NASA

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.

In the eastern Hellas region of Mars, the soil conceals
buried glaciers.

NASA

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.