In 1976, NASA’s twin Viking landers arrived on Mars, equipped with four experiments designed to offer foolproof evidence of life on the Red Planet. They were looking for biosignatures, or fingerprints of life. As they took their first scoops of Martian soil, the whole world held its breath.
The first experiment incinerated a sample of soil and analyzed the resulting hot gas for organic carbon, but none was detected. The second and fourth sprinkled nutrients and then carbon on two more soil samples, hoping to incite a feeding frenzy in any dormant Martian microbes, but the results were the same as with controls.
But it was the third—the labeled release experiment—that found something in the sere red dirt would absorb a tracer of radioactive carbon and send it up in a steady plume of carbon dioxide, just as a living, respiring cell would. There was life on Mars, or so it would seem.
As the media reported Viking’s success, the stunned researchers examined their data. Crucially, they could not replicate the initial results of the labeled release experiment. Moreover, the test had been designed to detect the respiration of carbon-based life, but damningly, the first experiment suggested Martian soil was devoid of organic carbon, and that anything living there was, at minimum, not very hungry. The frustrated scientists were forced to announce that the tests were inconclusive; life had not been found on Mars after all.
In 2008, NASA’s Phoenix lander touched down near the northern Martian polar ice cap and found traces of perchlorate on the surface. When exposed to carbon, the highly reactive molecule can form carbon dioxide gas—similar to what Viking’s third experiment detected. At long last, the mystery appeared to be solved.
But for astrobiologists, who study the possibility of life beyond Earth, Viking was only the first glimpse of a problem that now haunts them: If we land on an uncharted terrestrial planet today, how can we know with certainty if we find life? What if we fail to recognize it altogether?
Where could life live?
The entire Viking program, and the billion dollars spent on it, had been dedicated to answering once and for all the question of life on Mars. “Though Viking’s ambiguous results were quite an embarrassment for some NASA scentists, they also taught astrobiology a number of lessons,” says Steven Benner of the Westheimer Institute of Science and Technology.
One of the mission’s major errors was failing to fully consider the nature of the Martian environment; the freeze-dried, radiation-bathed surface was quite hostile to the kinds of life the experiments were designed to detect. After Viking, NASA stepped back and crafted a policy that still holds today: search for habitability—how comfortable an environment would be to Earth organisms—before actually looking for life.
The primary tenet of that policy is “follow the water,” which holds that wherever liquid water might be—or once have been—on other planets, life may have arisen. This is a reasonable assumption since life on Earth began in our planet’s oceans, and no known organism on Earth can survive indefinitely without water. Today, Mars’ atmospheric pressure is so low that liquid water cannot exist on the surface, but multiple signs point to abundant surface water in the remote past, and liquid water may still persist beneath the ground.
Thus, when investigating landing sites for rovers on Mars, planetary scientists look for remnants of ancient lakes and rivers where traces of life, past or present, might be seen. Astrobiological interest in other planets and moons beyond Mars, such as Europa and Enceladus, is also based on the belief that these places harbor or have harbored liquid water.
Habitability, however, is not actually a biosignature—an unmistakable signal that can only be explained by the presence of life. And as far as aiding the search for living planets, Ariel Anbar, a biogeochemist at Arizona State, says “follow the water” is, on its own, too broad to be a successful strategy. “There are lots of current and past water-rich environments on Mars and other planets in the solar system,” he says. “We need to start bringing in additional criteria for guiding the search for life.”
What would life use?
When he saw the results from the Viking experiments, the mission’s lead scientist Gerald Soffen said, “That’s the ball game. No organics on Mars. No life on Mars.” Soffen’s words summed up the consensus at the time: All life is definitively based on carbon.
Since then, though, the consensus has slightly shifted: Earth’s life is based on carbon, but alternate biochemistries are an intriguing possibility. Somewhere out there, there might be forms of life whose bodies are spun of silicon atoms instead of carbon or who breathe only methane.
