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LOS ANGELES — Data collected 25 years ago on the surface of Mars by NASA’s twin Viking landers show evidence of life, a scientist claimed Friday. 

Other scientists quickly cast doubt on the claim by Joseph Miller, citing a variety of other explanations for the data radioed back to Earth from the landers as they performed experiments on Martian soil in an effort to find any trace of life on the Red Planet. 

Miller, an associate professor in the Department of Cell and Neurobiology at the University of Southern California in Los Angeles, said he detected distinct rhythms in the levels of gas given off during the range of experiments that sought to prompt the growth of microbial life in samples of Martian soil doused with water and nutrients. 

The experiments were conducted aboard the robotic Viking landers, which set down on the surface of Mars 25 years ago this month. 

If any living organisms were present in the samples, they were expected to process the nutrients and release the carbon they contained in a gas form, which could then be detected. 

Gas was released during the experiments, but a majority of scientists have long held it was the result of chemical reactions within the newly moistened – and highly corrosive – soil found on the planet’s surface. 

But Miller said the fluctuations were so regular they resembled the natural day-night, or circadian, rhythms found in terrestrial life forms, including bacteria. 

“The most likely explanation is biology,” Miller said. 

Other scientists regarded the announcement with skepticism. 

“The explanation is that there were peroxides that formed in soil, and these are ultimately responsible for reactions, the results of which he saw,” said Arden Albee, project scientist on NASA’s Mars Global Surveyor mission. 

Miller, an expert in circadian rhythms, said he has analyzed about 30 percent of the Viking data. So far, he said, he has found that gas levels produced during the various experiments varied as much as 3 percent during regular periods that matched the length of a Martian day, slightly longer than an Earth day. He said he is ”90 percent” sure the results point to the existence of life. 

Bruce Jakosky, a professor of geological sciences at the University of Colorado, Boulder, said the regularity could be explained by a number of other reasons, including regular fluctuations in the spacecraft’s temperature, the atmospheric pressure on Mars or activity aboard the robotic lander itself. 

Miller will present his results on Sunday at an astrobiology symposium held during the International Society for Optical Engineering’s 46th annual meeting in San Diego.