Further examination of the hydrogen content in the upper atmosphere of Venus

Abstract

In a recent paper, Earth et al. [1968] computed the molecular hydrogen concentration in the upper atmosphere of Venus from the component of the Lyman-alpha emission attributed to photodissociation of molecular hydrogen as measured by Mariner 5. The outcome of these computations, as seen in Figure 3 of Barth et al. [1968], resulted in part from the assumptions of a molecular hydrogen density of 2.8 × 109 molecules cm−3 at a planetocentric distance of 6500 km, with an isothermal region of temperature 650°K above this level. As a consequence, the optical thickness of molecular hydrogen at 6500 km, based upon the photoionization cross sections of Lee et al. [1952], would result in significant ultraviolet energy in the wavelength region below 804 A being absorbed above 6500 km. Correspondingly, the rapid settling of the heavy CO2 molecule in the lower thermosphere would severely limit the effectiveness of CO2 as a cooling agent at these high altitudes. Under such circumstances, it is difficult to understand both the isothermal region above 6500 km and the relatively low exospheric temperature of Venus. To verify these points the author reconstructed the upper atmosphere of Venus by solving numerically the heat conduction equation in conjunction with separate diffusion equations for each constituent similar to the method outlined in Chamberlain [1962], Chamberlain et al. [1966], and McGovern [1967].