Gilbert V. Levin, whose experiment on NASA’s Viking mission in the 1970s seemed to suggest that there might be life in the soil of Mars, died on July 26 at a hospital in Bethesda, Md. He was 97.

The cause was an aortic dissection, a tear in the blood vessel leading from the heart, his son Henry said.

Most planetary scientists dismissed Dr. Levin’s conclusions. But he remained steadfast that his experiment had operated as designed, and that no one could offer a plausible alternative explanation. Over the years, scientific opinion has become more receptive to the possibility that life could have arisen on Mars and may even persist there today.

Half a century ago, the notion of sequencing the genomes of living organisms was a fanciful impossibility, so Dr. Levin and another investigator, Patricia A. Straat, proposed a clever way to test whether the soil of Mars contained any microbes. NASA agreed to fly their apparatus on the two Viking spacecraft, which were launched in 1975 and which both successfully landed on Mars the next year.

In their experiment, food for microbes was added to samples of Martian soil, and the organic molecules in the food contained unstable carbon-14 atoms. The idea was that if microbes digested the food, they would produce easily detectable radioactive carbon dioxide that would rise out of the soil. Dr. Levin called it the labeled release experiment, because the carbon dioxide had been labeled by the carbon-14 atoms.

That is exactly what happened at both Viking landing sites, 4,000 miles apart.

Other samples were then heated to 320 degrees Fahrenheit to sterilize them. If microbes were generating the radioactive gases, there should have been no gas rising from the sterilized soil.

That, too, is exactly what happened.

If a nonbiological process were at play, the radioactive carbon dioxide should have been seen after the sterilization as well.

“The data curves signaled the detection of microbial respiration on the red planet,” Dr. Levin wrote in Scientific American in 2019. “It seemed we had answered that ultimate question.”

But after an initial flurry of excitement, the consensus among scientists was that Mars was lifeless, and that therefore there had to be some other explanation for Dr. Levin’s results.

The images of the Martian landscape were desolate. More crucially, another experiment measured a complete lack of organic molecules, the carbon-based building blocks of life.

Over the decades, the pendulum has swung the other way; it is now believed that Mars was once warm and wet and potentially habitable. Some scientists think microbial life could still be hanging on, although most likely deep underground, where it would be protected from frigid temperatures and barrages of radiation.

In addition, a discovery by a later NASA mission to Mars offered possible evidence that Dr. Levin could have been right after all. In 2008, NASA’s Phoenix lander found chemicals known as perchlorates in the Martian soil. Viking’s organic molecule detector had heated the soil to release organics. But heating organic molecules in the presence of perchlorates destroys them, so even if they were there, Viking’s experiment might have missed them.

“I think he was ahead of his time,” said Rita R. Colwell, a former director of the National Science Foundation who also served on the board of Spherix, the company Dr. Levin founded in 1967.

Dr. Levin wanted to send a more sophisticated version of his experiment to Mars, which would have provided a conclusive answer. For the follow-up, the experiment would run twice: once with the radioactive food provided in what is called left-handed molecules, and once with right-handed versions, which would be mirror images of the left-handed ones.

On Earth, life uses just left-handed molecules, so presumably Mars microbes would also just eat one type of the food, generating the radioactive carbon dioxide, and leave the other untouched, not producing any gas. A nonbiological process would produce radioactive gas with both right-handed and left-handed molecules.

But NASA officials balked at the proposal. They “won’t fly it,” Dr. Levin complained in an interview with The New York Times in 2011. He added: “Changing a paradigm is a tough thing. We’ve run this experiment thousands of times on Earth. It’s never given a false positive. It’s never given a false negative.”

Dr. Levin’s career ventured far from Mars as well.

At Spherix, he developed an inexpensive process to make a sweetener that tastes like sugar and is almost as sweet, but that is mostly devoid of calories. The sweetener, tagatose, is in fact a sugar that occurs naturally, in minute quantities, in milk and beets. Clinical studies indicate that it even works as a drug to treat adult diabetes. But Spherix was never able to get it manufactured in quantity at a viable cost, and efforts to have it approved as a diabetes drug have also foundered.

Dr. Levin spent much of his life trying to find someone to manufacture and sell tagatose, but he had no more success with that than he did with his efforts to convince people of life on Mars. Tagatose was briefly an ingredient in Diet Pepsi Slurpees at 7-11 stores, but it never found much of a market.

Gilbert Victor Levin was born on April 23, 1924, in Baltimore to Henry and Lilian (Richman) Levin. His father was an immigrant from Lithuania who owned an antique store. Gilbert Levin attended Johns Hopkins University, but World War II interrupted his college education, and he served three years as a radio operator in the merchant marine before returning to Johns Hopkins in 1946.

He completed his bachelor’s degree in civil engineering in 1947 and a master’s degree in sanitary engineering in 1948. He then worked as a public health engineer in Maryland, California and Washington, D.C.

The origins of the labeled release experiment date to Dr. Levin’s public health work, he recalled in an article published in Johns Hopkins magazine in 2002. In the 1950s, he was trying to think of a better way of detecting microorganisms — for instance, the presence of disease-causing bacteria in drinking water. The standard procedure was to immerse a sample into a broth of nutrients. Microbes would eat the nutrients and produce bubbles of carbon dioxide. Dr. Levin thought that if the nutrients contained carbon-14 atoms, a Geiger counter could detect the radioactivity from the decay of the unstable carbon dioxide molecules long before the bubbles appeared.

The Atomic Energy Commission provided enough financing to allow Dr. Levin to show that the method worked. He co-founded an environmental consulting firm, Resources Research Inc., in 1955 and then returned again to Johns Hopkins, where he obtained a doctorate in environmental engineering in 1963.

Dr. Levin met T. Keith Glennan, the first administrator of NASA, at a Christmas party and asked if the space agency, recently established at the time, was interested in looking for life on Mars. He proposed the idea of the labeled release experiment, and NASA began financing its development.

After the sale of his consulting company, Dr. Levin worked at Hazelton Laboratories in Vienna, Va. In 1967, he was offered a professorship at Colorado State University, but instead he founded Biospherics Research, which he later renamed Spherix. Tagatose was to be the company’s blockbuster project, but success never arrived.

“He lost control of the company when there was a large section of the board of directors that took over the company,” Henry Levin said.

Spherix, now known as Alkido Pharma, abandoned tagatose and moved on to create other businesses, like using artificial intelligence to develop new medicines.

In addition to his son Henry, Dr. Levin, who had homes in Chevy Chase, Md., and Palm Beach, Fla., is survived by another son, Ron; a daughter, Carol Sanchez; and six grandchildren. His wife, Karen, died in 2019.

What Dr. Levin’s Mars experiment did or did not discover remains unresolved.

“He made a major step forward,” said James L. Green, NASA’s chief scientist. “He kept us thinking about, well, what could it be?”

A real answer awaits the return of rock and dirt samples from Mars that NASA’s new Perseverance rover will soon begin collecting. But Perseverance has no way to launch the samples back to Earth. The mission to pick them up and bring them back is still on the drawing board, and scientists will not be able to examine those samples until the 2030s.

When they do, they will be able to employ far more sophisticated instruments than what Dr. Levin had when he came up with the idea of using radioactive carbon-14 in a nutrient bath.

And maybe then his unwavering belief that his experiment had discovered microscopic Martians will be vindicated.

“I really hope it did,” Dr. Green said. “It would be the discovery of the century.”