Editing Exosphere

{{Short description|Outermost layer of an atmosphere}}
{{For|the racehorse|Exosphere (horse)}}
{{Use dmy dates|date=August 2015}}

[[File:EarthAtmosphereBig.jpg|thumb|80px|right|Diagram showing the five primary layers of the Earth's atmosphere: exosphere, [[thermosphere]], [[mesosphere]], [[stratosphere]], and [[troposphere]]. The layers are to scale. From the Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius.]]

The '''exosphere''' is a thin, atmosphere-like volume surrounding a [[planet]] or [[natural satellite]] where [[molecule]]s are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less.<ref name="NASA-20150817">{{cite web |last=Steigerwald |first=William |title=NASA's LADEE Spacecraft Finds Neon in Lunar Atmosphere |url=http://www.nasa.gov/content/goddard/ladee-lunar-neon |date=17 August 2015 |publisher=[[NASA]] |access-date=18 August 2015 }}</ref> In the case of bodies with substantial atmospheres, such as [[Atmosphere of Earth|Earth's atmosphere]], the exosphere is the uppermost layer, where the atmosphere thins out and merges with [[outer space]]. It is located directly above the [[thermosphere]]. Very little is known about it due to a lack of [[research]]. [[Mercury (planet)|Mercury]], the [[Moon]], [[Ceres (dwarf planet)|Ceres]], [[Europa (moon)|Europa]], and [[Ganymede (moon)|Ganymede]] have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The Earth's exosphere is mostly [[hydrogen]] and [[helium]], with some heavier [[atoms]] and [[molecules]] near the base.<ref>{{cite book |doi=10.1016/B978-0-12-804492-6.00001-0 |chapter=Chemicals and the Environment |title=Environmental Organic Chemistry for Engineers |date=2017 |last1=Speight |first1=James G. |pages=1–41 |isbn=978-0-12-804492-6 }}</ref>

==Surface boundary exosphere==
[[Mercury (planet)|Mercury]], [[Ceres (dwarf planet)|Ceres]] and several large natural satellites, such as the [[Moon]], [[Europa (moon)|Europa]], and [[Ganymede (moon)|Ganymede]], have exospheres without a denser atmosphere underneath,<ref name="NASA-20150306">{{cite web |last=Day |first=Brian |title=Why LADEE Matters |url=https://www.nasa.gov/mission_pages/LADEE/news/why-ladee-matters.html |date=20 August 2013 |publisher=NASA Ames Research Center |access-date=19 April 2015 |archive-date=18 April 2015 |archive-url=https://web.archive.org/web/20150418060132/http://www.nasa.gov/mission_pages/LADEE/news/why-ladee-matters.html |url-status=dead }}</ref> referred to as a '''surface boundary exosphere'''.<ref>{{cite web |title=Is There an Atmosphere on the Moon? |url=https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphere.html#.VX6V-d9PbGI |date=30 January 2014 |publisher=NASA |access-date=4 August 2016 |archive-date=2 November 2019 |archive-url=https://web.archive.org/web/20191102182404/https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphere.html#.VX6V-d9PbGI |url-status=dead }}</ref>  Here, molecules are ejected on [[parabolic trajectory|elliptic trajectories]] until they collide with the surface. Smaller bodies such as asteroids, in which the molecules emitted from the surface escape to space, are not considered to have exospheres.

==Earth's exosphere==
[[File:Earth’s geocorona from the Moon.jpg|thumb|The Earth and its hydrogen envelope of its exosphere, the [[geocorona]], as seen from the Moon. This ultraviolet picture was taken in 1972 with a camera operated by [[Apollo 16]] astronauts on the Moon.]]
The most common molecules within Earth's exosphere are those of the lightest atmospheric gases. [[Hydrogen]] is present throughout the exosphere, with some [[helium]], [[carbon dioxide]], and [[atomic oxygen]] near its base. Because it can be hard to define the boundary between the exosphere and outer space, the exosphere may be considered a part of the [[interplanetary medium]] or [[outer space]].

Earth's exosphere produces Earth's [[geocorona]].

