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About 4.5 billion years ago, Earth had a primarily hydrogen and helium atmosphere. Over time, outgassing created a secondary atmosphere rich in nitrogen and as oceans formed, CO2 decreased. Oxygen increased about 2 billion years ago due to plant photosynthesis, reaching present levels around 500 million years ago. The current dry atmosphere consists of 78% nitrogen, 20.95% oxygen, and trace amounts of other gases like argon, carbon dioxide, methane, nitrous oxide, ozone, and aerosols.
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Early atmosphere ~4.5 billion years ago Mostly Hydrogen and Helium Escaped into space Outgassing of water vapor and CO2 created a secondary atmosphere with some nitrogen As liquid water formed the oceans and CO2 was removed from atmosphere The atmosphere became dominated by nitrogen. ~ 2 billion years ago oxygen increased in the atmosphere because of plant life and photosynthesis. Oxygen reach it’s present level, making up 21% of the dry atmosphere about 500 million years ago.
78 % Nitrogen N2 20.95 % Oxygen O2 Dry atmosphere 0.93 % Argon Ar H2O 0 to 4 % Carbon Dioxide CO2 0 to .037 % (or 370 ppm) Methane CH4 1.8 ppm Nitrous Oxide N2O .3 ppm Ozone 0.04 ppm or 40 ppb) Aerosols (particles) highly variable
