A claim that NASA overlooked evidence of life on Mars in data collected by the Viking missions 25 years ago has been met with interest and scepticism.

Joseph Miller, a visiting professor at the University of Southern California, re-analysed data collected by probes sent to the Martian surface by the Viking 1 and 2 spacecraft in 1976. He believes tests performed on soil samples reveal a cycle of chemical activity similar to the daily rhythms seen in living organisms on Earth.

The Viking mission included several experiments designed to look for life. In one, a nutrient solution labelled with radioactive carbon was added to a soil sample in a covered petri dish. The idea was that any living organisms would consume the nutrients and release the radioactive carbon in gas form. This gas would be detected by a radiation monitor.

The original researchers found evidence of gas release, but less than they anticipated would be given off by living organisms, says Colin Pillinger of the UK’s Open University.


“At the time, the scientists involved in the experiments did an awful lot of soul-searching about what the experiments meant,” Pillinger says. But in the end, they concluded they did not have evidence for life.

Perfect timing

So the data sat collecting dust for more than two decades before Miller, an expert on circadian rhythyms, became interested. Simply accessing the data turned out to be a challenge – it was stored in a long-forgotten format on magnetic tapes. But eventually Miller tracked down printouts from the original experiments.

Poring over the records, he found that the efflux of gas from the soil samples fluctuated on a 24.66 hour cycle – remarkably close to the 24.68 hour Martian day. Miller interprets this as strong evidence of a biological process.

He presented his findings, based on about a third of the original Viking data, at an astrobiology symposium at the annual meeting of the International Society for Optical Engineering.

However, not everyone embraces Miller’s interpretation. “If he’s found some sort of diurnal variation, that’s very interesting, but let’s not jump to conclusions,” says John Bridges, a Mars expert at the Natural History Museum in London.

Pillinger is also cautious. Part of the reason the original team discounted the possibility of life was that they didn’t find organic compounds in the soil samples, he says. “Without organic material, they couldn’t believe they had an organic effect.” he says. “To have proof of life, you have to find the body.”