The chemical compositions of the primordial atmospheres of Venus, Earth and Mars have long been a topic of debate between the experts. Some believe that the original atmospheres were a product of outgassed volatiles from the newly accreted terrestrial planets and that these atmospheres consisted primarily of carbon dioxide, nitrogen, water vapor and residual hydrogen and helium (e.g., Lewis and Prinn, Planets and their Atmospheres, Academic Press, Orlando, FL, 1984, pp. 62-63, 81-84, 228-231, 383). Still others think the earliest atmospheres were composed of the gas components of the solar nebula from which the solar system formed (i.e., hydrogen, helium, methane, ammonia and water). I consider the latter to be the correct scenario. Presented herein is a proposed mechanism by which the original atmospheres of Venus, Earth and Mars were transformed to atmospheres rich in carbon dioxide and nitrogen. An explanation is proposed for why water is so common on the surface of Earth and so scarce on the Surfaces of Venus and Mars. Also presented are the effects the "great impact" (single cataclysmic event that wits responsible for producing the Earth-Moon system) had upon the early atmosphere of Earth. The origin, Structure and composition of the impacting object are determined through deductive analyses.
