A theoretical study of the evolution of the atmospheres and surface temperatures of the terrestrial planets.

A theoretical model which permits the computation of average surface temperature trends over the whole life-time of any terrestrial planet is described. The evolution of the atmosphere is considered in detail, and the changes in the physical parameters which affect the surface temperature are discussed. The variables of particular importance are the planetary albedo, the flux factor (a parameter previously considered constant), the infrared absorption spectrum of the atmosphere, the surface infrared emissivity and the probable evolutionary changes in the solar luminosity. Each of these parameters is discussed in detail and the possible feedback mechanisms which may link them to the evolution of the planetary atmosphere/surface system are considered. The computational techniques utilized in the development of the numerical model are described. The errors are discussed and the model is found to be widely applicable and to produce smoothed temperature tracks which agree well with present-day data for both the Earth and Mars. Numerical methods are used to extend laboratory parameterizations for the variation of absorption with absorber amount and partial pressure for two gases: carbon dioxide and water vapour. Trace constituents and the effect of broadening: by neutral gases are discussed. The resulting temperature curves exhibit a number of interesting features. In particular, a number of the model planets considered are found to exhibit remarkably stable surface temperatures as their atmospheres evolve. This stability results, in part, from the compensatory nature of the evolution of some of the planetary parameters. The rate and mode of degassing is found to be particularly important for Mars and may also be important for Venus. The temperature curve for the Earth remains above the freezing point of water throughout the life-time of the planet, and thus the predicted results agree well with geological data. Shorter-term fluctuations leading to glaciations, etc., are also discussed.