It contains roughly (by molar content/volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, trace amounts of other gases, and a variable amount (average around 1%) of water vapor.
This mixture of gases is commonly known as air.
There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades into space.
Three quarters of the atmosphere's mass is within 11 km of the planetary surface.
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