[meteorite-list] Evidence of Martian Life Dealt Critical Blow

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:12 2004
Message-ID: <200111201641.IAA09212_at_zagami.jpl.nasa.gov>

Office of Media Relations and Public Information
Arizona State University
Tempe, Arizona

Contact:
James Hathaway, 480-965-6375, Hathaway_at_asu.edu

Source:
Peter Buseck, 480-965-3945

Embargoed until November 20, 2001

Evidence of Martian Life Dealt Critical Blow

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).

IMAGE CAPTION: [http://clasdean.la.asu.edu/news/images/buseck/]
Multiple views of a magnetite (Fe3O4) nanocrystal from within a
magnetotactic bacterium. This tomographic reconstruction shows the
three-dimensional shape of the crystal viewed from several directions.
The images were obtained in a transmission electron microscope with
a field-emission gun. A chain of such crystals from within a single
bacterial cell runs from the upper left to the lower right.
Received on Tue 20 Nov 2001 11:41:05 AM PST