In a paper to be published in the July issue of the journal Meteoritics and Planetary Science, the researchers report evidence that crystals found in the meteorite were formed by epitaxial processes at temperatures that were likely too high for biological organisms to exist. The findings cast new doubt on claims by NASA's Johnson Space Center (JSC) researchers (led by David S. McKay) that the so-called "Mars rock" contains forms consistent with nanofossils.
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.
Epitaxial formation rules out intracellular precipitation of the magnetites by Martian organisms, a theory hypothesized by NASA scientists who believe the meteorite contains nanofossils, the Tech researchers said. And the implied high-temperature origin virtually eliminates the possibility that fossilized Martian organisms exist in this meteorite, they added.
This article is the third in a series of this research team's technical papers 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). NASA has sponsored all of this research, as well as work by JSC scientists who made the nanofossil claims.
"These three papers in combination basically invalidate much of their (JSC's) evidence," said Dr. John Bradley, an adjunct professor in the Georgia Tech School of Materials Science and Engineering and executive director of MVA Inc., a microanalytic company in Norcross, Ga. "Early skepticism has evolved into international consensus among meteoriticists and planetary scientists, with the exception of the JSC team, that this rock does not contain Martian nanofossils. I do not know of a single other individual who believes it at this point."
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 JSC 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 JSC researchers countered that the differences resulted from scientists using different microscopy techniques and thus seeing different objects.
So 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 paper, 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.
Also, researchers found 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.
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