Editing Atmosphere

{{Short description|Layer of gases surrounding an astronomical body held by gravity}}
{{Redirect|Air}}{{Hatnote group|{{Redirect-distinguish|Qualities of air|Air quality}}}}
{{About|atmospheres of celestial bodies|Earth's atmosphere|Atmosphere of Earth|other uses}}
{{Redirect|Atmospheric environment|the scientific journal|Atmospheric Environment}}
{{More citations needed|date=February 2024}}
[[File:Top of Atmosphere.jpg|thumb|upright=1.5|The atmospheric gases around Earth [[Rayleigh scattering|scatter blue light]] (shorter wavelengths) more than light toward the red end (longer wavelengths) of the [[visible spectrum]]; thus, a [[sky blue|blue]] glow over the horizon is seen when [[Earth observation|observing Earth]] from [[outer space]]. The [[Moon]] is visible in the background.]]

An '''atmosphere''' ({{etymology|grc|''{{Wikt-lang|grc|ἀτμός}}'' ({{grc-transl|ἀτμός}})|vapour, steam||''{{Wikt-lang|grc|σφαῖρα}}'' ({{grc-transl|σφαῖρα}})|sphere}})<ref name="etymology"/> is a layer of [[gas]]es that envelop an [[astronomical object]], held in place by the [[gravity]] of the object. A planet retains an atmosphere when the gravity is great and the [[temperature]] of the atmosphere is low. A [[stellar atmosphere]] is the outer region of a star, which includes the layers above the [[opacity (optics)|opaque]] [[photosphere]]; stars of low temperature might have outer atmospheres containing compound [[molecule]]s.

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"/>

==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]]

==Structure of atmosphere==

===Earth===
[[File:469649main earths-atmosphere-orig full.jpg|thumb|Earth's atmosphere profile]]
The [[atmosphere of Earth]] is composed of layers with different properties, such as specific gaseous composition, temperature, and pressure.

The [[troposphere]] is the lowest layer of the atmosphere. This extends from the planetary surface to the bottom of the [[stratosphere]]. The troposphere contains 75–80% of the mass of the atmosphere,<ref name="natgeoatmosphere"/> and is the atmospheric layer wherein the weather occurs; the height of the troposphere varies between 17&nbsp;km at the equator and 7.0&nbsp;km at the poles.

The [[stratosphere]] extends from the top of the troposphere to the bottom of the [[mesosphere]], and contains the [[ozone layer]], at an altitude between 15&nbsp;km and 35&nbsp;km. It is the atmospheric layer that absorbs most of the [[ultraviolet radiation]] that Earth receives from the Sun.

The [[mesosphere]] ranges from 50&nbsp;km to 85&nbsp;km and is the layer wherein most [[meteors]] are incinerated before reaching the surface.

The [[thermosphere]] extends from an altitude of 85&nbsp;km to the base of the [[exosphere]] at 690&nbsp;km and contains the [[ionosphere]], where solar radiation ionizes the atmosphere. The density of the ionosphere is greater at short distances from the planetary surface in the daytime and decreases as the ionosphere rises at night-time, thereby allowing a greater range of radio frequencies to travel greater distances.

The [[exosphere]] begins at 690 to 1,000&nbsp;km from the surface, and extends to roughly 10,000&nbsp;km, where it interacts with the [[magnetosphere]] of Earth.

==Pressure==
 {{main|Atmospheric pressure}}

Atmospheric pressure is the [[force]] (per unit-area) perpendicular to a unit-area of planetary surface, as determined by the [[weight]] of the vertical column of atmospheric gases. In said atmospheric model, the [[atmospheric pressure]], the weight of the mass of the gas, decreases at high altitude because of the diminishing mass of the gas above the point of [[barometer|barometric]] measurement. The units of air pressure are based upon the [[Atmosphere (unit)|standard atmosphere]] (atm), which is 101,325&nbsp;[[Pascal (unit)|Pa]] (equivalent to 760&nbsp;[[Torr]] or 14.696&nbsp;[[pounds per square inch|psi]]). The height at which the atmospheric pressure declines by a factor of ''[[e (mathematical constant)|e]]'' (an [[irrational number]] equal to 2.71828) is called the [[scale height]] (''H''). For an atmosphere of uniform temperature, the scale height is proportional to the atmospheric temperature and is inversely proportional to the product of the mean [[molecular mass]] of dry air, and the local acceleration of gravity at the point of barometric measurement.

