shapes of the meteoritic crystals
are accurately known and whether the claim of an exact match – the
only remaining evidence for bacterial life on the meteorite – is
accurate.

Buseck’s team also criticizes several other underpinnings of the
Martian life claim. The NASA group selected only 27 percent of all
the magnetite crystals present in the Martian meteorite for comparison
with bacterial crystals. The Buseck group implicitly questions both
the objectivity of their selection and the effect of such a limited
comparison on their conclusions.

Further, Buseck and McCartney’s team demonstrates that the shapes
of bacterial magnetite grains vary more than scientists had previously
thought. The shapes and sizes differ among bacterial strains and
even within individual bacteria. That expanded variety makes it
more likely that bacterial and meteoritic magnetite grains could
appear to match by simple chance.
Lacking sufficiently precise data and resting on a restricted analysis,
the NASA team’s claims must be considered best guesses, Buseck and
his co-authors argue.

However, they have not eliminated the possibility that the Martian
crystals could have a biological origin. With more advanced technology
now at their disposal, Buseck and his collaborators plan more conclusive
studies of the magnetite crystals from both the meteorite and several
strains of terrestrial bacteria.

"We will look at them in far greater detail than others have
been able to do before," says Buseck.

Buseck and McCartney’s co-authors on the paper are Rafal Dunin-Borkowski,
Paul Midgley, Matthew Weyland (all of Cambridge University, England),
Bertrand Devouard (of Blaise Pascal University, France), Richard
Frankel (of California Polytechnic State University), and Mihály
Pósfai (of the University of Veszprém, Hungary).

Contact: James Hathaway, 480-965-6375
Hathaway@asu.edu
Embargoed until November 20, 2001

Source: Peter Buseck, 480-965-3945