Findings from NASA's Viking landers

One of the oldest unanswered alien mysteries comes from 1976, when NASA's two Viking landers touched down on Mars. As well as being the first landers to operate on Mars, they also performed the first direct search for life there – and the results remain controversial.

On board each stationary lander was the Labeled Release

(LR) instrument, which scooped up bits of soil and mixed it with water. If the soil contained life, it was thought the nutrients in the water would be metabolised, releasing detectable carbon dioxide or methane gas. Amazingly, the instruments came out as positive, but two other experiments on the landers came out negative. One looked for organic material and found none, while another heated soil to look for organic residue, again finding nothing. Thus, the possibility they had found life was ruled out.

The results have since been re-interpreted though, with some scientists pointing out the other two experiments were not sensitive enough to support life. Others suggest the methods used on the Viking landers may have killed any life before it was found. It will take future missions, like ESA's EXOMARS rover or NASA's Mars 2020 rover, both scheduled to land in 2021, to provide more answers.

“The possibility they had found life was ruled out”