These two landers were launched in 1975 and began transmitting photographs from Mars in 1976. The program ended in 1983 by which time both landers had run out of power. They took soil samples, sampled Mars' air and carried out biological experiments.
The surface of Mars taken by Viking 2
LANDER DESIGN CONSIDERATIONS
Because the landers didn't move, they didn't need wheels and motor drives. However, the legs needed to settle onto the Martian soil without hitting rocks. So the engineers designed the lander with three legs for stability. Viking 2 managed to land which one leg up on a rock.
The Vikings were powered by radioisotope batteries (nuclear powered).