Editing Atmosphere (section)

==Occurrence and compositions==
===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 [[Protoplanet|planetary nucleus]] can develop from interstellar [[molecular cloud]]s or [[protoplanetary disks]] into [[Rocky planet|rocky]] [[astronomical objects]] with varyingly thick atmospheres, [[gas giant]]s or [[Fusor (astronomy)|fusor]]s.

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"/>

The composition of Earth's atmosphere is determined by the by-products of the life that it sustains. Dry air (mixture of gases) from [[Atmosphere of Earth|Earth's atmosphere]] contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally a variable amount of water vapor is also present, on average about 1% at sea level.<ref name="tornadosfsu"/>

The low temperatures and higher gravity of the Solar System's [[giant planet]]s—[[Jupiter]], [[Saturn]], [[Uranus]] and [[Neptune]]—allow them more readily to retain gases with low [[molecular mass]]es. These planets have hydrogen–helium atmospheres, with trace amounts of more complex compounds.

Two satellites of the outer planets possess significant atmospheres. [[Titan (moon)|Titan]], a moon of Saturn, and [[Triton (moon)|Triton]], a moon of Neptune, have atmospheres mainly of [[nitrogen]].<ref name="Lorenz2014"/><ref name="Ingersoll1990"/> When in the part of its orbit closest to the Sun, [[Pluto]] has an atmosphere of nitrogen and methane similar to Triton's, but these gases are frozen when it is farther from the Sun.

Other bodies within the Solar System have extremely thin atmospheres not in equilibrium. These include the [[Moon]] ([[sodium]] gas), [[Mercury (planet)|Mercury]] (sodium gas), [[Europa (moon)|Europa]] (oxygen), [[Io (moon)|Io]] ([[sulfur]]), and [[Enceladus (moon)|Enceladus]] ([[water vapor]]).

The first exoplanet whose atmospheric composition was determined is [[HD 209458]]b, a gas giant with a close orbit around a star in the [[constellation]] [[Pegasus (constellation)|Pegasus]]. Its atmosphere is heated to temperatures over 1,000&nbsp;K, and is steadily escaping into space. Hydrogen, oxygen, carbon and sulfur have been detected in the planet's inflated atmosphere.<ref name="HD209458"/>

===Atmospheres in the Solar System===
* [[Atmosphere of the Sun]]
* [[Atmosphere of Mercury]]
* [[Atmosphere of Venus]]
* [[Atmosphere of Earth]]
** [[Atmosphere of the Moon]]
* [[Atmosphere of Mars]]
* [[Ceres (dwarf planet)#Atmosphere|Atmosphere of Ceres]]
* [[Atmosphere of Jupiter]]
** [[Io (moon)#Atmosphere|Atmosphere of Io]]
** [[Callisto (moon)#Atmosphere and ionosphere|Atmosphere of Callisto]]
** [[Europa (moon)#Atmosphere|Atmosphere of Europa]]
** [[Ganymede (moon)#Atmosphere and ionosphere|Atmosphere of Ganymede]]
* [[Atmosphere of Saturn]]
** [[Atmosphere of Titan]]
** [[Enceladus (moon)#South polar plumes|Atmosphere of Enceladus]]
* [[Atmosphere of Uranus]]
** [[Titania (moon)#Atmosphere|Atmosphere of Titania]]
* [[Neptune#Atmosphere|Atmosphere of Neptune]]
** [[Atmosphere of Triton]]
* [[Atmosphere of Pluto]]