1. Present atmosphere vs. Original one
1. Differences of the elements between Universe and Earth
1. Universe>consists almost entirely of hydrogen and helium. Lighter elements.
2. Earth>consists of heavier elements. Abundant hydrogen. Helium is rare
2. The earth was formed by the accretion of planetesimals
1. Planetesimals had no atmosphere
2. The atmosphere of earth has been derived by out gassing of combined and occluded gases within panetesimals
3. Primitive atmosphere
1. Two hypothesis
2. Original atmosphere consisting largely of methane and ammonia
3. Present-day volcanic gases may indicate the nature of the primitive atmosphere

-In either case, free oxygen was absent

1. The geochemical processes which are series of gains and losses have operated to modify its composition

-Atmospheric gain-

1. Gases released by igneous activity
2. Oxygen and hydrogen produced by the photochemical dissociation of water vapor
3. Oxygen produced by photosynthesis
4. Helium produced by radioactive breakdown of uranium and thorium
5. Argon produced by the radioactive breakdown of potassium

-Atmospheric loss-

1. Oxygen removal by oxidation of ferrous to ferric iron, sulfur compounds to sulfates, hydrogen to water, and similar reactions

6. Carbon dioxide removed by the formation of coal, petroleum, and the death and burial of organisms
7. Carbon dioxide removed by formation of calcium and magnesium carbonates
8. Nitrogen removed by formation of oxides of nitrogen in the air and by the action of nitrify bacteria in the soil
9. Hydrogen and helium by escape from the Earth’s gravitational field

1. Atmosphere gets oxygen through photosynthesis
2. Estimation of oxygen content of atmosphere
1. About 1% of the present level 2 billion years ago
2. About 10% of the present level at the beginning of the Cambrian Period

*The present content by Devonian Times