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 the upper stratosphere, and in turn creates a zone similar in some respects tothe present ozone layer. Above the mesopause, products resulting from the photolysis of CH4, mainlyatomic hydrogen, ethane, acetylene and propane, would dominate. These hydrocarbons are very efficientradiators in the infrared; a small concentration of these constituents is sufficient to cool the thermosphereeffectively. At these relatively low densities diffusion is the controlling process in governing the distributionof the constituents above the mesopause. Subsequently, solving for the temperature of the exosphereassuming conductive equilibrium resulted in the following conclusions: 1) Over a wide range of conditions a primitive CH4 atmosphere could have produced exospheric temperatures between 500 and 1000K. 2) Since the exospheric temperature is sensitive to the hydrogen content of the thermosphere, the ultimatefactor controlling the temperature of the exosphere was the outgassing rate of hydrogen. 3) After possibly an initial turbulent period of escape, the maximum molecular hydrogen concentrationin the lower atmosphere was near 10-15 with a corresponding maximum exospheric temperature ofapproximately 900K. 4) During the latter stages of the primitive atmosphere when hydrogen outgassing was small, the exospheric temperature approached a minimum of 600-100K.Diminishing hydrogen outgassing eventually resulted in insufficient NH3 to protect water vapor fromphotodissociation, thereby releasing oxygen which converted CH4 to CO- and CO. Evolving from theprimitive atmosphere after 108-109 years was an environment dominated by nitrogen with a few millibarsof CO2, water vapor and possibly CO.