Scientists assumed that a nonbiological reaction had caused Levin’s instruments to register life, and the lack of organics seemed to seal the deal. NASA lost interest in what seemed a quixotic quest, and Mars exploration was abandoned. “And from then until now, the gospel is that Mars doesn’t have any organics in its soil,” McKay says. The irony is that we now know such compounds litter the solar system, present everywhere from Saturn’s moon Titan to common meteorites; organics are probably present even on Mars’s little moons, Phobos and Deimos.

In 2008 the NASA Phoenix lander added fuel—literally—to the debate. The robot detected perchlorates, charged particles consisting of a single chlorine atom surrounded by four oxygen atoms, in the arctic soil taken from near the planet’s north pole. That molecule helped Phoenix get to Mars in the first place, since perchlorate is a powerful rocket fuel. Some researchers took the presence of perchlorate as another sign that life on Mars’s surface is unlikely, since the compound is a powerful oxidizer, acting like a bleach at high temperatures.

But McKay believes the find is an exciting hint of life’s presence. “This is the most important discovery since Viking,” he contends. “This made our whole world change.” In his reading, the Viking gas chromatograph scooped up soil, heated it, and in so doing activated the perchlorate, which then destroyed the very organics the spacecraft was searching for. Only a single type of molecule, which could have been produced by the perchlorate reacting with organics, appeared in each sample. “The results were misinterpreted,” McKay says. “And our whole community is in denial.”

Schulze-Makuch agrees, saying the Viking gas chromatograph lacked sensitivity; it also failed to register life when a version of the device on Earth was fed a sample from the Dry Valleys of Antarctica, a seemingly barren place that actually hosts some microorganisms. He adds that perchlorates could serve as a potent energy source as well as a way for life to access water. In the Atacama Desert of Chile, one of the world’s most desiccated spots, perchlorate in the soil can condense water out of the atmosphere. Intriguingly, small droplets—which appeared to be water—were spotted in photos on the legs of the Phoenix lander.

So while some researchers see the perchlorate find as another sign of life’s unlikelihood on Mars, Schulze-Makuch is positively elated. “From an extremophile perspective, this story is a plus for life,” he says. For example, instead of using a water-based substance as a basis for its cellular processes, a Martian organism might use hydrogen peroxide, a molecule similar to perchlorate that is abundant on Mars. The debate may finally be resolved when NASA’s Mars Science Laboratory arrives in the autumn of 2012. “Our great hope is that we will find organics either in the soil or inside rocks” drilled by the robot, McKay says.

The renewed possibility of organics on Mars has led researchers to reconsider the conclusions of Levin. “The community is not convinced he’s right, but he’s a first-rate scientist, and you can’t just dismiss what he says,” Jakosky notes. At UCLA, Schopf argues that laboratory tests long ago showed how nonbiological soil chemistry could have produced the results seen on Viking. But he admits that the mainstream view could prove false. “Levin isn’t necessarily wrong,” he says. “There are lots of examples of people who stick to their guns and are proved right—think of Alfred Wegener and continental drift.”  

Like Levin, the team that claims it found evidence of life in a Mars rock dubbed Allan Hills 84001 remains unbowed in the face of widespread scientific skepticism. That rock was blasted off the surface of the Red Planet millions of years ago and fell to Earth as a meteorite, landing in Antarctica. At its famous 1996 press conference, a group from NASA Johnson Space Center led by David McKay—no relation to Chris—laid out four lines of chemical and physical evidence that they believed made a strong case for life on Mars.

After years of further analysis and debate, many astrobiologists think that three of those four can be shown to be the result of nonliving chemical or geological activity. The fourth line is tenuous but more intriguing. The meteorite is full of grains of an iron oxide mineral called magnetite, each grain a mere 20 to 120 nanometers across. On Earth, organisms called magnetotactic bacteria routinely manufacture such tiny crystals. But in 2003 two papers noted that there are other ways to make magnetite crystals, such as slamming a rock from space onto the Martian surface. The intense heat could degrade carbonates and form similar structures.

In November 2009 the McKay team struck back with a new paper. Using advanced microscopy, they noted that the crystals in the Allan Hills sample are too pure to be explained by a thermal event. “We do not believe it is too incautious to restate our original hypothesis that such magnetites constitute strong evidence of early life on Mars,” said lead author Kathie Thomas-Keprta at the time. A member of the original Allan Hills team, she insists that there is no longer an alternative to the existence of life in the formation of the crystals. “We’re left with only one hypothesis standing,” she says. Her next step is to find corroborating evidence in other meteorites.

