NASA spends billions on Mars missions that aim to find signs of life on the Red Planet, but one scientist thinks evidence of alien life on Mars has already been found more than 40 years ago.

NASA's Viking Program

In an opinion piece for Scientific American, former NASA scientist Gilbert Levin wrote that humans may have already found proof of the existence of alien life on Mars during an experiment that he led for NASA's Viking mission in 1976.

The Viking program sent two space probes to Mars to conduct experiments and capture photos of the Martian surface.

LR Life Detection Experiment

Levin claimed that the mission detected positive results in the Labeled Release (LR) life detection experiment that he led.

The test involved mixing a sample of Martian soil and a nitrogen-based nutrient solution label with a unique radioactive compound.

The idea is that if there are microorganisms in the soil, they would metabolize the nutrients and cause the production of radioactive carbon dioxide gas or radioactive methane.

Levin recalled that the initial results of the experiment were positive for microorganisms. He said that thousands of reliable tests using Earth-based soil and microbial cultures that were conducted at the time also supported the results.

He likewise pointed out evidence that was obtained after NASA's Viking mission had supported his conclusion. These include the evidence of surface water, ammonia, methane and worm-like features that appear in images taken by the Curiosity rover.

Review Of LR Data

Unfortunately, NASA discarded the results of the LR experiment because the Viking Molecular Analysis Experiment failed to detect organic matter. The U.S space agency concluded that LR detected only something that was imitating life, but not life.

Levin cited how subsequent NASA Mars landers failed to bring life detection instrument to follow up on the results of the decades-old test. He now wants an independent group of researchers to review the Viking LR data.

"A panel of expert scientists should review all pertinent data of the Viking LR together with other and more recent evidence concerning life on Mars," Levin wrote. "Such an objective jury might conclude, as I did, that the Viking LR did find life.