A limping Mariner 10 spacecraft was coaxed into a third and final encounter with Mercury in March of 1975. Due to several problems with the aging spacecraft, only ~450 useful images of the planet were acquired, though many are at significantly higher resolution than previous encounters.
In this high resolution image (FDS 528884) the sun is low to the horizon thus enhancing topographic features with prominent shadows. As can be seen here, the surface of Mercury is heavily cratered similar to the Moon.
The prominent scarp that snakes up the image was named Discovery Rupes. Like Hero Rupes, this feature is thought to have been formed as the planet compressed, possibly caused by cooling of the planet.
The vertical (tall narrow) format of the third encounter images resulted from problems with the tape recorder and transmitter on the spacecraft. Only the middle quarter of each frame could be sent back.
The Mariner 10 mission, managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, explored Venus in February 1974 on the way to three encounters with Mercury-in March and September 1974 and in March 1975. The spacecraft took more than 7,000 photos of Mercury, Venus, the Earth and the Moon.