After passing Mercury the first time and making a trip around the Sun, Mariner 10 again flew by Mercury on September 21 at 1:59 PMPDT. This encounter brought the spacecraft in front of Mercury in the southern hemisphere.
In this frame south is down, the south pole is located on the right hand edge of the large crater that has only its rim sticking up into the light (Chao Meng Fu crater). When this frame (FDS 166902) was acquired Mariner 10 was about 83,000 km from Mercury.
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.