In a landmark 2007 report, the National Research Council outlined how scientists could explore these possibilities and how NASA could incorporate them into its search. Among many other questions, The Limits of Organic Life in Planetary Systems asks how a genetic system might be built without DNA, whether Darwinian evolution can occur with non-biological polymers, and whether solvents other than water could harbor the kind of chemical processes believed to have led to life on Earth. Astrobiologists are now exploring these cutting-edge questions in conjunction with synthetic biologists.
But there is much that is still unknown about the possibility of even Earth-like life on other planets. Scientists planning the payloads of exploration vehicles consequently focus on the biosignatures of familiar life.
Earth’s ancient past is as distant and strange as the surface of Mars, but far more accessible. And certain arctic or desert environments on Earth resemble the barrens of other planets. Thus, scientists like Anbar, a biogeochemist who heads a NASA Astrobiology Institute grant called “Follow the Elements,” are studying past and present life on Earth to better understand how to detect alien organisms.
Anbar’s team seeks to understand the chemical requirements for life, especially in the case of extremophiles thriving in unusual niches. They also search the geologic record for biosignatures, looking for deposits of organic material and considering how past organisms might have altered their environment. Other teams examine the ratios of isotopes within the rock—in the case of elements like carbon, life preferentially uses certain isotopes over others, which means measuring relative levels of isotopes can indicate life’s past presence. Still other groups, like the Carnegie Institute of Washington’s Arctic Mars Analog Svalbard Expedition, which tests equipment to be used on the surface of Mars, travel to harsh, remote locations to see what life has managed to survive there and how it can be detected. Calibrating Mars missions’ instruments with real living organisms also helps ensure that future tests for life will be properly sensitive, says Andrew Steele, who is the principal investigator of AMASE.
Even the atmospheres of planets can give insight into what life might exist on the surface, says Victoria Meadows, the head of NASA’s Virtual Planet Laboratory, which simulates planet formation and evolution in order to better inform the long-distance detection of habitable worlds. “Methane on Mars, which was detected remotely [earlier this year], is maybe one of the most promising signs of life on the planet we’ve found so far,” she says, noting that organisms that excrete gas can drastically influence the composition of a planet’s atmosphere. Subterranean communities of microbes could perhaps be producing the Martian methane.
Unfortunately, as promising as chemistry is for seeking signs of life in a planet’s soil and sky, there’s a catch. As NASA learned the hard way with the Viking labeled release experiment, abiotic processes can also cause phantom biosignatures. The methane in Mars’ atmosphere, for instance, might come from geological processes rather than methane-producing bacteria, Meadows says. Several biomolecules, such as the amino acids that form proteins, are perfectly capable of being made without life. A major area of astrobiology is devoted to just crossing these potential biosignatures off the list.
In 1996, a Martian meteorite found in Antarctica’s Allan Hills caused a global uproar when scientists found it contained what they thought were the remains of ancient life. The meteorite was laced with organic material and studded with crystals that resembled minuscule fossils. It is now clear, however, that the crystals and some of the organic material could be made abiotically, Steele, who has studied the meteorite extensively, says. What’s more, the rest of the organics are likely contamination from Earth, where, unlike anywhere else in the solar system, there is no shortage of life.
Building a holistic view of life
When it comes to deciding whether a planet is home to life, no single approach is foolproof. The one thing that every astrobiologist comes back to is that in the quest to find extraterrestrials, context is everything. Habitability doesn’t guarantee life; neither does the presence of the correct elements; nor do organics in the fossil record or crystals similar to those produced by life. “The ‘smoking gun’ would be if you found an old hubcap: Something so complicated that only life could have produced it. Otherwise, everything depends on context,” Anbar says. Even three decades after Viking, this is still the case.
On Earth, we have context aplenty. “When questions of life on Earth are settled, they are settled with a large amount of information, of interlocking situational cues,” Benner says. By studying life’s origins and limitations here on Earth, astrobiologists will probably get closer to being able to recognize it elsewhere. Scientists’ fantasies of alternate biochemistries may even be satisfied without visiting other worlds, as some researchers are actively searching for life on Earth that uses strange combinations of elements to produce bizarre biosignatures. Benner speculates that maybe someday we’ll have enough context elsewhere in solar system to be able to help detect life. While we wait, there are deserts and depths aplenty here on Earth.
Originally published November 9, 2009