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.”