==Escape==
{{Main|Atmospheric escape}}
[[Surface gravity]] differs significantly among the planets. For example, the large gravitational force of the giant planet [[Jupiter]] retains light gases such as [[hydrogen]] and [[helium]] that escape from objects with lower gravity. Secondly, the distance from the Sun determines the energy available to heat atmospheric gas to the point where some fraction of its molecules' [[thermal motion]] exceed the planet's [[escape velocity]], allowing those to escape a planet's gravitational grasp. Thus, distant and cold [[Titan (moon)|Titan]], [[Triton (moon)|Triton]], and [[Pluto]] are able to retain their atmospheres despite their relatively low gravities.

Since a collection of gas molecules may be moving at a wide range of velocities, there will always be some fast enough to produce a slow leakage of gas into space. Lighter molecules move faster than heavier ones with the same thermal [[kinetic energy]], and so gases of low [[molecular mass|molecular weight]] are lost more rapidly than those of high molecular weight. It is thought that [[Venus]] and [[Mars]] may have lost much of their water when, after being [[Photodissociation|photodissociated]] into hydrogen and oxygen by solar [[ultraviolet]] radiation, the hydrogen escaped. [[Earth's magnetic field]] helps to prevent this, as, normally, the solar wind would greatly enhance the escape of hydrogen. However, over the past 3 billion years Earth may have lost gases through the magnetic polar regions due to auroral activity, including a net 2% of its atmospheric oxygen.<ref name="seki2001"/> The net effect, taking the most important escape processes into account, is that an intrinsic magnetic field does not protect a planet from atmospheric escape and that for some magnetizations the presence of a magnetic field works to increase the escape rate.<ref name="gunell2018"/>

Other mechanisms that can cause [[atmospheric escape|atmosphere depletion]] are [[solar wind]]-induced sputtering, [[impact event|impact]] erosion, [[weathering]], and sequestration—sometimes referred to as "freezing out"—into the [[regolith]] and [[polar ice cap|polar caps]].

== Terrain ==
Atmospheres have dramatic effects on the surfaces of rocky bodies. Objects that have no atmosphere, or that have only an exosphere, have terrain that is covered in [[impact crater|craters]]. Without an atmosphere, the planet has no protection from [[meteoroid]]s, and all of them collide with the surface as [[meteorites]] and create craters.

For planets with a significant atmosphere, most [[meteoroids]] burn up as [[meteors]] before hitting a planet's surface. When meteoroids do impact, the effects are often erased by the action of wind.<ref name="forbesmeteor"/>

[[Wind erosion]] is a significant factor in shaping the terrain of rocky planets with atmospheres, and over time can erase the effects of both craters and [[volcanoes]]. In addition, since [[liquid]]s cannot exist without pressure, an atmosphere allows liquid to be present at the surface, resulting in [[lake]]s, [[river]]s and [[ocean]]s. [[Earth]] and [[Titan (moon)|Titan]] are known to have liquids at their surface and terrain on the planet suggests that [[Mars]] had liquid on its surface in the past.

===Outside the Solar System===
{{Main|Extraterrestrial atmosphere}}
* Atmosphere of [[HD 209458 b]]

==Circulation==
{{Main|Atmospheric circulation}}
The circulation of the atmosphere occurs due to thermal differences when [[convection]] becomes a more efficient transporter of heat than [[thermal radiation]]. On planets where the primary heat source is solar radiation, excess heat in the tropics is transported to higher latitudes. When a planet generates a significant amount of heat internally, such as is the case for [[Jupiter]], convection in the atmosphere can transport thermal energy from the higher temperature interior up to the surface.

