The Viking I and Viking 2 missions were to both orbit Mars and land on the planet's surface. There were two spacecraft for each mission.
Among other things, measurements from the Viking orbiters helped
establish the enormous size of Olympus Mons, which was first imaged
by Mariner 9. Viking made more complete measurements of the shape
of Mars, measurements which help scientists understand the interior of
the planet and the existence of the bulge called Tharsis Ridge.