Close-up of Martian surface, taken by Viking 2 Lander, Credit: NASA

While some of the experiments revealed unexpected chemical activity in the soil, those results were not convincing signs of life.

However, some scientists don't rule out the possibility of life on Mars. Future expeditions, like the Phoenix mission, will continue the search. And maybe sometime, humans will be the ones setting foot on the planet.

“I feel that the best exploration is ultimately going to be done by humans,” Steve Squyres, Principal Investigator for the Mars Exploration Rover Mission, told Science Friday in March 2004. “I think we’re making some significant steps towards that with our robotic exploration.”

The Viking 2 lander made its final data transmission in 1980, while Viking 1 operated until 1982, when contact was lost.

On July 20, 1976, the Viking 1 lander separated from its orbiter and arrived on Chryse Planitia, a smooth and circular plain on the surface of Mars. Viking 2 followed suit on September 3, landing on Utopia Planitia, another Martian plain.

The Viking landers accumulated over 4,500 close-up images of Mars's surface. They also carried instruments to study the composition of the planet, including its chemical and magnetic properties. In addition, the landers searched for life on Mars by conducting biology experiments on the soil.

Viking 2 Lander, Credit: NASA