Atmospheric Composition

The atmosphere emerged 500 million years after the formation of the Earth.  It emerged from the vapor of gases that were expelled during the degassing of the planets interior.  The gases consisted of hydrogen(H₂), water vapor, methane (CH₄), and carbon oxides.  Prior to 3.5 billion years ago the atmosphere probably consisted of carbon dioxide (CO₂), carbon monoxide (CO), water (H₂O), nitrogen (N₂), and hydrogen. Our present day atmosphere is relatively a stable mix of several types of gases.  The total atmosphere has a mass of about 5.15 x 10¹⁵ tons and surrounds the planet holding on by gravity only. The proportions of gases, excluding water vapor, are nearly uniform up to approximately 80km above the Earth's surface.  The major components of this region, by volume, are oxygen (21%), nitrogen (78%), and argon (.93%).  Small amounts of other gases are also present.  These remaining trace gases exist in such small quantities that they are measured in terms of a mixing ratio.  This ratio is defined as the number of molecules of the trace gas divided by the total number of molecules present in the volume sampled.  Atmospheric temperature and chemistry are believed to be controlled by the trace gases.  There is evidence that environmentally significant trace gases are changing because of both natural and human factors (man-made gases like CFC's).  Natural gas levels like carbon dioxide, nitrous oxide, and methane are changing through actions like the burning of fossil fuels, expulsion from living and dead biomass, release from metabolic processes of microorganisms in the soil, wetlands, and oceans of our planet. 
  
Gas Composition (image) 

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