The day Gil Levin says he detected life on Mars, he was waiting in his lab at NASA's Jet Propulsion Laboratory, watching a piece of paper inch out of a printer.
Levin snatched the sheet and scrutinized the freshly inked graph. A thin line measuring radioactive carbon crept steadily upward, just as it always did when Levin performed the test with microbes on Earth. But this data came from tens of millions of miles away, where NASA's Viking lander was — for the first time in history — conducting an experiment on the surface of Mars.
"Gil, that's life," his co-investigator, Patricia Straat, exclaimed when she saw the first results come in. There was jubilation at JPL. Afterward, Levin said, he drove into the mountains above Los Angeles, sat on the ground and stared up at the night sky.
"I was sort of trembling, you know?" he recalled. It was July 30, 1976.
Forty years later, Levin and Straat still believe that their experiment was evidence of microbiotic Martians. But few people agree with them. To NASA, and to most scientists, the 1976 Viking mission was a technical triumph but a biological bust. Scientists, such as Carl Sagan, who had wagered that large organisms "are not only possible on Mars; they may be favored," were disappointed to see images the lander sent back of a dry, barren planet. Two experiments aimed at finding life turned up negative, and NASA concluded that the results of Levin's test, called the Labeled Release experiment, could be explained by chemical processes rather than biological ones.
“I was sort of set aback,” recalled NASA chief astrobiologist Penny Boston, who was still in college at the time. “I was thinking, ‘Gosh, I want to work in exobiology, as we called it at the time, and now it seems like it’s just a pile of rocks, and there’s no life there at all.'”
Viking put a 20-year damper on Mars exploration. Even when NASA did return to the Red Planet, it completely quit trying to test for living organisms directly.
But hope was in the air at Langley Research Center last week, where NASA held a two-day conference to honor the 40th anniversary of the Viking landing. After decades of pointedly not looking for it, the space agency is more optimistic than it's been since 1976 that it might find life on Mars yet.
"Every new piece of information we get about the planet seems to point to greater and greater habitability," Boston said. "It just seems more and more likely."
The issue with the Viking experiments is that they expected to find too much too soon, speaker after speaker explained over the course of the conference. Detecting life with Viking would have been a breakthrough of unprecedented proportions, and science doesn't usually happen that way. Most "breakthroughs" come after years of accumulating incremental increases in knowledge.
So, for the past four decades, "we’ve engaged in creeping up on the problem," Boston said.
Some evidence in favor of a livable Mars came from the same mission that seemed to quash the possibility: Viking itself. While the two landers relayed bleak photos and disappointing data from the surface, the orbiters that were launched along with them revealed landscapes that looked strikingly like ones on our own planet.
Ellen Stofan, now NASA's chief scientist, was then a summer intern at JPL assigned to map Mangala Valles, a system of crisscrossing channels near Mars's equator.
"What was so fascinating were all these features that were so familiar from our studies of the Earth," she recalled. "Things like teardrop-shaped islands, abandoned oxbow sections of channels, features that by looking at rivers on Earth we could understand that these features on Mars had been carved by water, and in some cases by great floods of water, coursing across the Martian surface."
Images from the Viking orbiters confirmed what the Mariner 9 satellite found when it arrived at the planet five years earlier: Mars once had water, a key ingredient for the evolution of life as we know it. But that water existed hundreds of millions, perhaps even billions, of years ago, offering little promise that organisms might still exist.
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