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The Earth's atmosphere is made up of different gasses, but the main two gasses are nitrogen and oxygen. In fact, nitrogen makes up for about 78% of the atmosphere, oxygen makes up for about 21% of the atmosphere, and all other trace gasses, including carbon dioxide, methane, xenon, sulfur dioxide, ozone, etc., combine to make up for about 1% of the atmosphere. In addition, there is a small amount of water vapor that remains trapped in the planet's atmosphere.
The question of where there contents came from is a bit more complex. The Earth has gone through three different atmospheric changes since the beginning of the planet. The planet's first atmosphere consisted of mostly hydrogen and helium, with some ammonia and methane mixed in. These light gasses would have eventually dissipated into outer space and were eventually replaced with the second atmosphere, which was mostly composed of nitrogen and carbon dioxide released from volcanic activity and asteroid impacts. Finally, the third major atmosphere is the current one where oxygen was introduced by the photosynthesis of evolving plant and bacteria life on the planet.
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