Could Viking Have Missed Life?

Gil Levin has never wavered. The principal investigator of Viking’s labeled release experiment is now approaching ninety, but he’s very active and stays current with research on Mars. Levin had an unusual career path, starting as a sanitation engineer before join­ing NASA. In addition to authoring 120 scientific papers, he owns fifty patents for items ranging from artificial sweeteners to thera­peutic drugs.46 Levin insists that at two locations on Mars, and in seven out of nine tests, his experiment detected biological activity. No purely chemical reactions have been identified that would fully reproduce the labeled release results, which explains the cautious and equivocal wording of the science mission summary.

Levin is in the minority, but he’s not alone in believing the Vi­king results bear revisiting. Rafael Navarro-Gonzalez and his team went to the highest and driest parts of the world, places like the Atacama Desert and Antarctic dry valleys, and duplicated the tests that Viking did more than thirty-five years ago.47 In northern Chile, they found arid soils with levels of organic material that would have been undetectable by the Viking instruments, yet there were bacteria in the soil. In other words, Viking was not sensitive enough to detect either organics or life in terrestrial locations that are the closest analogs to Mars.48 The team speculated that the organic material on Mars might have been too stable to turn into a gas, even at the blistering temperature of 500°C (932°F) reached inside the oven of the Viking experiment. They also noted that iron in the soil might oxidize the organic material and prevent its detec­tion by the mass spectrometer. This would account for the carbon dioxide that was detected in the experiments.

Mars is an alien chemical and physical environment, so it makes sense to think “outside the box” when considering how biology might operate there. If we only look for life as we know it, that’s all we’ll be able to detect and we may miss life with a different biochemical basis. Dirk Schulze-Makuch and Joup Houtkooper have speculated that Mars might be home to microbes that use water and hydrogen peroxide as the basis for their metabolism.49 Hydrogen peroxide is a toxic household disinfectant and seems implausible as a basis for life, but the researchers note that it’s more life-friendly under the extreme conditions typical of Mars. It attracts water and when mixed with water freezes at -57°C, yet it doesn’t form cell-destroying ice crystals at even lower tem­peratures. Hydrogen peroxide is tolerated by many terrestrial microbes and is the basis of the metabolism for Acetobacter per – oxidans. It’s used as a defensive spray by the bombardier beetle and even performs useful functions in the cells of some mammals. As Schulze-Makuch pointed out in 2007, “We can be absolutely wrong, and there might not be organisms like that at all. But it’s a consistent explanation that would explain the Viking results. . . . If the hypothesis is true, it would mean we killed the Martian mi­crobes during our first extraterrestrial contact, by drowning—due to ignorance.”50

Another speculation was spurred by the discovery of perchlo­rate in a Martian polar region by the Phoenix lander in 2008.51 Phoenix carried out a wet chemical analysis of Martian soil, find­ing it to be alkaline and low in the type of salts found on Earth, but it had enough perchlorate to act as antifreeze and allow the soil to hold liquids for short periods during the summer. Perchlorate is strongly oxidizing and so generally considered to be toxic for life, but some microbes metabolize it.52 In 2010, the Viking results were reinterpreted in the light of the Phoenix discovery, giving support to Gil Levin’s lonely position.53 When perchlorate was added to Atacama Desert soils and analyzed in the manner of the Viking samples, it released chlorine compounds. When these were seen in the Viking experiments back in the 1970s, they were presumed to be cleaning fluid contaminants from Earth. But if perchlorate is present in the mid-latitude Martian soil, it would explain the data. Since perchlorate becomes a strong oxidant when heated, it would have destroyed any organics and so explain why Viking didn’t detect any. It’s remarkable that the Viking legacy continues to provide so many surprises and unanswered questions.

A leitmotiv in the reinterpretation of the Viking biological ex­periments is the amazing range of life on Earth—ihere are mi­crobes that can tolerate or thrive in conditions that would be fatal to plants and animals. Collectively, forms of life found in physi­cally extreme conditions are called extremophiles. The envelope of habitability is much larger than we thought in the 1970s. There are microbes than can live below the freezing point of water and with toxic and alkaline conditions as seen on Mars. Even high doses of ultraviolet radiation are not an impediment to life, since mi­crobes can survive conditions in the upper stratosphere that match the radiation environment of Mars.54 In science, proof is the gold standard, but it’s a very high bar to clear, usually requiring copious amounts of evidence. While Viking didn’t find life on Mars, it was unable to prove the converse hypothesis, that biology is absent. It’s premature to declare Mars dead. Surprisingly, this harsh planet inspired a new way of thinking about life on Earth.