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Moreover, the suspected fossils predated the meteorite's arrival on Earth by many, many years. Scientists pegged the age of the carbonate globules at 3.6 billion years, strongly suggesting that they formed in crevices of the rock while it was still part of the Martian crust. That argument makes sense to Carl Sagan. "This is a time," he says, "when Mars was warmer and wetter than it is today, with rivers, lakes and possibly even oceans. This is just the epoch in Martian history when you expect that life may have arisen."
As to the origin of the meteorite, the researchers have little doubt that it was Martian. They base their conclusion largely on the composition of gases trapped in tiny pockets within the meteorite. The NASA team found a strikingly close match between the constituents of the rock gases and those in the current Martian atmosphere, which the unmanned Viking landers sampled in 1976, transmitting the data back to Earth. Summarizing the findings, NASA's McKay concedes that "there are alternative explanations for each of the lines of evidence that we see." But after 2 1/2 years of study, the team became convinced that the evidence, taken as a whole, points to the existence of early life on Mars.
UCLA paleobiologist William Schopf, best known for discovering the world's oldest fossils, spoke for many who would urge caution. Invited by NASA to represent the natural (and healthy) skepticism of the scientific community, he repeated a familiar Sagan quotation: "Extraordinary claims require extraordinary evidence." Said Schopf: "I happen to regard the claim of life on Mars, present or past, as an extraordinary claim. And I think it is right for us to require extraordinary evidence in support of the claim." It was clear that to Schopf such evidence was not yet forthcoming. He noted that PAHS are routinely found in interstellar and interplanetary debris, as well as in other meteorites. "In none of those cases," he said, "have they ever been interpreted as being biological."
Turning to the putative fossils in the electron-microscope images, Schopf pointed out that they are a hundred times smaller than any found on Earth, too minuscule to be analyzed chemically or probed internally. Also, he noted, "there was no evidence of a cavity within them, a cell." Nor was there any evidence of life cycles or cell division. This led him to believe that the structures NASA was touting as fossilized life-forms were probably made of a "mineralic material" like dried mud. "The biological explanation," he concluded, "is unlikely."
Schopf acknowledged that the NASA team had done first-rate scientific research, but he regarded it as only a "preliminary" report. "All I'm saying is that there's additional work to be done." On that point Goldin agreed. "We want these results investigated," he said, "and we're prepared to make samples of the rock available" to credible researchers with sound experimental proposals.
Whatever the outcome of these investigations, the Mars mystique will probably endure. Throughout history humans have been intrigued by the baleful glare of the Red Planet in the night sky. To ancient civilizations it was the god of war, dubbed Ares by the Greeks and Mars by the Romans. When the first telescopes revealed that the planets were neither specks of light nor gods, but worlds, perhaps like Earth, the notion grew that Mars might harbor life.