The Life on Mars Debate

Citing growth patterns associated with non-biological origin, researchers dispute claims of "nanofossils" in Martian meteorite.

By Jane M. Sanders

It was about 13,000 years ago, according to carbon-14 dating, when a chunk of very old Martian meteorite landed in the Far Western Icefield of the Allan Hills Region of Antarctica.

photo by Stanley Leary "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," says Dr. John Bradley, adjunct professor in Georgia Tech's School of Materials Science and Engineering.

It inhabited that cold ground until 1984 when geologist Roberta Score found it and immediately realized it was unusual.

But she probably did not realize at the time that the rock would become the source of a long-running scientific debate — whether the meteorite contains evidence of life on the Red Planet.

In the latest volley of argument in the debate, a team of researchers, including a Georgia Institute of Technology adjunct professor, cite mineral growth patterns associated with non-biological origin of crystals in the meteorite.

In a paper published in the July issue of the journal Meteoritics and Planetary Science, the researchers report evidence that these crystals 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 Dr. David S. McKay) that the so-called "Mars rock" contains nanofossils.

Using transmission electron microscopy, the Georgia Tech researchers discovered that magnetite crystals in the meteorite, known as ALH84001, were atomically intergrown with carbonates by a process known as epitaxy. This process is an ordered growth of one material on top of another. The finding suggests that the magnetites and carbonates grew together at temperatures much greater than 120 degrees Celsius, researchers say.

Epitaxial formation rules out intracellular precipitation of the magnetites by Martian organisms, a theory hypothesized by NASA scientists who believe the meteorite contains nanofossils, says Dr. John Bradley, an adjunct professor in Georgia Tech's School of Materials Science and Engineering and executive director of the microanalytic company MVA Inc. in Norcross, Ga. And the implied high-temperature origin virtually eliminates the possibility that fossilized Martian organisms could be present in this meteorite, he adds.

