Taken about 40 minutes before Mariner 10 made its close approach to Mercury on September 21, this picture (FDS 166684) shows a large (230 kilometers, 142 miles) double-ringed basin (center of picture) located in the planet's south polar region - 75 degrees S. Lat. 120 degrees W. Long. Mercury saw the basin from a different viewing angle on Mariner 10's first sweep last March. This picture was taken from about 55,000 kilometers (44,000 miles). North is toward upper left.
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.