Chemistry of the Planets

Prior to the first Apollo landing on the Moon, man's knowledge of the com­ position of solar system bodies was limited to remote spectroscopic observa­ tions of planetary atmospheres, approximate density determinations of the planets, geochemical and geophysical data on the Earth, and laboratory studies on meteorites. Now we also have in hand hundreds of kilograms of surface samples selected from several widely separated areas on the near side of the moon, and spacecraft investigations of the planets have progressed to the point where instrumented atmospheric entry and landing probes have reached the surfaces of Venus and Mars. This explosion in our knowledge of the composition of solar system bodies, while still in an early stage, nonethe­ less suffices to force a reevaluation of our understanding of the composition, origin, and evolution of the planets, satellites, asteroids, and comets. The crucial questions to which we shall address our attention are: