Exploration of the planets: an invited discourse presented before the Sixteenth General Assembly of the International Astronomical Union, Grenoble, France, August, 1976.

The paper reviews in situ observations of Venus, Jupiter, and Mars by various spacecraft, including Mariner 2, 9, and 10, Pioneer 10 and 11, Venera 8, 9, and 10, and Viking. Detailed attention is given to the Viking mission, emphasizing adaptive aspects of the Viking spacecraft, landing sites, instruments, the geology of Mars, measurements of atmospheric constituents, the nature of rocks and sediment, and the biology experiments. It is noted that the organic-chemistry experiment has yielded entirely negative results, while two of the three microbiological experiments have yielded positive results. It is concluded that at the very least, a nonbiological chemistry on the Martian surface has been found which duplicates many of the essential steps in animal respiration and in green-plant photosynthesis.