One of the main goals of the twin Viking landers, which touched down on Mars 50 years ago, was to search for life. They found it. But then they didn’t. But maybe they really did.

If that sounds confusing – well, it is. Scientists have been debating the findings since the beginning – and the debate is still going on.

Viking 1 landed on July 20th, 1976, in a smooth plain north of the equator. It was the first fully successful landing. Viking 2 touched down six weeks later and 4,000 miles away.

Both landers conducted several experiments to look for life. They scooped up some of the powdery dust and dumped it into chemical laboratories for analysis.

In one experiment, they added water, nutrients, and a radioactive form of carbon. Any microbes in the samples should gobble up the nutrients and “exhale” the carbon. And that’s what appeared to happen – a sign of life.

But other experiments came up empty. They looked for organic compounds – the building blocks of life. And they found nothing. According to most scientists at the time, that ruled out the possibility of life. Instead, the positive results were interpreted as complex chemical reactions.

But many scientists continue to say the landers did find life. And over the past decade, there’s been a lively conversation about the results. So life at the Viking landing sites remains an open question.

Script by Damond Benningfield