For those seeking life on Mars, it is the best of times and the worst of times. Nearly 35 years after NASA’s twin Viking robots eased down onto its ruddy surface, there is still no incontrovertible evidence that living organisms ever existed on the fourth planet from the sun. Few researchers accept one scientist’s claims that the 1976 Viking experiment detected life. The brief frenzy over possible fossils in a Mars meteorite has fizzled. And even after billions of dollars’ worth of adorable rovers and eagle-eyed orbiters have prodded and probed the planet, the results have been at best ambiguous and at times downright confusing.
Yet a growing number of space scientists are upbeat, even buoyant, about the likelihood that Mars is a living world. “A variety of discoveries are creating a kind of buzz,” says Chris McKay, an astrogeophysicist at NASA Ames Research Center in Mountain View, California. “And people seem more enthusiastic. It’s group psychology.” There has been no single major breakthrough in the search, but a subtle change is taking place within the clubby community dedicated to finding and bringing back organisms—dead or alive—from the Red Planet.
“It is not now considered a stupid idea to look for life on Mars,” says Bruce Jakosky, a planetary geologist at the University of Colorado at Boulder. “In recent years the case has been made again and again that life is or was possible there.” Undergirding this new optimism are reams of data—from Earth-based telescopes as well as Mars orbiters, landers, and rovers—that have slowly painted a much more complete and complicated picture of the Mars environment stretching back billions of years, providing intriguing hints that microbes might have once evolved there, and might yet endure.
For younger researchers who were children when Viking landed, it is hard to conceive of a solar system where Earth is the only life-bearing place. They take it for granted that organisms can endure extreme environments. Weird and wonderful forms of life have been found deep within the Earth’s crust, swimming in boiling pools, and clinging to vents deep under the ocean surface. That versatility heartens those looking beyond our own planet. “If it smells like life and looks like life, then it could be life,” says Dirk Schulze-Makuch, a 46-year-old astrobiologist at Washington State University in Pullman. “There’s a strong sense that we should get missions going to nail this down. I’d be surprised if Mars were sterile.”
That assessment would have raised eyebrows 10 years ago, but it is no longer outside the mainstream. Even William Schopf senses a shift in attitude. The UCLA paleobiologist was the house skeptic at NASA’s 1996 press conference introducing the Mars meteorite and its alleged fossils of microbes. While he remains skeptical, Schopf believes that if biology ever took hold on Mars, it is probably still there. “If we’ve learned one thing in recent years,” he says, “it is that life is resilient.”
No one knows that better than John Baross, an astrobiologist at the University of Washington in Seattle. A very resilient life form nearly killed him. When the bacteria attacked, his body turned as red as a fireplug as his temperature climbed to 104 degrees Fahrenheit. In eight days he lost 38 pounds as the invader released a toxin that ate away at the muscles in his legs and back. While doctors frantically pumped his bloodstream full of antibiotics, the organism hid behind a thin protective skin of sugary slime that made it impervious to the medicine.
Baross at the time happened to be studying just such slime, known as a biofilm. His graduate students sent the hospital extensive information on the bacterium and its protective cloak to share with the puzzled physicians. “We knew more about it than they did,” Baross recalls wryly. Finally, an exhaustive battery of tests pinpointed the invader’s primary location: the liver. Doctors were then able to tailor the meds to overwhelm it. Even so, it took Baross seven painful months to recover.
Baross’s personal encounter with the bacteria deepened his belief that such biofilms might extend from the bottom of the ocean into interplanetary space. Two thousand feet below the sea, in the cracks of the Mid-Atlantic Ridge, he and his students recently discovered single-celled organisms flourishing in highly alkaline water close to the boiling point. The gooey film encasing these organisms is the key to their survival.
Such sticky mucilage is among the oldest of known organisms, dating back more than 3 billion years. And Baross’s lab work shows that the mid-Atlantic biofilms have an astonishing capacity for transferring genes. That facility may have been just what early life needed to give rise to the widely varied genomes that walk, swim, and fly on Earth today.
Other researchers are busy scouring our planet to test the limits to organic life. They have found Cephalosporium (fungi that live in highly acidic environments), Euglena and Chlorella (algae that grow in heavy metals), and a cockroach that can survive massive doses of gamma radiation. Some archaea—a domain of microbial life that was little understood when the Viking landers reached their destination—live in even more extreme situations, flourishing in temperatures far above the boiling point of water and surviving in thick brine.
The many extreme-life discoveries led NASA to ask the National Academy of Sciences for help in knowing what to search for beyond our planet. Baross chaired the investigating committee. The group reported that carbon-based life dependent on liquid water and using DNA “is not the only way to create phenomena that would be recognized as life.” Quickly dubbed the “weird life report,” the study dramatically concluded that many locales in the solar system could support life drawing on a variety of liquids and energy sources. “I think life existed on Mars,” Baross says. And if it did, he—like Schopf—thinks that it, or evidence of it, is probably still there.
Life’s resilience and the sheer diversity of terrestrial organisms were not obvious on July 20, 1976, when the first Viking lander touched down on Mars’s Chryse Planitia lowlands, programmed to find life as we then knew it. At the time, that meant looking for water, warmth, and the right nutrients for delicate organisms. Scientists didn’t dream that life could flourish in brine pockets of sea ice or in mine water filled with heavy metals. “In hindsight, what we did with Viking was incredibly naive,” Jakosky says. “We have since learned that life can be exceedingly difficult to detect.”
And yet one man insists that the Viking search yielded a positive result. “In my mind the question is resolved,” says Gilbert Levin, the leader of one of the Viking experiments, the Labeled Release Life Detection Experiment. Ever since the data came in from Viking, he has argued that his tests gave evidence consistent with life on Mars; now, after further analysis, he believes they prove its existence. His colleagues have responded with doubt, even derision, over the years. But growing knowledge about extremophiles on Earth and the environment on Mars has given Levin, who just turned 86, hope that his assertion will finally be taken seriously.
Levin’s recipe for smoking out Martian life was elegantly simple: Scoop up Martian dirt with the Viking arm, seal it in a chamber, add an organic compound with a trace of radioactive carbon, and wait. Any bacteria similar to those on Earth would exhale radioactive gas. Next, take a second sample and subject it to high heat to kill off any microorganisms, then add more radioactive compounds to the chamber. If there is no subsequent radioactive release, that demonstrates that there are living microbes on Mars.
Indeed, the chambers on both Viking landers signaled a radioactive reading in the first round followed by none in the second. Levin celebrated with champagne and cigars. But within days that finding was contradicted by results from the gas chromatograph spectrometer, which detected no sign of organic compounds, much less evidence of life. And without organic compounds—molecular combinations of carbon and hydrogen—life as we know it is not imaginable.
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.”