===Lower boundary===
{{Main|Thermopause}}
The lower boundary of the exosphere is called the ''thermopause'' or ''exobase''. It is also called the ''critical altitude'', as this is the altitude where [[Barometric formula|barometric conditions]] no longer apply. Atmospheric temperature becomes nearly a constant above this altitude.<ref>Bauer, Siegfried; Lammer,  Helmut. ''Planetary Aeronomy: Atmosphere Environments in Planetary Systems'', [[Springer Publishing]], 2004.</ref> On Earth, the altitude of the exobase ranges from about {{convert|500|to|1000|km|lk=on}} depending on solar activity.<ref>{{cite web |title=Exosphere - overview |url=http://scied.ucar.edu/shortcontent/exosphere-overview |date=2011 |publisher=UCAR |access-date=April 19, 2015 |archive-url=https://web.archive.org/web/20170517071138/https://scied.ucar.edu/shortcontent/exosphere-overview |archive-date=17 May 2017 |url-status=dead }}</ref>

The exobase can be defined in one of two ways:

If we define the exobase as the height at which upward-traveling molecules experience one collision on average, then at this position the [[mean free path]] of a molecule is equal to one pressure [[scale height]]. This is shown in the following. Consider a volume of air, with horizontal area <math>A</math> and height equal to the mean free path <math>l</math>, at pressure <math>p</math> and temperature <math>T</math>. For an [[ideal gas]], the number of molecules contained in it is:
: <math> N = \frac{pAl} {k_{B}T} </math>

where <math>k_B</math> is the [[Boltzmann constant]]. From the requirement that each molecule traveling upward undergoes on average one collision, the pressure is:

: <math> p = \frac{m_{A}Ng} {A} </math>

where <math>m_{A}</math> is the mean molecular mass of the gas. Solving these two equations gives:

: <math> l = \frac{k_{B} T} {m_{A}g} </math>

which is the equation for the pressure scale height. As the pressure scale height is almost equal to the density scale height of the primary constituent, and because the [[Knudsen number]] is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where <math>\mathrm{Kn}(h_{EB}) \simeq 1</math>.

The fluctuation in the height of the exobase is important because this provides atmospheric drag on satellites, eventually causing them to fall from [[orbit]] if no action is taken to maintain the orbit.

===Upper boundary===
[[File:IBEX ENA Feature FInal Fig3.jpg|thumb|Earth's exosphere, [[energetic neutral atoms]] (ENA) and [[magnetosphere]].]]
In principle, the exosphere covers distances where particles are still [[Gravity|gravitationally]] bound to [[Earth]], i.e. particles still have ballistic orbits that will take them back towards Earth. The upper boundary of the exosphere can be defined as the distance at which the influence of solar [[radiation pressure]] on atomic [[hydrogen]] exceeds that of Earth's gravitational pull. This happens at half the distance to the Moon or somewhere in the neighborhood of {{convert|200,000|km}}. The exosphere, observable from space as the [[geocorona]], is seen to extend to at least {{convert|100,000|km}} from Earth's surface.<ref>{{cite web| url=https://scied.ucar.edu/learning-zone/atmosphere/exosphere#:~:text=Since%20the%20exosphere%20gradually%20fades,about%20halfway%20to%20the%20Moon.|title=The Exosphere|website=University Corporation for Atmospheric Research|access-date=October 5, 2022}}</ref>

==Exosphere of other celestial bodies==
If the atmosphere of a celestial body is very tenuous, like the [[atmosphere of the Moon]] or [[atmosphere of Mercury|that of Mercury]], the whole atmosphere is considered exosphere.<ref name=Showman_2014/>

