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Life on Mars?

It's hard enough to identify fossilized microbes on Earth. How would we ever recognize them on Mars?

On August 7, 1996, reporters, photographers and television camera operators surged into NASA headquarters in Washington, D.C. The crowd focused not on the row of seated scientists in NASA’s auditorium but on a small, clear plastic box on the table in front of them. Inside the box was a velvet pillow, and nestled on it like a crown jewel was a rock—from Mars. The scientists announced that they’d found signs of life inside the meteorite. NASA administrator Daniel Goldin gleefully said it was an “unbelievable” day. He was more accurate than he knew.

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The rock, the researchers explained, had formed 4.5 billion years ago on Mars, where it remained until 16 million years ago, when it was launched into space, probably by the impact of an asteroid. The rock wandered the inner solar system until 13,000 years ago, when it fell to Antarctica. It sat on the ice near AllanHills until 1984, when snowmobiling geologists scooped it up.

Scientists headed by David McKay of the JohnsonSpaceCenter in Houston found that the rock, called ALH84001, had a peculiar chemical makeup. It contained a combination of minerals and carbon compounds that on Earth are created by microbes. It also had crystals of magnetic iron oxide, called magnetite, which some bacteria produce. Moreover, McKay presented to the crowd an electron microscope view of the rock showing chains of globules that bore a striking resemblance to chains that some bacteria form on Earth. “We believe that these are indeed microfossils from Mars,” McKay said, adding that the evidence wasn’t “absolute proof” of past Martian life but rather “pointers in that direction.”

Among the last to speak that day was J. William Schopf, a University of California at Los Angeles paleobiologist, who specializes in early Earth fossils. “I’ll show you the oldest evidence of life on this planet,” Schopf said to the audience, and displayed a slide of a 3.465 billion-year-old fossilized chain of microscopic globules that he had found in Australia. “These are demonstrably fossils,” Schopf said, implying that NASA’s Martian pictures were not. He closed by quoting the astronomer Carl Sagan: “Extraordinary claims require extraordinary evidence.”

Despite Schopf’s note of skepticism, the NASA announcement was trumpeted worldwide. “Mars lived, rock shows Meteorite holds evidence of life on another world,” said the New York Times. “Fossil from the red planet may prove that we are not alone,” declared The Independent of London.

Over the past nine years, scientists have taken Sagan’s words very much to heart. They’ve scrutinized the Martian meteorite (which is now on view at the Smithsonian’s National Museum of Natural History), and today few believe that it harbored Martian microbes.

The controversy has prompted scientists to ask how they can know whether some blob, crystal or chemical oddity is a sign of life—even on Earth. Adebate has flared up over some of the oldest evidence for life on Earth, including the fossils that Schopf proudly displayed in 1996. Major questions are at stake in this debate, including how life first evolved on Earth. Some scientists propose that for the first few hundred million years that life existed, it bore little resemblance to life as we know it today.

NASA researchers are taking lessons from the debate about life on Earth to Mars. If all goes as planned, a new generation of rovers will arrive on Mars within the next decade. These missions will incorporate cutting-edge biotechnology designed to detect individual molecules made by Martian organisms, either living or long dead.

The search for life on Mars has become more urgent thanks in part to probes by the two rovers now roaming Mars’ surface and another spaceship that is orbiting the planet. In recent months, they’ve made a series of astonishing discoveries that, once again, tempt scientists to believe that Mars harbors life—or did so in the past. At a February conference in the Netherlands, an audience of Mars experts was surveyed about Martian life. Some 75 percent of the scientists said they thought life once existed there, and of them, 25 percent think that Mars harbors life today.

The search for the fossil remains of primitive single- celled organisms like bacteria took off in 1953, when Stanley Tyler, an economic geologist at the University of Wisconsin, puzzled over some 2.1 billion-year-old rocks he’d gathered in Ontario, Canada. His glassy black rocks known as cherts were loaded with strange, microscopic filaments and hollow balls. Working with Harvard paleobotonist Elso Barghoorn, Tyler proposed that the shapes were actually fossils, left behind by ancient life-forms such as algae. Before Tyler and Barghoorn’s work, few fossils had been found that predated the Cambrian Period, which began about 540 million years ago. Now the two scientists were positing that life was present much earlier in the 4.55 billion-year history of our planet. How much further back it went remained for later scientists to discover.

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