Finding life beyond the Earth would be a major scientific discovery with significant implications for all areas of science and human thought. Yet, only one direct search for extraterrestrial life has ever been conducted.

The NASA Viking spacecraft, which landed on Mars, conducted this search in the summer of 1976. Viking consisted of two twin orbiters and landers, with experimental chambers in the landers to conduct three biology experiments.

Over the past half-century, the measurements made during the Viking biology experiments have been the subject of many discussions, analyses and speculation. Today, scientists are still discussing the results of these experiments in an attempt to answer the age-old question of whether there is life beyond the Earth.

The year 2025 marks 50 years since the two spacecraft launched, three weeks apart. These landers achieved humankind’s first two successful soft landings of operational and functioning spacecraft on the surface of another planet.

I’m an atmospheric scientist who worked on the Viking missions in the 1970s at the NASA Langley Research Center, the laboratory that developed and managed the highly successful Viking missions. The Viking missions’ scientific discoveries painted a new picture of Mars’ atmosphere, surface and planetary history.

Launching and landing the Viking spacecraft

The two Viking spacecraft both consisted of an orbiter and a lander. Viking 1 entered Mars’ orbit on June 19, 1976, and successfully landed on the surface on July 20, 1976, which was also the seventh anniversary of the first human Moon landing. Viking 2 followed, landing on Sept. 3, 1976, at a site farther to the northwest.

Viking wasn’t just looking for life.

These crafts contained equipment to take pictures; map heat energy, wind and weather; study the chemical composition of the surface, dust and atmosphere; and collect and analyze soil samples.

Measurements that Viking took of the atmosphere suggested that Mars used to have a much denser atmosphere but over time lost it. It also observed that the wind picks up tiny dust particles, blowing them into the atmosphere. This process colors the planet’s sky permanently pink.

The Viking landers also discovered that at any location on Mars, the atmosphere’s surface pressure varies seasonally. The planet has frozen north and south poles, like on Earth. At the Martian poles in summer, the frozen carbon dioxide sublimates – transforming from a frozen solid to a gas – and then at the winter pole condenses back into a frozen solid.

That process, unique to Mars, affects the atmospheric pressure by changing how much carbon dioxide is in gas form instead of solid form over the planet’s surface.

Biology experiments

Each of the three Viking biology experiments brought a soil sample from the Martian surface into a sterilized test chamber and exposed the sample to a different nutrient under different atmospheric conditions.

Researchers wanted to find out whether the soil contained microorganisms, so they monitored how the atmosphere in the chamber changed. Metabolic processes – like breathing – from organisms consuming the nutrient would change the chemical composition of the chamber’s atmosphere.

Depending on the experiment, the nutrient contained either carbon, carbon dioxide or carbon monoxide – all of which were radioactive. With radioactive samples, researchers could track the level of radioactivity in the chamber to see if metabolic reactions in the soil samples were raising or lowering it.

For all three experiments, the researchers could use radio commands to heat up the test chamber, which was still inside the Viking spacecraft on Mars. This would destroy any potential microorganisms in the soil and stop the production of any gases they were creating metabolically.

In the first experiment, called the carbon assimilation experiment or the pyrolytic release experiment, the researchers simulated the Martian atmosphere in one of Viking’s test chambers. They filled the chamber with gases such as carbon dioxide and carbon monoxide and made these gases radioactive to see how the atmosphere changed from interactions with the soil sample.

In the second experiment, The labeled release experiment, researchers directly injected the soil sample with a nutrient containing radioactive carbon. They monitored the experimental chamber for radioactive carbon dioxide and measured the level of radioactive carbon dioxide after injecting the soil samples. In this experiment, the investigators saw results that could have come from a biological source.

The third experiment, the gas exchange experiment, filled the chamber with helium, which doesn’t react with anything. They exposed the soil to different types of nutrients. Some had been incubating in wet conditions, others in humid conditions and others still in dry conditions.

Again they monitored the chamber for potential metabolically produced gases. When the soil samples touched the wet nutrient, the humidity immediately caused some changes in the chamber’s chemical environment. Most of these changes were just caused by the water evaporating.

In one case, superoxides in the soil, which are O₂ molecules that have taken on an extra electron, reacted with water. Other changes had to do with oxygen molecules in the soil breaking down. All of these changed the atmosphere in the chamber but likely wouldn’t have been caused by microorganisms.

The researchers repeated this experiment by resetting the chamber’s atmosphere and adding in fresh nutrients, but they didn’t change the soil sample. This time, the soil released only carbon dioxide into the chamber, which likely came from the organic materials in the nutrient they added breaking down.

The results from this third experiment led the researchers to conclude that there likely weren’t microorganisms in the soil. But together, the results from the three experiments weren’t exactly straightforward.

Only the labeled release experiment results suggested a biological source for the observed results. The carbon assimilation experiment and the gas exchange experiment suggested that nonbiological or inorganic chemical reactions caused the observed results.

Lead researchers on the project concluded that there was no unambiguous discovery of life by the Viking landers, but it cannot be completely ruled out.

The molecular analysis experiment

Unlike the biology experiments, which experimented on soil samples, another Viking experiment, the molecular analysis experiment, directly searched the Martian surface for organic matter. Organic materials are carbon compounds bonded with hydrogen, oxygen or nitrogen that come either directly or indirectly from living organisms.

To everyone’s surprise, this experiment did not detect any organic compounds on the surface of Mars. Researchers had known for years that meteorites containing organic materials had hit Mars repeatedly throughout its history, so to find none at all seemed strange.

Some scientists theorized that Martian soil might contain a compound that quickly converts any organic material on the surface to carbon dioxide. A compound like this would have evaporated any evidence before scientific instruments had the chance to find it.

In 2008, decades after this finding, NASA found a compound that may be doing just that. Their Phoenix lander detected high concentrations of a compound called perchlorate in the soil.

When perchlorate is heated – as it was in the Viking molecular analysis experiment – it can chemically destroy organic compounds, and scientists figured it’s the likely culprit behind the strange result from the molecular analysis experiment.

A new model for life on Mars

Scientists are still using the findings from these experiments today. Recently, Steven A. Benner, the director of the Foundation for Applied Molecular Evolution, developed a new model for present-day life on Mars based on the three Viking biology experiments’ measurements.

His model predicts that microorganisms could have used the radioactive carbon nutrient in the experiment chamber to create their own food, releasing radioactive carbon dioxide in the process. It also suggests that at night, microorganisms could be absorbing oxygen and expelling carbon dioxide. That could explain the oxygen released from the Mars soil sample when moistened.

The Benner model suggests that there could be living microorganisms on the surface of Mars, but future research and measurements will need to confirm this very intriguing possibility.

Dr. Joel S. Levine is a consultant and subject matter expert for the NASA Engineering and Safety Center in the areas of space and planetary environments. Dr. Levine worked for NASA from 1970 to 2011 and worked on the Viking Mission to Mars, the subject of this article. Dr. Levine was appointed Mars Scout Program Scientist in the Mars Exploration Program, NASA Headquarters and appointed co-chair of the NASA panel on the Human Exploration of Mars Science Analysis Group (HEM-SAG), planning for the first human mission to Mars.