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The [[atmosphere of Earth]] is composed of [[nitrogen]] (78%), [[oxygen]] (21%), [[argon]] (0.9%), [[Carbon dioxide in Earth's atmosphere|carbon dioxide]] (0.04%) and trace gases.<ref name="earthcomposition"/> Most organisms use oxygen for [[respiration (physiology)|respiration]]; lightning and bacteria perform [[nitrogen fixation]] which produces [[ammonia]] that is used to make [[nucleotides]] and [[amino acids]]; [[plant]]s, [[algae]], and [[cyanobacteria]] use carbon dioxide for [[photosynthesis]]. The layered composition of the atmosphere minimises the harmful effects of [[sunlight]], [[ultraviolet]] radiation, [[solar wind]], and [[cosmic ray]]s and thus protects the organisms from genetic damage. The current composition of the atmosphere of the Earth is the product of billions of years of biochemical modification of the [[paleoatmosphere]] by living organisms.<ref name="globalchange"/>
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===Origins===
Atmospheres are clouds of gas bound to and engulfing an astronomical focal point of [[Sphere of influence (astronomy)|sufficiently dominating mass]], adding to its mass, possibly escaping from it or collapsing into it.
Because of the latter, such [[
Composition and thickness is originally determined by the stellar nebula's chemistry and temperature, but can also by a product processes within the astronomical body outgasing a different atmosphere.
===Compositions===
[[File:Solar system escape velocity vs surface temperature.svg|thumb|upright=1.2|Graphs of escape velocity against surface temperature of some Solar System objects showing which gases are retained. The objects are drawn to scale, and their data points are at the black dots in the middle.]]▼
The atmospheres of the planets [[Venus]] and [[Mars]] are principally composed of [[carbon dioxide]] and [[nitrogen]], [[argon]] and [[oxygen]].<ref name="universetoday"/>
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===Atmospheres in the Solar System===
▲[[File:Solar system escape velocity vs surface temperature.svg|thumb|upright=1.2|Graphs of escape velocity against surface temperature of some Solar System objects showing which gases are retained. The objects are drawn to scale, and their data points are at the black dots in the middle.]]
* [[Atmosphere of the Sun]]
* [[Atmosphere of Mercury]]
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