NASA

Scientists believe that a reanalysis of soil samples taken during a Viking mission nearly 30 years ago again shows strong evidence of microbial life on Mars.

In a paper this month published in the International Journal of Aeronautical and Space Sciences, scientists described performing new experiments and drawing on more recent understanding of the Mars environment to reach a conclusion opposite to one reached decades ago regarding the building blocks of life on Mars.

Controversy has surrounded the original experiments, called the Labeled Release (LR), done on the soil samples during the Viking mission in 1976, according to the paper's authors. The study doesn't say that Viking definitively found past life on Mars, but that these latest experiments indicate the presence of carbon-based compounds.

"We believe that these results provide considerable support for the conclusion that the Viking LR experiments did, indeed, detect extant microbial life on Mars," the study concludes.

The researchers did statistical analysis of the data from the Viking soil sample and found signs of circadian rhythms, as a bacteria would have. "The presence of circadian rhythmicity and a high degree of mathematical complexity or order in the LR data most likely means Viking discovered microbial life on Mars over 35 years ago," neuropharmacologist and study co-author Joseph Miller told Science Blog.

Most likely, any bacteria would be living under the soil because, unlike Earth, the Mars atmosphere doesn't block ultraviolet radiation, he said.

Viking was the first mission dedicated to detecting life on another planet. The conclusion that there was no life on Mars in the 1970s led to a long break in Mars probes, but scientists such as Miller argue that new missions should set out to find signs of life again.

The study published in IJASS isn't the first time scientists have questioned the original Viking results. In a previous chemical analysis, scientists concluded that a type of compounds called perchlorates were present in Martian soil, contrary to the initial conclusions. Scientists in 2011 repeated a Viking experiment on perchlorate-rich soils from Chile's Atacama Desert and found signs of carbon-based gases. When they detected those gases from Viking soils, it was dismissed as a contaminant, according to Discovery News.

Updated to indicate original soil tests were done during Viking mission.