The Exospheric Temperature of a Primitive Terrestrial Atmosphere with Evolving Oxygen Content

Abstract

The evolution of oxygen in a primitive terrestrial atmosphere from the dissociation of water vapor is determined from the production and dissipation of hydrogen. The temperature of the exosphere however is directly related to the absorption of solar radiation by an increasing amount of oxygen. In this paper a simple model of the dependence of the exospheric temperature on the oxygen content is proposed. It is found, in the available range of values of EUV solar flux, that an exospheric temperature up to 0.1 of the present atmospheric level for oxygen (PAL) is low enough for Spitzer?limited flow to he established. The assumption is made that the limiting flux of hydrogen is determined by the total hydrogen mixing ratio at lower heights at various oxygen levels, and escape fluxes are computed for mixing ratios of 10, 102 and 103 ppm. It is argued that if the hydrogen density were not high enough to influence the thermospheric temperature the rise time in the oxygen content produced by photodissociation could he much slower.