Mars Biology Hindsight: Lessons from a 50-Year-Old Debate

Scientists are once again discussing the results of the Viking mission, which conducted the first biological experiments on the surface of Mars over half a century ago. In a new study, researchers suggest that NASA’s lander might have detected signs of microbial life as early as 1976, but these data were misinterpreted. In July and September of 1976, two Viking series spacecraft transmitted images from Mars’ surface and conducted a series of soil sample experiments for the first time in history. One of them was aimed at detecting metabolic activity: they added water with nutrients and radioactive carbon to the soil. If microorganisms were present in the sample, they would metabolize the nutrients and release radioactive carbon dioxide. In the first experiment, the instruments recorded the release of 14CO2 with a probability of 99.7% compared to the control sample. Furthermore, other tests observed oxygen release and CO2 exchange with the atmosphere. However, subsequent measurements did not yield equally clear results, and scientists could not reach a consensus on the explanation. At the time, most specialists considered that the detected organic compounds were contaminants from Earth. Viking project leader Gerald Soffen concluded, “No organics – no life.” Thus, the mission was officially declared to have found no signs of life.

The authors of the new work believe this conclusion was premature. In a letter to Science magazine, they note that the cumulative data from the Viking mission experiments align with signs of microbial respiration. One of the arguments against a biological interpretation has long been the presence of perchlorates – compounds capable of chemically reacting with organics. It was assumed that they could have caused the gas release. However, later on, the Phoenix probe showed that perchlorates are a natural part of the Martian soil. Laboratory experiments also demonstrated that when heated, these compounds degrade organic substances into chlorinated organics, which change the interpretation of the Viking data.

Researchers also highlight another detail. After the first positive signal, re-adding nutrients did not lead to new gas emissions. Previously, this was considered proof of the reaction’s non-biological nature. However, according to the authors, a possible explanation is that microorganisms could have died due to an excess of the added substances. Similar effects are observed in extremophiles in the Atacama Desert on Earth, where sudden moistening leads to mass microbe death. The new work proposes a hypothesis about the existence of special bacterial autotrophs on Mars, capable of remaining dormant for extended periods. According to the authors, such organisms might accumulate oxygen and carbon dioxide, quickly activate when moisture and heat appear, and start fixing carbon. The researchers named this hypothetical life form BARSOOM – from the name of Mars in Edgar Burroughs’ novel. They emphasize that this is not about proving the existence of Martian microbes, but an attempt to rethink old data.

Mars exploration has seen significant advancements since the Viking mission. Current and upcoming missions are equipped with enhanced scientific instruments capable of conducting more sophisticated analyses. For instance, NASA’s Perseverance rover, which landed on Mars in 2021, uses a suite of tools designed to look for signs of past life and collect samples for future return to Earth.

As scientists prepare for manned missions to Mars and the potential return of soil samples to Earth, the conversation about the Viking mission’s findings has gained renewed importance. Researchers believe that future Mars studies should consider the likelihood of indigenous biology and the risk of accidentally transferring it to our planet.