The Atmospheres of Mars and Venus

Of all the planets which may exist in the Universe, only nine have been studied by man. As a result, one cannot classify planets with the same confidence that one has in classifying stars; there is no theory of planetary evolution comparable in development to the theory of stellar evolution. Nevertheless, many of the goals of planetary science and stellar astronomy are the same: to classify objects according to their most fundamental properties in order to understand their present physical state and their evolution. From this point of view, the terrestrial planets comprise a group which can usefully be considered together. By comparing the similarities and differences between them, we may hope to gain insight into the evolution of the entire group. Perhaps the most fundamental basis for identifying the terrestrial planets as a group is the similarity of their densities. The mean densities of Mercury, Venus, Earth, and Mars are 5.4, 5.1, 5.5, and 4.0 g/cm8, respectively, and their masses are 0.054, 0.815, 1.000, and 0.108 in units of the Earth’s mass. From these data, it is believed that the terrestrial planets consist mainly of the cosmically abundant metals Fe, Si, Mg, Ca, Al, and their oxides. Their atmospheres consist mainly of the cosmically abundant elements H, C, O, N, in compounds such as COs, H~O, N~, O~. Thus the terrestrial planets are further distinguished by the high degree of oxidation of their atmospheres. It is generally assumed that the compounds COs, H~O, etc, observed at the surfaces of the terrestrial planets, represent the volatile fraction released from the interiors after planetary accretion. This review will focus on the atmospheres of Mars and Venus. These planets’ atmospheres consist mainly of CO~, and so it is convenient to consider them together. In contrast, Mercury appears to have no atmosphere. This may be due either to a different internal history or to an inability to hold an atmosphere. The Earth’s atmosphere is beyond the scope of this review, but we shall compare the abundances of CO2, H,O, N~, etc, on the Earth,