Based on a SPACE.com story
It is fitting that the Antarctic-recovered Martian meteorite, ALH84001, is potato-shaped. After years of argument, the "Mars rock" continues to be just that -- a scientific hot potato.
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| The microscopic formations above are believed by some
scientists to be microfossils. Credit: NASA |
The inside story is that the meteorite may contain evidence of ancient life on the Red Planet. Trying to anchor that belief in a sea of skepticism remains a daunting and challenging, but dutiful task for those making the assertion.
But this scientific saga has taken more blasts, twists and turns than the meteorite took to get to Earth in the first place. Without doubt, the Carl Sagan axiom that "extraordinary claims require extraordinary proof" serves as the foundation for an ongoing and spirited debate.
The 32nd Lunar and Planetary Science Conference, held here at NASA’s Johnson Space Center (JSC) March 12-16, served up another generous helping of claim, controversy and downright grouchiness.
Under the microscope
The argument swirling around the Mars rock centers on whether or not there is evidence of fossil remains of Martian microbes in the meteorite. These fossils are purported to be in the form of tiny magnetite crystals, like those used by aqueous bacteria on Earth. The magnetite is utilized as a compass by the bacteria to steer toward food and energy sources. Furthermore, magnetotactic bacteria produce magnetite crystals having a distinct size and shape. These magnetite crystals form chains within their cells.
Kathie Thomas-Keprta, a Houston-based Lockheed Martin scientist, reported during the conference that single magnetite crystals inside the meteorite are similar to those formed by ‘modern’ magnetotactic bacteria now living on Earth.
Thomas-Keprta and her research colleagues contend that the crystals are interpreted as Martian magnetofossils and "constitute evidence of the oldest life yet found."
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| Magnetic microscopy image of ALH84001. The exterior (upper
left) has been remagnetized in the Earth's field after the heat of entry,
while the interior of the meteorite retains the weaker, mixed magnetism
it acquired on Mars. Credit: Francis MacDonald, Caltech |
Thomas-Keprta said that in support of her view, early Mars likely had freestanding bodies of liquid water, both organic and inorganic carbon (e.g. atmospheric carbon dioxide), energy sources, and likely was replete with a planetary magnetic field sufficient to support the growth of magnetotactic bacteria.
Imre Friedmann, a senior research fellow at NASA’s Ames Research Center in California’s Silicon Valley, also weighed in heavy in support of ALH84001 containing relics of early Martian life. He led an international team discovering chains of magnetite crystals in the Mars rock, stating that they are of biological origin. The chains were preserved in the meteorite long after the bacteria themselves decayed, he said
"We conclude that the chains in ALH84001 are magnetofossils, remnants of magnetotactic bacteria. No other consistent interpretation would account for our observations," Friedmann reported at the conference.
Let the data speak
"I think it’s fair to say that the data now are looking even stronger than they did three to four years ago," said David McKay, JSC senior scientist for exploration and science. McKay headed the team -- that also included Thomas-Keprta -- which announced to the world in August 1996 that ALH84001 held evidence for past Martian life.
"We’re particularly interested in the magnetite data. The only thing that it fits is a biological magnetite on Earth. This magnetite is so unique and unusual that we just don’t see how it can be made except with the help of biology. We’re really quite confident that we have evidence for primitive life in the meteorite," McKay told SPACE.com.
"We’ve come a long way in understanding some of the more specific properties in the rock," said Everett Gibson, a JSC geochemist, and another original member of the Mars rock team. "We as a team are more convinced now than when we presented our results in 1996," he said.
"Let the data speak. There’s a lot of people arguing on emotions as opposed to facts," Gibson told SPACE.com. "Sure the critics are more vocal, but where’s their data?" he said.
Critics corner
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| The meteorite at the center of the debate: ALH84001 Credit: NASA |
Taking issue with any bounty of Martian microbes in the Mars rock is Allan Treiman, a research scientist at the Lunar and Planetary Institute in Houston. His detailed studies point to abiotic -- without the presence of life -- and non-Martian origins for the features claimed to be biological in ALH84001.
The reported bacteria-shaped objects were formed on Earth, Treiman said, likely stemming from the Antarctic environment itself, as well as artifacts from sample preparation, or weathering effects on the Mars rock from thousands of years residing in Antarctica before being picked up.
Joining Treiman in the skeptics' corner is John Bradley, adjunct professor at the Georgia Institute of Technology in Atlanta. "Unfortunately, there are many signatures in the fossil record here on Earth, and probably on Mars, that look very similar to bacterial signatures. But they are not unique to bacterial processes," he said.
"Extraordinary evidence hasn’t been furnished at this point. The bottom line is, even though they’ve found structures in this meteorite, some of which are consistent with bacterial magnetite, these structures are probably not unique to bacterial magnetite," Bradley said.
"In science, the onus is on them, not the critics to prove it right," Bradley said.
Truth is…
There are "fundamental flaws" in the research by those claiming to have found evidence for life in ALH84001, said Ralph Harvey, assistant professor in geology at Case Western Reserve University in Cleveland, Ohio.
"It has all boiled down now to this magnetite. The only defense used is to claim that these must be biological because nobody has ever published a detailed description of these in an inorganic setting. The truth is that we haven’t looked," Harvey said. "It’s hard to prove a negative hypothesis. It’s hard to test it, but it is essential. It’s the difference between science and faith," he said.
Harvey said that the research community is "tired of following their footsteps on the wrong path. We’re not going to spend our careers testing straw men that they put up. I think they are caught in a ‘moving hypothesis’ mode," he said.
"I’m absolutely uncompelled by the argument for life until alternative hypotheses are tested…that these things are inorganic," Harvey said.
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| This is an electron microscope image of a small chip of
ALH84001. In the center of this image are several tiny structures that are
possible microscopic fossils of primitive, bacteria-like organisms. Credit: NASA |
The debate rests on whether the features found in ALH84001 can be made non-biologically, said William Boynton, a space scientist at the University of Arizona in Tucson. "It’s not something that can be rigorously ruled out. But to date, nobody has seen things exactly like this either in nature or synthesized in the lab," he said.
"I think it’s actually coming down now to more a question of philosophy rather than science. What does it take to constitute a proof?," Boynton said. "In my view, there’s nothing that can be done to move this debate much further until we bring samples back from Mars," he said.
Secret of the rock
Since the Mars rock was unveiled to the public in August 1996, David McKay and his team have continued to chip away at disbelievers that ALH84001, indeed, has something important to say about life on Mars.
But will there ever be closure on the debate? McKay believes so, but it means pushing the state-of-the-art in analyzing the meteorite.
"It’ll be another four or five years before we think we can convince the community or refute everything we’ve said. But we have to go to a whole new level of analysis," McKay said. "I personally think that in five years we will have solved this question. But that’s a minority viewpoint and has to do with using a lot of techniques that are not commonly used today," he said.
"We can’t really predict what the outcome will
be. But we know it’s worth pursuing. It is worth taking the time and effort
or we wouldn’t be doing this," McKay said. "I think the secret
of the rock is still buried in the rock. We just have to figure out how to get
it out," he said.