On July 20, 1976, the NASA Viking 1 lander touched down on the surface of Mars. It was the first time humanity had ever gotten a close look at our neighbor planet. Scientists had no idea what they might find - strange rock formations, unfamiliar soil, maybe even alien life?

The experiments aimed at identifying Martian organisms turned up negative, or inconclusive, and decades of further exploration failed to find signs of life on the red planet. But at an event at NASA Langley Research Center Wednesday in honor of Viking's 40th birthday, NASA scientists said the search isn't over yet.

We spoke with Jim Green, the director of NASA's planetary sciences division, about the history of Viking and humankind's future on Mars.

Read more: NASA’s 1976 Viking mission to Mars did all that was hoped for it — except find Martians