==Importance==
From the perspective of a planetary [[geologist]], the atmosphere acts to shape a planetary surface. [[Wind]] picks up [[dust]] and other particles which, when they collide with the terrain, erode the [[Terrain|relief]] and leave [[Deposition (sediment)|deposits]] ([[Aeolian processes|eolian]] processes). [[Frost line|Frost]] and [[Precipitation (meteorology)|precipitations]], which depend on the atmospheric composition, also influence the relief. Climate changes can influence a planet's geological history. Conversely, studying the surface of the Earth leads to an understanding of the atmosphere and climate of other planets.

For a [[meteorologist]], the composition of the Earth's atmosphere is a factor affecting the [[climate]] and its variations.

For a [[biologist]] or [[paleontologist]], the Earth's atmospheric composition is closely dependent on the appearance of life and its [[evolution]].

==See also==
{{Portal|Weather}}
* [[Atmometer]] (evaporimeter)
* [[Atmospheric pressure]]
* [[International Standard Atmosphere]]
* [[Kármán line]]
* [[Sky]]

==References==
{{reflist
|refs=

<ref name="etymology">{{cite web |url=https://perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Da%29tmo%2Fs |title=ἀτμός |archive-url=https://web.archive.org/web/20150924182433/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Da%29tmo%2Fs |archive-date=24 September 2015 |date=2015-09-24 |first1=Henry George |last1=Liddell |first2=Robert |last2=Scott |work=A Greek-English Lexicon |publisher=[[Perseus Project|Perseus Digital Library]]}}</ref>

<ref name="earthcomposition">{{cite web |url=https://earthhow.com/earth-atmosphere-composition/ |title=Earth's Atmosphere Composition: Nitrogen, Oxygen, Argon and CO2 |date=2017-07-31 |website=Earth How |language=en-US |access-date=2019-10-22 |archive-date=2022-04-19 |archive-url=https://web.archive.org/web/20220419213949/https://earthhow.com/earth-atmosphere-composition/ |url-status=live }}</ref>

<ref name="globalchange">{{cite web |title=Evolution of the Atmosphere |url=https://globalchange.umich.edu/globalchange1/current/lectures/Perry_Samson_lectures/evolution_atm/ |website=globalchange.umich.edu |access-date=30 April 2023 |archive-date=9 August 2022 |archive-url=https://web.archive.org/web/20220809204821/https://globalchange.umich.edu/globalchange1/current/lectures/Perry_Samson_lectures/evolution_atm/ |url-status=live }}</ref>

<ref name="universetoday">{{Cite news|url=https://www.universetoday.com/35796/atmosphere-of-the-planets/|title=What is the Atmosphere Like on Other Planets?|last=Williams|first=Matt|date=2016-01-07|website=Universe Today|language=en-US|access-date=2019-10-22|archive-date=2019-10-22|archive-url=https://web.archive.org/web/20191022200133/https://www.universetoday.com/35796/atmosphere-of-the-planets/|url-status=live}}</ref>

<ref name="tornadosfsu">{{Cite web|url=http://tornado.sfsu.edu/geosciences/classes/m201/Atmosphere/AtmosphericComposition.html|title=Atmospheric Composition|website=Department of Earth & Climate Sciences |publisher= San Francisco State University |access-date=2019-10-22|archive-date=2020-04-20|archive-url=https://web.archive.org/web/20200420141730/http://tornado.sfsu.edu/geosciences/classes/m201/Atmosphere/AtmosphericComposition.html|url-status=dead}}</ref>

<ref name="HD209458">{{cite news | author1=Weaver, D. | author2=Villard, R. | title=Hubble Probes Layer-cake Structure of Alien World's Atmosphere |publisher=Hubble News Center | date=2007-01-31 | url=http://hubblesite.org/newscenter/archive/releases/2007/07/ | access-date=2007-03-11 |url-status = dead | archive-url=https://web.archive.org/web/20070314043755/http://hubblesite.org/newscenter/archive/releases/2007/07/ | archive-date=2007-03-14 }}</ref>

