Meteorite Studies Challenge Claims of Life on Mars

Casting a shadow of doubt over the most dramatic and contentious scientific claim in memory--that fossil life forms were found on a meteorite from Mars--two new studies suggest that organic molecules on the famous rock were actually picked up on Earth.

The highly charged debate over the true nature of the submicroscopic fossils is far from settled, however.

Researchers welcomed the new studies, published today in the journal Science, as important clues to unraveling the mystery of the rock, but worried that the scientific debate was being swept up in an emotional undertow--leading to exaggerated claims of certainty on both sides.

“What’s at stake is the answer to the most fascinating question that any of us could ever hope to ask,” said UCLA geochemist Laurie Leshin. “Is there life elsewhere?”

“I don’t believe the work is that convincing as a refutation,” said Stanford chemist Richard Zare, an author of the original paper claiming the discovery.

In one of the new studies, Jeff Bada at Scripps Institute of Oceanography in San Diego found that amino acids in the rock were similar to those on Earth. From this he concluded that the organic material in the rock probably came from Earth rather than Mars.

“He may be right,” said Zare. “But that he’s so sure of himself is very bothersome.”

The other study, from the University of Arizona, separated the organic carbon from the rest of the rock and found distinct chemical signatures of earthly carbon. However, the Arizona group also found some carbon compounds of mysterious origin.

The evidence is subtle at best, and decoding the ambiguous markings on the rock is akin to reading smoke signals sent in a foreign language 4 billion years ago. The upshot, said planetary scientist Allen Treiman of the Lunar and Planetary Institute in Houston, may be that scientists “are overreaching their data. . . . They’re trying to get exciting results out fast and maybe they’re doing it faster than they should be.”

The new findings join a slew of recent papers offering conflicting claims on the Mars rock, and highlighting the extreme difficulty in deciphering its message.

Remarkably, scientists do agree on the rock’s unlikely history. Formed more than 4 billion years ago on a warmer, wetter and possibly quite Earth-like Mars, it got blasted off the rusty planet by a passing asteroid 16 million years ago, then wandered about the solar system until it fell onto a blue patch of Antarctic ice about 13,000 years ago.

A researcher out for a joy ride on a snowmobile saw it in 1984. The rock--officially known as Allan Hills 84001--was misclassified for 10 years before scientists got around to studying it in depth.

In August 1996, NASA announced the discovery of possible life on ancient Mars, based on at least three lines of evidence from the meteorite: concentrated rings of organic molecules called PAHs; minerals possibly formed as “droppings” from primitive life, and submicroscopic bacteria-like shapes. All have been seriously challenged, but not absolutely refuted.

Getting to the truth is hard because scientists are pushing the edge of technology. “We’re using the highest-tech techniques there are, and some of these measurements have never been demonstrated to work,” Leshin said. “So you always have to take a critical eye. We’re on the hairy edge of what we can say.”

Moreover, there isn’t much rock to go around, and the “fossils” are too small to be explored by even the most powerful conventional microscopes, forcing scientists to devise new kinds of analyses. Researchers are “just grasping for atoms here,” Leshin said.

But even when the research is solid and clear, scientists aren’t sure what to make of it. Both new studies, for example, reveal that organic matter on the rock is remarkably Earth-like, suggesting that it had been contaminated during its stay in Antarctica.

But Zare and others insist that they have admitted all along that the rock would pick up contaminants. “What they’ve proved is what we already suggested,” Treiman said. “It’s not a surprise. [The rock]’s been in Antarctica for 13,000 years, sitting in pools of water, and whatever organics are around are going to get into the meteorite. We’ve been saying that all along.”

The real question is: is all the organic material from Earth? Or only part? The PAHs studied by the NASA and Stanford researchers make up less than 1% of the organic material in the rock.

Of the two studies, chemist Bada’s, from Scripps, struck researchers as most controversial. Where Zare and colleagues looked at PAHs--which are commonly present in charred steak and soot as well as life forms--Bada looked specifically at amino acids, the building blocks of proteins.

“PAHs do not have any role in biochemistry,” he said. “We should look at molecules that are intimate parts of biochemistry.”

Like gloves, amino acids come in left- and right-handed forms. Bada found that the acids on the Mars rock were left-handed, just like those on Earth.

But because any life based on amino acids has a 50-50 chance of choosing a left-handed biology purely by chance, Bada’s discovery isn’t conclusive, critics said. Moreover, life on Mars and Earth may have had a common origin, or primitive life from either planet might have landed on the other during the solar system’s early, violent history, said Caltech geophysicist Joseph Kirschvink, whose recent research proved that life could have survived even ALH84001’s improbable journey.

“If life is hanging out on these meteorites and dropping in on us, then life may have jumped back and forth from Earth to Mars many times,” he said.

More troubling for the Mars life thesis, Bada found that the three most common amino acids on the rock were the same three that are most prevalent in the Antarctic ice, suggesting that they came from a common source.

But Zare said he isn’t surprised. Amino acids dissolve in water and would be expected to seep inside the rock, he said. “But PAHs are greasy. They don’t dissolve in water.”

According to Zare, that makes Bada’s work nice, but irrelevant. “It doesn’t have anything to do with PAHs.”

However, Zare admitted that the other paper “is a lovely study” that “convincingly shows” most of the organic material in the rock is from Earth. “It makes me worried about the interpretation of our results,” he said. “But it doesn’t refute them.”

In that study, Arizona researchers Timothy Jull and Warren Beck heated organic material from the Mars rock at different temperatures, and measured the amounts of different versions, or isotopes, of carbon present in the burn-off. Since some carbon isotopes are radioactive and change into other forms over time, measuring the ratios of two isotopes is a good way to date carbon-based materials. In contrast to Bada’s work, the compounds analyzed by Jull and Beck did contain PAHs.

Jull and Beck found that most of the organic carbon in the Mars rock was less than 13,000 years old, which means it must have come from Earth. “The bulk of the organic carbon is younger than the date of its fall,” said Beck. “Therefore it must be terrestrial.”

The key word, however, is “bulk.” It is still not clear whether all of the organic material was picked up during the rock’s 13,000 years on Earth, or whether some came from Mars, as Zare and others suspect. Moreover, Jull and Beck found a small amount of organic material that did not appear Earth-like.

“We did find a mystery component in one experiment whose origins we don’t understand,” said Beck.

The question is unlikely to be resolved at least until a NASA spacecraft is scheduled to bring back the first new Mars samples in 2008. “That’s 10 years of wringing our hands,” said Bada. Meanwhile, scientists are test-driving the techniques they will need to analyze what they see.

“By throwing everything but the kitchen sink at these meteorites, when we get those samples back, we’ll really be able to do a good job,” he said. “Then we’ll answer the question once and for all.”