Mariner 10 photograph of the surface of Mercury, the innermost planet, showing a region of typical, heavily-cratered terrain known as the Kuiper Quadrangle. The large, white-floored impact crater at upper left is Lermontov and the small white- floored crater at lower right is Kuiper. Mariner 10 was the first spaceprobe to visit the planet, making three separate encounters during 1974-5.