The last mission to Mars launched in 1971 was Mariner 9, which lifted off on May 30th, two days after Mars 3. However, due to a tighter trajectory, Mariner 9 reached Mars on November 14th, shortly before both Mars 2 and Mars 3, which reached Mars on November 22nd and December 2nd, respectively.
Mariner 9 was designed to continue the atmospheric studies begun by Mariner 6 and 7, and to map over 70% of the Martian surface from the lowest altitude (1500 km) and at the highest resolutions (1 km per pixel to 100 m per pixel) of any Mars mission up to that time. An infrared radiometer was included to detect heat sources as evidence of volcanic activity. Mariner 9 was also to study temporal changes in the Martian atmosphere and surface as well as Phobos and Deimos.
Upon arrival, Mariner 9 observed that a great dust storm was obscuring the whole globe of the planet. Ground controllers sent commands to the spacecraft to wait until the dust had settled and the surface was clearly visible before compiling its global mosaic of high-quality images of the Martian surface. The storm persisted for a month, but after the dust cleared, Mariner 9 proceeded to reveal a very different planet than expected -- one that boasted gigantic volcanoes and a grand canyon stretching 4,800 km (3,000 miles) across its surface. This canyon, Valles Marineris, Latin for the "Valley of the Mariner," was named after Mariner 9.
Mariner 9 exceeded all primary photographic requirements by photo-mapping 100 percent of the planet's surface. The spacecraft also provided the first closeup pictures of the two small, irregular Martian moons: Phobos and Deimos.
The Mariner 9 mission was concluded on October 27, 1972, when the spacecraft ran out of altitude control gas. Mariner 9 remains in orbit around Mars where it is expected to survive until the year 2022, when it may finally crash onto the Martian surface.
The above photo is of the Olympus Mons caldera. This photo was taken in 1972 on Mariner 9's 230th orbit, approximately 4,000 km above the Martian surface. The basic morphology is similar to terrestrial calderas, although the scale is much larger. The smallest (and youngest) collapse pit, at right, is 30 km across. Concentric fractures and terraces are visible in the larger parts of the caldera. The caldera is about 29 km above the mean Martian surface. North is at 7:00.