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NASA’s twin Viking spacecraft, which landed in 1976, famously dug in the sand and performed experiments looking for living microbes but came up empty, perhaps because scientists didn’t understand Martian chemistry.

"You can assign them to Mars based on the gaseous inclusions that they have, that are sort of the equivalent of the gases that were shown by the Viking spacecraft" to exist in Mars' atmosphere, Ruvkun said.

However, says Dr McConnochie, the temperature in the Viking spacecraft chamber was much warmer than it would be outside, even during spring and summer.

"Amazingly, they were positive. As the experiment progressed, a total of four positive results, supported by five varied controls, streamed down from the twin Viking spacecraft landed some 4,000 miles apart."

As the experiment progressed, a total of four positive results, supported by five varied controls, streamed down from the twin Viking spacecraft landed some 4,000 miles apart.