The ionospheres of Mars and Venus

Abstract

Observations made by spacecraft in the Mariner and Venera series have provided considerable knowledge of the structure of the ionospheres and atmospheres of Mars and Venus. This paper begins with brief but complete discussions of these measurements and their interpretations. Specifically, we summarize the characteristics and use of UV radiometry, magnetometers, and ion probes for determining solar wind properties near planets, as well as the application of the occultation experiment. Some of the significant results were the very small, perhaps zero, magnetic moment of Venus and the possibly small but nonzero magnetic moment of Mars (Mars 2 and Mars 3 spacecraft); observation of daytime ionospheres on both Mars and Venus with peak number densities of 105 and 6 × 105 cm−3, respectively; observation of the top side scale heights for both planets; observation of a ledge of ionization with a large scale height and the anemopause above Venus; revelation by the two‐frequency radar occultation experiment carried by Mariner 5 of the existence of a nighttime Venusian ionosphere with a peak number density of 10³ cm−3 and a very large plasma scale height above the peak; and measurement by Mariner 6, 7, and 9 and Mars 2 and 3 of EUV radiation intensities (scattered solar radiation and dayglow emission), which yielded data for reasonably precise estimates of the concentration profiles of CO2, CO, O, C, and H as well as the thermospheric temperature (350°K for Mariner 6 and 7) on Mars. Corresponding observations by Mariner 5 and Venera spacecraft have not yielded as much information about Venus; Mariner 5 did measure the atomic hydrogen profile, whereas Venera 4 established an upper limit on atomic oxygen on the night side. The structure of the neutral atmosphere at high altitudes is of key significance to ionospheric structure. Although CO2 is the major constituent up to and including the main layer of the ionosphere, lighter gases such as O, He, and H probably play important roles at heights between the main layer and the anemopause. The occurrence of these gases, their high‐altitude profiles, and the closely related problem of the photolytic stability of the Martian and Venusian atmospheres are discussed in considerable detail. The anemopause also has an important bearing on the structure of the upper ionosphere. Unlike the earth the solar wind impinges directly on the ionospheres of Mars and Venus, thus acting as a possible heat source. However, no adequate theory has yet been developed to predict details of the interaction. The crude method of computing the day side boundary is discussed: it requires the balancing of the solar wind dynamic pressure by the ionospheric static pressure. The ‘downwind’ side of the boundary is also discussed, but mainly from the standpoint of our ignorance of the wake structure. The principles for constructing models of ionospheres are presented from the standpoint of mass, momentum, and energy conservation. The relevant ion chemistry is presented, and the meaning and use of plasma scale heights are discussed. The general principles in varying degrees of simplification have been employed by several investigators to model the upper ionospheres of Mars and Venus. The models discussed are the model of Cloutier and co‐workers, who suggested that direct hydrodynamic pressure of the solar wind on the top side Martian ionosphere depresses the ionospheric scale height to a value equal to the neutral atmosphere scale height; the models of Banks and Axford, of Whitten, and of Herman and co‐workers, who attempted to account for the ionospheric ledge above the main layer of the Venusian ionosphere by proposing the ionization of light constituents; the model developed by McElroy and Strobel to account for the nighttime ionosphere of Venus; and the models developed by a number of investigators for the thermal structure of the Martian and Venusian ionospheres. In presenting the essentials of each model, their strengths and weaknesses are also discussed. The paper concludes with a summary of current knowledge of the upper ionospheres of Mars and Venus and the information needed to understand fully their structures.