## Section2.3Evolution of the atmosphere

There is evidence that the atmosphere has not always possessed its present-day composition, but has evolved over geological timescales. Most significant is the appearance of oxygen as a major atmospheric component. The atmosphere of the has thus traditionally been called a second atmosphere in contrast to a hydrogen-rich primary atmosphere originally inherited from its time of formation. Broadly speaking, the history of the Earth's atmosphere is as follows:

• initial primordial atmosphere inherited from from formation; this atmosphere is rich in hydrogen and helium, light gases that readily lost to atmospheric escape;

• over time, gases such as N$$_2\text{,}$$ CO$$_2\text{,}$$ and H$$_2$$O build up via volcanic outgassing;

• with the advent of photosynthetic life, the amount of oxygen begins to increase; the rapid rise in the oxygen abundance (around 2.3 Ga) is known as the Great Oxygenation Event;

• throughout this history, the removal of CO$$_2$$ into carbonate sediments and rocks allows the build up of oxygen (and explains the relative lack of carbon dioxide in the Earth's atmosphere);

It should be noted that the presence of oxygen is highly unusual, and tied to the presence of life via photosynthesis and respiration: $$6\textrm{CO}_2+6\textrm{H}_2\textrm{O}\xrightarrow{h\nu} \textrm{C}_{6}\textrm{H}_{12}\textrm{O}_6+6\textrm{O}_2$$

$$\textrm{C}_{6}\textrm{H}_{12}\textrm{O}_6+6\textrm{O}_2\longrightarrow 6\textrm{CO}_2+6\textrm{H}_2\textrm{O}$$

These reactions can be used to summarize the main interaction between the atmosphere and the biosphere.