This photograph of Mercury was taken by the Mariner 10 spacecraft from a range of 34,860 miles (55,340 kilometers) during the first Mercury encounter on March 29, 1974. It shows an area in the northern hemisphere in the general area of photography to be obtained during the third encounter, Sunday, March 16, 1975. Seen in the photograph is a transition from heavily cratered terrain to a smooth plain. These smooth plains areas on Mercury are thought to be volcanic in origin with lava flows filling in heavily cratered areas. The photograph area is about 308 miles (490 kilometers) wide. (FDS 156)

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.