Faculty Column Nanofossils or non-biological magnetites? Mars rock debate opens news era of study in astrobiology. By John Bradley Adjunct Professor, School of Materials Science and Engineering No meteoriticist, planetary scientist or astronomer will ever forget the televised press conference in August 1996. It seemed as if the solar system temporarily ground to a halt as NASA Administrator Dan Goldin made the stunning announcement to the nation and the world that a consortium of NASA and university scientists had found evidence of past Martian life in a potato-sized rock from Mars. In a paper that appeared a few days later in the journal Science, the scientists claimed to have found this evidence within the cracks and crevices of a meteorite called ALH84001, one of a dozen or so SNC or "SNIK" meteorites known to have been blasted off the surface of the planet Mars and eventually captured by Earth. The meteorite landed in Antarctica some 13,000 years ago where it remained until it was picked up by a meteorite search party in 1984. Administrator Goldin called it "an exciting time to be alive." President Bill Clinton spoke of the philosophical significance of the discovery and its vindication of the U.S. space program. One of the authors of the paper explained, "We did it for the American people." And another, describing how a bug-infested rock from Mars could have fertilized Earth, declared, "We may all be Martians!" Meanwhile, all of them emphasized the need for the evidence to be independently examined by the scientific community at large. The evidence for past Martian life centers on tiny (micrometer-sized), pancake-shaped carbonate deposits that decorate the inner fracture surfaces of the meteorite. The authors of the paper proposed that these carbonates were deposited from fluids saturated with carbon dioxide at temperatures no higher than 100 degrees Celsius, a temperature around which microorganisms flourish here on Earth. Intimately associated with the carbonates are even smaller (nanometer-sized) iron sulfide and iron oxide (magnetite) grains, which they concluded are biominerals (i.e., inorganic minerals that are produced by bacteria). (The sizes, shapes and crystal structures of some of these grains are indeed similar to those produced by bacteria here on Earth.) Also identified were organic compounds known as polyaromatic hydrocarbons (PAHs), which they proposed are the organic remains of long-dead Martian organisms. But perhaps the most startling evidence of all were worm-shaped objects within the fracture zones of the meteorite. They proposed these are fossilized remains or "nanofossils" of the Martian organisms themselves. The authors conceded that individually none of their findings is strongly indicative of life, but that collectively they make a compelling case for ancient Martian microbial life in the meteorite. In other words, they reasoned that several "possiblys" add up to a "probably." The stage was set for controversy even before publication of the now famous Science paper. A month earlier (in July 1996), a paper published in the journal Nature suggested that the carbonate pancakes in ALH84001 may have formed at temperatures greater than 650 degrees Celsius, much hotter than the 100 degrees Celsius proposed and much too hot for biological processes. (This key issue of the formation temperature of the carbonates is still being debated.) Others have since presented evidence that at some point the carbonates were shocked and melted, further complicating the issue of their original temperature of formation. As for the tiny iron oxide (magnetite) grains, even though they resemble biominerals, they probably formed at much higher temperatures by crystal growth processes called spiral growth and epitaxy under strictly non-biological conditions. The tiny sulfide grains in the meteorite were likely misidentified and, in any case, appear irrelevant to the question of life on Mars. PAHs have now also been found in Antarctic ices and other Antarctic meteorites, which raises the thorny issue of terrestrial organic contamination. (PAHs are so ubiquitous in the terrestrial environment that their absence is perhaps more significant than their presence.) Carbon-14 isotopic measurements have revealed that most of the organic carbon in ALH84001 is terrestrial. Even the worm-like "Martian nanofossils" have taken it on the chin: Recent evidence suggests they are simply (non-biological) mineral ledges and that their worm-like segmented surfaces are laboratory artifacts caused by the conductive metal coatings the scientists applied to their specimens before imaging in the electron microscope. (Similar worm-like structures have recently been observed in lunar rocks and, because the Moon is sterile, nobody is arguing that they might be the remains of lunar organisms.) It's been almost two years since the press conference. The claims in the paper that started it all have been thoroughly examined by the scientific community at large. A quiet consensus has emerged that the ALH84001 meteorite contains no evidence of past Martian life. Apparently the scientific process of checks and balances is alive and well. Now the focus has shifted to Mars itself. For better or worse, the ALH84001 debate has raised the ultimate cosmological question, "are we alone?". It may require several missions (and perhaps even a drilling rig on its surface) to unambiguously answer the question of whether life exists or ever did exist on the Red Planet. An armada of Mars probes and landers are in the works; NASA has created the new Astrobiology Institute (at its Ames, Calif., facility, which was rumored to be up for closure); the public appears interested; and Congress has not yet blinked. Many in the meteoritics community (most of us are chemists, geologists or physicists) are "going biological" over the prospect of a stream of new research dollars for updating laboratory equipment and support for a new generation of planetary (bio)scientists. Mr. Goldin was right, it is an exciting time to be alive.

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 the JSC scientists who believe the meteorite contains nanofossils.

"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," Bradley says.

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 says. 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 says.

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 says. "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 focused 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 say. Magnetite crystals, apparently formed by several different 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 that the ideal lattice "misfit" between two crystal structures should not be more than 15 percent. In this case, the lattice "misfit" was only 11 to 13 percent, which is ideal for epitaxial growth, Bradley says.

Furthermore, many of the epitaxially formed magnetite whiskers in the meteorite appear to be free of internal defects, the researchers say. 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 say. 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 says.

Despite this paper and the other two Bradley team publications, the debate over nanofossils in Martian meteorite ALH84001 will continue, Bradley says.

"Unless the JSC team concedes, the debate will never die," Bradley says. "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 says.

While the debate or debates continue, Bradley says, he is nonetheless encouraged because public interest in the life on Mars question has renewed enthusiasm for science in America.

For more information, you may contact Dr. John Bradley, MVA Inc., 5500 Oakbrook Pkwy., Suite 200, Norcoss, GA 30093. (Telephone: 770/662-8509) (E-mail: jbradley@mvainc.com).

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Last updated: October 7, 1998