by Steve Koppes
The scientific firestorm was immediate. One critic estimated that the evidence for life in the meteorite had only five chances in 100 of proving correct and in the years since, the scientific consensus hasnt improved.
If I had to revise it, I would revise it downward, said critic Laurie Leshin, an assistant professor of geology at Arizona State University. All of their lines of evidence have fallen by the wayside except one. Theyre still clinging to that, and it needs to be investigated more.
It was in this atmosphere that Romanek and a few of his like-minded colleagues recently refueled the debate with research presented in the December 2000 issue of Geochimica et Cosmochimica Acta, the February 2001 issue of Precambrian Research and the Feb. 27, 2001, issue of Proceedings of the National Academy of Sciences, and at the 32nd annual Lunar and Planetary Science meeting in Houston this past March.
Going the distance
Like a runner, he also knows well how to go the distance alone even when it means holding fast to an unpopular scientific theory. Romanek and his ALH84001 colleagues have endured a great deal of criticism from their peers over the past five years. A few critics even stubbornly refuse to accept the concept of meteorites from Mars at all let alone that one of them might contain fossil evidence of microbial life. Still, Romanek shows no signs of bitterness. He speaks enthusiastically of his work and graciously of his critics.
Most scientists now accept the evidence that some meteorites originated on Mars. The mix of gases and isotopes that Martian meteorites give off when heated matches the strange atmospheric composition that the Viking landers recorded on Mars in 1976.
But the evidence that life once existed in them, even Romanek admits, is weak. But its evidence nonetheless, and it has allowed a really healthy discourse on the subject, said Romanek, an associate professor of geology and associate research scientist at UGAs Savannah River Ecology Laboratory. I think its pointed the scientific community in the right direction for the kinds of questions that we have to answer if we really want to know whether theres life anywhere else in the universe.
How the story of ALH84001 will end, no one knows, but it began about 4.5 billion years ago, shortly after the birth of the solar system. Volcanoes extruded extensive pools of lava in the restless early history of Mars, as they did on Earth. The Martian lava cooled, solidifying into an igneous rock.
By human standards, an eternity passed. Then, scientists theorize, 16 million years ago, a comet or asteroid slammed into Mars at a low angle, spraying bits of rock into orbit. After whizzing through the inner solar system for ages more, Earths gravity grabbed at least one of the rocks, plunking it down in Antarctica 13,000 years ago. There, protected from rust in Mother Natures deep freeze, glacial movements slowly pushed ALH84001 and thousands of other meteorites that have fallen during the millennia downhill toward the sea.
Civilizations rose and fell; humans explored the North and South poles, the peak of Mount Everest, the deepest depths of the ocean floor and the surface of the moon. In 1984, Roberta Score of NASAs Johnson Space Center in Houston, Texas, collected ALH84001 on a meteorite-collecting expedition to the Antarctics Allan Hills region. The specimen looked green to Score, an observation she recorded in her field notes.
U.S. expeditions collect as many as 1,000 meteorites annually in Antarctica, where the space rocks have become exposed on the barren expanse by fierce, howling winds. Leshin, the ASU professor who has examined about half of the 18 Martian meteorites discovered so far, including ALH84001, once lived in a tent for two months on an Antarctic expedition.
Clearly, this sample looked different to the folks in the field, so the curators were drawn to it, Leshin said. Consequently, ALH84001 became the first meteorite that Johnson Space Center scientists catalogued that year. The irony is that it is really a bland, gray-looking rock. It only looked green through Robbies goggles.
The Johnson Space Centers David Mittlefeldt recognized the meteorites Martian origin in 1994. At the time, Romanek happened to be serving a postdoctoral research fellowship the academic equivalent of a medical internship at the space center.
As an undergraduate at Furman University, Romanek had written his senior thesis on planetary geology using images from the Voyager space probe. Except for that one paper, he had focused all of his research on Earth rocks. That was about to change.
By analyzing the meteorites stable isotopes, Romanek figured he could determine the temperature at which the meteorite formed. Stable isotopes are the nonradioactive siblings of the atomic world, forms of a common element that differ only in the number of neutrons at their core (see Scientific Sleuthing).
There are chemical and isotopic fingerprints in a rock to give you insights into the processes that those rocks experienced, including the temperatures, maybe, at which they were formed, Romanek said.
