Four Stages of the Earth's Atmosphere

Figure 3 gives four stages of the evolution of the earth's atmosphere. Presumably the earth was formed as a product spinning off from the sun, which is mostly hydrogen and helium. When the earth formed and cooled, its earliest atmosphere (Atmosphere I) probably consisted of primarily ammonia, and compounds of such elements as bromine, chlorine, fluorine, and sulfur. Constituents of this atmosphere would have been products of outgasing and are similar to the kinds of gases coming out of volcanoes: hydrogen sulfide, hydrochloric acid, hydrofluoric acid, hydrogen sulfide and ammonia. If you remember any chemistry, you will recognize that these are not nice chemicals to be around.

However this atmosphere didn't last long, quickly being replaced by an atmosphere of water, CO₂ , and nitrogen (Atmosphere II). As the earth cooled further to conditions below the critical pressure and critical temperature of water, the water of course started to condense and make oceans and so water gradually over a period of time disappeared from the atmosphere. This is the most interesting period for us, the period between Atmospheres II and III, the latter revealing the appearance of oxygen, possibly produced by photochemical breakdown of water. The presence of oxygen to shield the earth's surface from ultraviolet light allowed for the arrival of photosynthesizing plants that consumed the abundant CO₂ and gave off oxygen, thereby diminishing the former and enhancing the relative abundance of the latter to give the atmosphere we have today. A more extensive and precise list of current atmospheric constituents is given in a table that will be discussed in Unit 1-4.

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