Earth's atmosphere

The early Earth's atmosphere was mostly Co₂ with virtuall no oxygen. A lot of the early carbon dioxide was absorbed into the oceans and locked up in sedimentary rocks, and fossil fuels. Green plants also steadily removed it and released oxygen. Nitrogen gas began to be produced by organisms such as denitrfying bacteria.

This build up of oxygen killed off early organisms that couldn't cope, but allowed the evolution of more complicated organisms. The oxygen also created the ozone layer ( O₃). Present day composition -

78% Nitrogen.
1% Argon.
21% Oxygen.
0.04% Carbon dioxide.

Also - Varying amounts of water vapour, and other noble gases in small amouts.

Earth's Atmospheric Layers

There are several ways to classify the atmospheric layers of the Earth.

Very broadly,

• homosphere - surface to approx 80~100km
• heterosphere - approx 80~100km and above

Distinct layers,

• troposphere - surface to 18km, capped by the tropopause
• stratosphere - 18km to 50km, capped by the stratopause
• mesosphere - 50km to 80km, capped by the mesopause
  (also known as the lower ionosphere, or the airglow, or the ozone band)
• thermosphere - 80km to 400km, capped by the thermopause
• (ionosphere includes the mesosphere and thermosphere)
• exosphere - 400km to 64000km
  (also known as the magnetosphere)

Radio layers,

• D-layer or D region - 56km to 88km
  (absorbs energy of shortwave raido waves reflected by other layers)
• Heaviside-Kennelly layer, E-layer or E region - 88km to 154km
  (during the day, reflects radio waves)
• Appleton layer, F-layer or F region - 154km to 400km
  (reflects radio waves of up to 50MHz)