It appeared to Romanek that the meteorites carbonates formed at temperatures ranging from 32° to 175° F, low enough for microbial life to flourish. Knowing that similar rocks on Earth often contain fossilized bacteria, Romanek wondered if the Martian meteorite might have them as well.
Thats how it all started. Romanek took the results of his research on the carbonates to Everett Gibson, a senior scientist at Johnson Space Center. He immediately closed the door and said, Youve got something here.
Gibson took the data to David McKay, the Johnson Space Center scientist who ultimately would head the research team that wrote the original Science paper on ALH84001 in 1996. McKays associate, Kathie Thomas-Keprta of Lockheed Martin, took a close-up look at the meteorite with a tunneling transmission electron microscope. It was Thomas-Keprta who isolated the magnetite, an iron-related mineral commonly produced by Earth bacteria.
Romanek, meanwhile, showed that the magnetite had to have formed on Mars. The magnetite was contained inside some pancake-like carbonate structures that contain an isotopic composition similar to what scientists would expect to see on Mars.
Of four branches of evidence for ancient life in ALH84001 presented in the 1996 Science paper, the magnetite survives today as the most viable. Bacteria produce magnetite, a mineral that aligns with Earths magnetic field, to help them navigate. Nonbiological processes could explain most of the magnetite grains, Leshin said, but others are still open to question. Nobodys ever shown that they couldnt be inorganic, she acknowledged.
Two new papers presenting further evidence for the magnetites biological origins appeared in the Feb. 27, 2001, issue of Proceedings of the National Academy of Sciences.
In one paper, Romanek, Thomas-Keprta and
others compared what they regard as the Martian meteorites
biologic magnetites with a wide variety of terrestrial magnetites,
formed under high-temperature conditions. Magnetites
Romanek stopped short of claiming this proves the case for life on Mars. But it might serve as a potential biological indicator when applied to samples returned from future Mars missions, he said.
The other Proceedings paper, which contained data produced independently of Romanek and his colleagues, also bolstered a biological interpretation for the magnetites. The paper described magnetite chains that resembled microscopic strings of pearls. Such chains would only form inside an organism, according to the papers lead author, Imre Friedmann of NASAs Ames Research Center near San Francisco.
In his debates with critics, Romanek has learned the importance of scrutinizing supportive evidence with the same skepticism that he applies to contradictory data. He regards Friedmanns study with caution.
It certainly is suggestive and is one of these criteria that we may be able to utilize in a sample-return coming back from Mars, Romanek said.
Questioning the evidence
Of the latter evidence, Leshin maintains that their isotopic composition is unlike anything we see on Earth, at least to this point, she said. Theres a lot we dont really understand yet about this rock. But most of the environments that we can imagine making these things dont look very biological.
These environments include the extremely hot ones that would follow impact shock from comets or asteroids. And ALH84001, everyone agrees, has undergone intense shock. Its been fragmented and put back together again, and the place where these carbonates occur are these zones where its been stuck back together again, Leshin explained. The carbonates themselves could be a result of shock.
To explore the issue further, Romanek recently spent a week in Inuvik, in the Canadian Arctic, collecting samples of a type of carbonate called cryogenic calcites, which forms in extremely cold environments.
They have very unusual carbon isotope compositions. Some of them actually approach the compositions that we measure in the Martian meteorite, Romanek said. Were hypothesizing that this freezing of brines and waters may actually be a very good analogue for the types of processes that could form carbonates on Mars.
With funding from NASAs Astrobiology Institute, Romanek and NASAs Rick Socki already have succeeded in growing cryogenic calcites under controlled laboratory conditions. Their experiments showed that if enough briny waters existed on the Martian surface, carbon 13 could indeed become enriched in ways that resemble the carbonates in ALH84001. This finding, presented in March at the annual Lunar and Planetary Science meeting, lends further credence to Romaneks contention that the carbonates formed at lower temperatures.
Regardless of the outcome or whether scientists ever overcome President Clintons big if simply examining the evidence has been a boon for this branch of science, even Romaneks critics concede.
You really do have smart people thinking about these problems now, said Leshin. Its a positive thing, I would say, overall.
For the foreseeable future, scientists will continue to debate the merits of this research. But Romanek, the distance runner, envisions the finish line once future space probes bring samples directly back from Mars.
If I can live to a ripe old age, he said, I bet you that therell be some pretty exciting discoveries in store regarding the potential for past life on Mars.
For more information, access http://cass.jsc.nasa.gov/pub/lpi/meteorites/mars_meteorite.html.