<ref name="natgeoatmosphere">{{Cite web |title=Atmosphere  |url=https://education.nationalgeographic.org/resource/atmosphere |access-date=2022-06-09 |website=National Geographic Society |archive-date=2022-06-10 |archive-url=https://web.archive.org/web/20220610180446/https://education.nationalgeographic.org/resource/atmosphere/ |url-status=live }}</ref>

<ref name="seki2001">{{cite journal | author1=Seki, K. | author2=Elphic, R. C. | author3=Hirahara, M. | author4=Terasawa, T. | author5=Mukai, T. | title=On Atmospheric Loss of Oxygen Ions from Earth Through Magnetospheric Processes | journal=Science | year=2001 | volume=291 | issue=5510 | pages=1939–1941 | url=http://www.sciencemag.org/cgi/content/full/291/5510/1939 | access-date=2007-03-07 | doi=10.1126/science.1058913 | pmid=11239148 | bibcode=2001Sci...291.1939S | citeseerx=10.1.1.471.2226 | s2cid=17644371 }}</ref>

<ref name="gunell2018">{{cite journal |last1=Gunell |first1=H. |last2=Maggiolo |first2=R. |last3=Nilsson |first3=H. |last4=Stenberg Wieser |first4=G. |last5=Slapak |first5=R. |last6=Lindkvist |first6=J. |last7=Hamrin |first7=M. |last8=De Keyser |first8=J. |year=2018 |title=Why an intrinsic magnetic field does not protect a planet against atmospheric escape |journal=Astronomy and Astrophysics |volume=614 |pages=L3 |doi=10.1051/0004-6361/201832934 |bibcode = 2018A&A...614L...3G |doi-access=free }}</ref>

<ref name="forbesmeteor">{{cite web|url=https://www.forbes.com/sites/marshallshepherd/2019/06/27/scientists-detected-an-incoming-asteroid-the-size-of-a-car-last-week-why-that-matters-to-us/#3fb514894869|title=Scientists Detected An Incoming Asteroid The Size Of A Car Last Week - Why That Matters To Us|date=2019-06-27|website=[[Forbes]]|access-date=2019-07-26|archive-date=2019-07-26|archive-url=https://web.archive.org/web/20190726062900/https://www.forbes.com/sites/marshallshepherd/2019/06/27/scientists-detected-an-incoming-asteroid-the-size-of-a-car-last-week-why-that-matters-to-us/#3fb514894869|url-status=live}}</ref>

<ref name="Lorenz2014">{{Cite journal|last=Lorenz|first=Ralph D.|date=2014|title=Titan: Interior, surface, atmosphere, and space environment, edited by I. Müller-Wodarg, C. A. Griffith, E. Lellouch, and T. E. Cravens. Cambridge, UK: Cambridge University Press, 2014, 474 p. $135, hardcover |isbn=978-0-521-19992-6 |journal=Meteoritics & Planetary Science|language=en|volume=49|issue=6|pages=1139–1140|doi=10.1111/maps.12317|issn=1945-5100|doi-access=free}}</ref>

<ref name="Ingersoll1990">{{cite journal|last=Ingersoll|first=Andrew P.|date=1990|title=Dynamics of Triton's atmosphere|journal= Nature|volume=344|issue=6264 |pages=315–317|doi=10.1038/344315a0|bibcode = 1990Natur.344..315I |s2cid=4250378 }}</ref>

}} <!-- end of reflist -->

==Further reading==
* {{Cite book |last=Sanchez-Lavega |first=Agustin |title=An Introduction to Planetary Atmospheres |publisher=[[Taylor & Francis]] |year=2010 |isbn=978-1420067323}}

==External links==
{{Commons category}}
*[https://web.archive.org/web/20090421144658/http://www.allstar.fiu.edu/aero/fltenv2.htm Properties of atmospheric strata – The flight environment of the atmosphere]
*[https://mappingaround.in/atmosphere-everything-you-need-to-know/ Atmosphere – Everything you need to know]

{{Atmospheres}}
{{Authority control}}

[[Category:Atmosphere| ]]
[[Category:Gases]]
[[Category:Planetary science]]