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.
Hero Rupes, the large scarp visible running across the bottom of the image, is thought to thought to have been formed by global tectonic forces, possibly due to shrinkage of the planet as it cooled. Mariner 10 was looking obliquely across Mercury's southern hemisphere when it acquired this dramatic shot near the beginning of its southern hemisphere pass (FDS166618).
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.