round planet enveloped in smooth cloud layers. Venus exhibited a slow rotational period of 243 days and had only 0.05 percent of Earth's magnetic field. The planet's atmosphere was composed mostly of carbon dioxide.

.....After the Venus flyby, Mariner's trajectory was bent in toward the Sun to accelerate and fling it out of Venus's gravitational field and onward to Mercury. Mariner 10 reached Mercury on March 29, 1974, passing over the planet at 705 kilometers (438 miles) above the surface. A second encounter with Mercury occurred on September 21, 1974, at an altitude of about 47,000 kilometers (29,200 miles). The sunlit side of the planet and the south polar region were photographed. A third and last Mercury encounter, at an altitude of 327 kilometers (203 miles), occurred on March 16, 1975. About 300 additional photographs were obtained along with magnetic field measurements. Photographs of the planet reveal an intensely cratered,