Baross argues that if Earth-like life-forms created these crystals, they would have to be highly sophisticated organisms. Given the harsh environment, he finds it unlikely that any Mars life evolved far beyond simple biofilms. Other researchers say that any number of chemical processes might be responsible. “It’s almost a fool’s errand,” Jakosky says. “You could spend the rest of your life altering formulas to show what can or cannot make magnetite.”

Thomas-Keprta believes the data and the momentum are on her side. “As a group, scientists now consider Mars to have once been habitable, and it may still be so,” she says. “We’re a long way from the dry and desiccated deserts of the past. Mars could have supported a biosphere.”

Some researchers believe the mere presence on Earth of a meteorite from its distant neighbor underscores the possibility that life has been transferred from planet to planet.

Perhaps the most intriguing sign of life since the Allan Hills announcement was the detection of methane in the thin Martian atmosphere. The discovery by a number of research teams did not grab headlines around the world as the Mars rock did, but it may prove more important in the quest to find life. On Earth, methane is emitted by two sources: living creatures, such as cows, and geological formations, such as mud volcanoes. So the methane on Mars may be the result of geological activity or life—or both.

Last year, using ground-based telescopes, a team led by Michael Mumma of NASA’s Goddard Space Flight Center reported that Mars’s methane is concentrated in vast plumes. The team charted the spread of several plumes across the northern hemisphere as summer approached and noted that they seemed to originate above volcanic regions. One plume expanded to include more than 20,000 tons of methane, comparable to the output of the Santa Barbara seep, one of the largest geological methane sources on Earth.

There is no doubt Mars once rumbled and spewed. The planet is home to the solar system’s largest volcano, Olympus Mons, which is as tall as three Mount Everests. But geologists have found little evidence of activity in recent eons; the newest volcanic deposits date back millions of years. Besides, volcanoes typically give off not just methane but a host of other gases, and those have not been detected.

Mumma says he stands by his data. But some scientists—Chris McKay, for example—see it as highly unlikely that the Red Planet is active enough to produce methane and believe there is no explanation for its high rate of dissipation in the atmosphere. “I’m forced to conclude that the methane data are probably wrong,” McKay says.

Baross, however, is excited by the possibility that the methane results from a chemical process called serpentinization, which can provide a rich environment for life. “It’s a driving force for recycling nutrients in the Earth system,” he says. Cold water reacts with oxidants to crack rock, producing heat and a host of mineral compounds. “It’s a really dynamic process, and if it is going on on Mars, then you may be circulating a lot of liquid water through rock.” That is a purely chemical process, but it is the same one that led to the growth of slimy organisms along the Mid-Atlantic Ridge. Back on Mars, NASA’s Spirit and Opportunity rovers have found evidence of minerals associated with serpentinization.

The topsoil of Mars is probably dead, due to the intense radiation and extreme temperatures (–195° to 70°F), but the prospects for life look much better below the surface. Baross wants a mission that drills below the places Mumma has pinpointed as methane sources, and that goes far deeper than the drilling planned for future missions. He envisions a sophisticated robot that could do the equivalent of deep-sea drilling, boring down hundreds of meters. Only then, he believes, can scientists answer the heady question of life on Mars.

For most life seekers, the ultimate goal is getting a few pounds of Martian rock back to a lab on Earth. “What we need is a sample return,” Jakosky says. NASA and ESA currently envision a joint mission to bring back the Martian goods around the middle of the next decade. But the cost—possibly $8 billion—makes that a tough sell in the current economic climate. Baross is also wary of grabbing a few rocks and repeating Viking’s legacy of ambiguous, yet-to-be-understood results.

In the meantime, Jakosky’s Mars Atmosphere and Volatile Evolution probe is slated to arrive in 2013, two years after the Mars Science Laboratory. If all goes as planned, joint European-U.S. projects will yield an orbiter ready for launch in 2016 and two rovers in 2018. None of these probes is likely to find the direct evidence of life that will settle the debate, though. “My guess is that skepticism will remain in the science community almost regardless of what is found,” Schopf predicts. “That’s not to say the missions won’t lead us in the right direction.”

A slow, steady pace is just what is needed, NASA managers insist. “The key is to do careful, long-range homework,” says Michael Meyer, lead scientist of NASA’s Mars Exploration Program. Gathering more detailed data on surface chemistry, the history of liquid water, climate cycles, and the exact constituents of the atmosphere are critical to building a case for—or against—life.

That approach may seem conservative to some, but it takes into account the hard-learned lessons of Viking. And it will provide the foundation for the extraordinary evidence required to support the most extraordinary of claims. “This could lead to the most stupendous discovery in the history of human existence,” Schopf says, speaking with the practiced patience of a scientist. “Little steps add up.”