The primitive earth - Thermal models of the upper atmosphere for a methane-dominated environment.

Abstract In discerning between alternate hypotheses pertaining to the dominant composition of the primitiveenvironment of the earth, the presence of free hydrogen in the atmosphere is of prime importance. Anatmosphere composed of CH4 and NH3, in contrast to a N2 and CO2 atmosphere, depends upon the availability of free hydrogen. However, the gravitational escape rate of hydrogen based on the current exospherictemperature maximum (~2000K) indicates that an atmosphere composed of CH4 and NH would havepersisted for only l0-l06 years, even with substantial outgassing of hydrogen from the crust. Therefore, anecessary prerequisite to resolving the question of the stability and durability of a primitive CH4-NHsatmosphere is a knowledge of the exospheric temperature of this early environment. Under chemical equilibrium in a primitive CH4-NHs atmosphere, CH4 would be the predominant constituent in the lower atmosphere. Ammonia shields water vapor from photodissociation, while absorbingthe majority of its energy in...