1. Origin, Evolution, and Composition of the Atmosphere

2. Abbreviation Definitions
Ga: billion years
Ma: Million years
H2Ov : water vapor
H2Ol : Liquid water
CO2 : Carbon Dioxide
N2 : nitrogen molecule
O2 : oxygen molecule

3. Origins of Earth Nebula Theory
Solar nebula, rotating proto-planetary disk, accretion = planetesimals = protoplanet = planets
Age of the Earth: 4.6 Ga

4. Possible sources of Earth’s present atmosphere
Volcanic outgassing
CO2 and H2Ov are the most abundant volcanic gases
Impact degassing
CO2 and H2O released during impacts of planetesimals as Earth accreted
Later impacts, especially by water-rich comets, during the heavy bombardment period (lasting to approx Ga)

5. Chemical Evolution of the atmosphere

6. Phase 1: Primordial atmosphere
4.6 Ga
Original nebula
Dominated by H, He , and hydrogen compounds (methane, ammonia)
Blown away by solar wind

7. Phase 2: Early secondary atmosphere
4.6 to 2.0 Ga
Formed by impact degassing, outgassing, and/or comet impacts
N2, CO2, and H2Ov
H2Ov largely removed by 4.0 Ga as Earth cools
Formation of oceans due to cooling of H2Ov
Carbon dioxide is “rained out” of the atmosphere to be eventually stored in marine sediments/limestone
H2Ov →cooling→H2Ol =oceans

8. Phase 3: Transitional secondary atmosphere
2.0 Ga to 400 Ma
Dominated N2
Continued removal of CO2 by biological processes and storage in marine sediments
Build-up of O2 over time
Evolution of photosynthesis, maybe as early as 3.8 Ga
CO2 + H2O →Sunlight →Carbohydrate (C-H “food”) + O2

9. Figure 1http://www. windows2universe

10. Phase 4: Modern atmosphere
~ 400 Ma to present
Dominated by N2 and O2
Presents levels of oxygen
Ozone layer formed
Life colonizes the land