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Planetary Science

Earth's atmosphere consists of 5 main levels, each with distinct characteristics. The atmosphere is primarily nitrogen (78%) and oxygen (21%), and smaller quantities of other gases.


Ground to 12 km. The atmosphere has a total mass of about 5.15×1018 kg, half of which is found in the first 5.6 km of the troposphere. This part of the atmosphere is dense and turbulent, and is where nearly all clouds and meteorological phenomena occur.


12 to 50 km. The tropopause separates the troposphere from the stratosphere. Air pressure at the top of the stratosphere is about 1000 pascals, or one-thousandth the air pressure at sea level. It is where the vital ozone layer is located, a protective shield of O3, preventing dangerous solar UV light reaching the Earth's surface. The temperature at the tropopause may be as low as -60°C, but rises to around 0°C at the top of the stratosphere.


50 to 80 km. Temperature decreases with increasing altitude in the mesosphere. The mesosphere is too high to be accessed by aircraft or balloon, and too low for satellites.


80 to 700 km. The exact height of the thermosphere varies with solar activity. The ionosphere (the part of the atmosphere ionised by solar radiation) is located in the lower part of the thermosphere. The atmosphere is very low density, and the molecules are very energetic. Temperature increases with altitude in the thermosphere. There is no water vapour, so there are no clouds. The ISS (International Space Station) orbits in the thermosphere, at 320 - 380 km.


700 to 10,000 km. The exosphere has practically no atmosphere, and is the zone where most satellites are located.

Composition of the Atmosphere

Earth's atmosphere consists of a mixture of different gases, mainly nitrogen and oxygen.

    Earth atmospheric gases by percentage of volume:
  • N2 = 78.084 %
  • O2 = 20.946 %
  • Ar = 0.9340 %
  • CO2 = 0.043 %
  • Ne = 0.00182 %
  • He = 0.000524 %
  • CH4 = 0.00018 %
  • Kr = 0.000114 %
  • H2 = 0.000055 %

There is also a variable amount of water vapour (0.001 - 5%), or an average of about 0.25%.

All of the gases in the atmosphere have a total mass of 5.1480×1018 kg (5 million billion tonnes).


O3. A form of oxygen molecule with three atoms of oxygen, instead of the two of the respirable oxygen. Ozone is a pollutant near ground level, but in the stratosphere forms a vital shield against UV light from the Sun. The so-called 'ozone hole' is a thinning of this protecton due to the release of chemicals, such as chlorofluorocarbon (CFCs), HCFCs, freons,and halons, known collectively as ODSs, ozone-depleting substances.