When the Viking Missions Landed
#1 landed on August 20, 1975 and was the first mission to successfully land on Mars. #2 was landed on September 9, 1975 and was still operating until 1980.
They found all the things that were essential to life on Earth. Those things were: carbon, nitrogen, hydrogen, oxygen and phosphorus.
Here are some pictures that Viking 1 took
Here are some pictures that Viking 2 took
What We Now Know After The Vikings' Discoveries
We now know what type of surface that the planet Mars has, what the climate is there and if there is (or rather not) any life on Mars.
Viking 1 was actually meant to be Viking 2 but Viking 2's battery was discharged and so the old Viking 2 became the one that we now call Viking 1.