Mariner 4, 6, 7, and 9 took the first images of the volcanoes of Mars, including Olympus Mons, and others atop the Tharsis Ridge, and the south polar region. They also studied the Tharsis Ridge and measured the shape of the planet.
The Mariner missions helped establish that, like the Moon, Mars had a
barren surface with no running water. The vast number of craters
helped establish that the surface of Mars was very old when compared to
that of the Earth.