Earth's atmosphere is a layer of various gasses that are held to Earth by gravity. The main two gasses that make up Earth's atmosphere (by volume) are Nitrogen (N2), which makes up 78.09% and Oxygen (O2), which makes up 20.95%. The remaining ~1% is made up of many trace gasses, including 5 of the 6 natural noble gasses and Carbon Dioxide (CO2). Water Vapor was not included. 

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