A scarp, or cliff, more than 300 kilometers (185 miles) long extends diagonally from upper left to lower right in this Mariner 10 picture (FDS 166842) of Mercury taken on September 21. Numerous similar structures have been discovered by Mariner 10 during the television sequences on the spacecraft's second flyby of the planet. These structures are believed to be formed by the compressive forces due to crustal shortening. The picture was taken from 64,500 kilometers (40,000 miles).
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.