During the 1970s, NASA sent the two Viking landers to Mars. The mission provided incredible insights into the Red Planet and also delivered some surprising results. One experiment gave the first possible hint that we'd detected traces of life on Mars. Back then the evidence was found inconclusive, but one of the experimenters, Dr Gilbert Levin, says that he’s convinced that we did find alien life back then. His ideas are published in Scientific American.

The experiment, called Labeled Release (LR), had a moderately simple setup. A sample of Martian soil was given a drop of diluted nutrients tagged with a radioactive carbon isotope. If lifeforms were emitting carbon dioxide on Mars, they would release the radioactive tag and the experiment would detect it.

Both Viking landers conducted the experiment. One collected a sunlight-exposed sample, the other collected a sample from under a rock. Both experiments reported a detection. The experiment was then repeated after a week using the same sample, but this time, nothing was detected. In 1976, Levin and his partner in the experiment Dr Patricia Ann Straat deemed the results inconclusive.

Given that the Viking Molecular Analysis Experiment failed to detect organic matter, NASA concluded that whatever caused the LR detection was a chemical reaction mimicking life. Over the last few years, Levin and Straat have actually reconsidered the findings, arguing that this was our first detection of alien life, and noting that this and many other findings from the last 43 years make life on Mars a strong possibility.

“What is the evidence against the possibility of life on Mars? The astonishing fact is that there is none," Levin writes in Scientific American. "Furthermore, laboratory studies have shown that some terrestrial microorganisms could survive and grow on Mars.” 

While we cannot take a lack of contrary evidence as proof that there is indeed life on Mars, Levin is right in saying that the possibility of it cannot be discounted. Levin also argues that NASA should conduct an LR-type experiment again. The next Mars mission hasn’t got one on board, but maybe the next one should.

“In keeping with well-established scientific protocol, I believe an effort should be made to put life detection experiments on the next Mars mission possible,” Levin writes. “I and my co-experimenter have formally and informally proposed that the LR experiment, amended with an ability to detect chiral metabolism, be sent to Mars to confirm the existence of life: non-biological chemical reactions do not distinguish between 'left-handed' and 'right-handed' organic molecules, but all living things do.”

The existence of life on Mars is a subject of study, speculation, and often science fiction, which has been debated for at least 140 years. We are yet to have certainty about the existence of life on the Red Planet, but maybe some proof has already been discovered.