Origin, Evolution, and Composition of the Atmosphere

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

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

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.. 3.8 Ga)

Chemical Evolution of the atmosphere

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

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

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

Figure 1http://www. windows2universe

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