Atlanta, GA (7/10/98)- New geologic analyses appear
to pour cold water on suggestions that strange crystals found on a Martian
meteorite represent bacterial fossils from the red planet.
In
August 1996, NASA researchers first announced the discovery
of possible signs of life on a small Martian meteorite found on Antarctica.
The announcement set off a firestorm of debate in the scientific community.
The pendulum now seems to be swinging in favor of those who doubt the 15
million year old rock ever carried ancient fossils with it.
Graphic: False-color backscatter
electron (BSE) image of fractured surface of a chip from ALH84001 meteorite
showing distribution of the carbonate globules.
New geologic research indicates that crystals found in the meteorite
were formed by epitaxial processes at temperatures that were likely too
high for biological organisms to exist. 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.
The finding
of epitaxial formation appears to rule out intracellular precipitation
of the magnetites by Martian organisms, a theory hypothesized by NASA scientists
who believe the meteorite contains nanofossils. And the implied high-temperature
origin virtually eliminates the possibility that fossilized Martian organisms
exist in this meteorite, according to Dr. John Bradley, an adjunct professor
in the Georgia Tech School of Materials Science and Engineering.
The current research supports the findings of two previous studies by
the same team 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).
"These three papers in combination basically invalidate much of their
(JSC's) evidence. Early skepticism has evolved into international consensus
among meteoriticists and planetary scientists, with the exception of the
Johnson Space Center team, that this rock does not contain Martian nanofossils.
I do not know of a single other individual who believes it at this point,"
said Dr. Bradley.
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 NASA 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 NASA researchers countered that the differences
resulted from scientists using different microscopy techniques and thus
seeing different objects.
Dr. 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 study, 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.
The researchers also studied 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.
The study appears in the July 1998 issue of the journal Meteoritics
and Planetary Science.
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