The origin and development of the terrestrial planets' atmospheres with special reference to the Earth

The atmospheres of the terrestrial planets are generally supposed to have been formed by secondary degassing processes. It is proposed instead, in this thesis, that the atmospheres formed as a necessary consequence of the final stages of planetary accretion. When the planetary embryos reached a critical size, the accretional material impacting the surface began to vaporize and some remained in this gaseous state. As the gases accumulated, further incoming material was decelerated, thus limiting the rate of atmosphere growth. When reasonable assumptions are made, concerning the nature of the impacting material, an acceptable model for the early terrestrial atmospheres results. Our knowledge of the noble gas abundances for the present Martian and terrestrial atmospheres does not preclude the possibility of atmosphere production from the impact of mainly primitive Cl meteorite type material. Subsequent development of these primitive atmospheres into those which currently exist is possible. The evolution of the Earth's atmosphere is discussed in detail, with emphasis on the development of the carbon dioxide cycle. The primitive CO2,-H 2O atmosphere would not have produced a runaway greenhouse condition on Earth, such as occurs on Venus, due to the rapid solution of carbon dioxide in water which condensed onto the surface. The creation of an atmospheric dust veil is proposed as a critical factor in maintaining surface temperatures within the range possible to permit the formation of bodies of surface water. The development of the carbon dioxide cycle through three main stages is proposed. Carbon dioxide partial pressures in the atmosphere remained in excess of the current value throughout the Precambrian, to maintain higher surface temperatures throughout this period: present values were not approached until the advent of deep sea organisms. Methods of regulating the carbon dioxide reservoir in the atmosphere are discussed for each stage and the importance of subduction processes in the present situation is stressed. Finally, the proposed evolutionary histories for the atmospheres of the Earth, Mars and Venus are outlined.