=== The Exosphere of Mercury ===
Many hypotheses exist about the formation of the surface boundary exosphere of [[Mercury (planet)|Mercury]], which has been noted to include elements such as [[sodium]] (Na), [[potassium]] (K), and [[calcium]] (Ca).<ref name=":0">{{Cite journal |last1=Leblanc |first1=F. |last2=Chassefière |first2=E. |last3=Johnson |first3=R. E. |last4=Hunten |first4=D. M. |last5=Kallio |first5=E. |last6=Delcourt |first6=D. C. |last7=Killen |first7=R. M. |last8=Luhmann |first8=J. G. |last9=Potter |first9=A. E. |last10=Jambon |first10=A. |last11=Cremonese |first11=G. |last12=Mendillo |first12=M. |last13=Yan |first13=N. |last14=Sprague |first14=A. L. |date=2007-06-01 |title=Mercury's exosphere origins and relations to its magnetosphere and surface |journal=Planetary and Space Science |series=Highlights in Planetary Science |volume=55 |issue=9 |pages=1069–1092 |doi=10.1016/j.pss.2006.11.008 |bibcode=2007P&SS...55.1069L }}</ref> Each material has been suggested as a result of processes such as impacts, solar wind, and [[degassing]] from the terrestrial body that cause the atoms or molecules to form the planet's exosphere.<ref name=":0" />

Meteoroids have been reported to commonly impact the surface of Mercury at speeds ranging up to 80&nbsp;km/s, which are capable of causing vaporization of both the meteor and surface regolith upon contact.<ref name=":1">{{Cite journal |last1=Berezhnoy |first1=Alexey A. |last2=Klumov |first2=Boris A. |date=2008-06-01 |title=Impacts as sources of the exosphere on Mercury |journal=Icarus |volume=195 |issue=2 |pages=511–522 |doi=10.1016/j.icarus.2008.01.005 |bibcode=2008Icar..195..511B }}</ref> These expulsions can result in clouds of mixed materials due to the force of the impact, which are capable of transporting gaseous materials and compounds to Mercury's exosphere. During the impact, the former elements of the colliding bodies are mostly devolved into atoms rather than molecules that can then be reformed during a cooling, quenching process. Such materials have been observed as Na, NaOH, and O<sub>2</sub>.<ref name=":1" /> However, it is theorized that, though different forms of sodium have been released into the Mercury exosphere via meteor impact, it is a small driver for the concentration of both sodium and potassium atoms overall.<ref name=":1" /> Calcium is more likely to be a result of impacts, though its transport is thought to be completed through [[Photodissociation|photolysis]] of its former oxides or hydroxides rather than atoms released during the moment of impact such as sodium, potassium, and [[iron]] (Fe).<ref name=":1" />  

Another possible method of the exosphere formation of Mercury is due to its unique [[magnetosphere]] and [[solar wind]] relationship. The magnetosphere of this celestial body is hypothesized to be an incomplete shield from the weathering of solar wind. If accurate, there are openings in the magnetosphere in which solar wind is able to surpass the magnetosphere, reach the body of Mercury, and [[Sputtering|sputter]] the components of the surface that become possible sources of material in the exosphere.<ref>{{Cite journal |last1=Potter |first1=A. E. |last2=Morgan |first2=T. H. |date=1990-05-18 |title=Evidence for Magnetospheric Effects on the Sodium Atmosphere of Mercury |journal=Science |language=en |volume=248 |issue=4957 |pages=835–838 |doi=10.1126/science.248.4957.835 |pmid=17811832 |bibcode=1990Sci...248..835P }}</ref><ref name=":2" /> The weathering is capable of eroding the elements, such as sodium, and transporting them to the atmosphere. However, this occurrence is not constant, and it is unable to account for all atoms or molecules of the exosphere.<ref name=":2">{{Cite journal |last1=Killen |first1=R. M. |last2=Potter |first2=A. E. |last3=Reiff |first3=P. |last4=Sarantos |first4=M. |last5=Jackson |first5=B. V. |last6=Hick |first6=P. |last7=Giles |first7=B. |date=2001-09-25 |title=Evidence for space weather at Mercury |journal=Journal of Geophysical Research: Planets |volume=106 |issue=E9 |pages=20509–20525 |doi=10.1029/2000JE001401 |bibcode=2001JGR...10620509K }}</ref>

==See also==
* [[Aeronomy]]
* [[Extraterrestrial atmospheres]]
* [[Extraterrestrial skies]]
* [[List of natural satellites]]

==References==
{{Reflist|refs=
<ref name=Showman_2014>{{cite book| last1=Showman | first1=A. P. | last2=Dowling | first2=T. E. | chapter=Earth as a Planet: Atmosphere and Oceans| chapter-url=https://books.google.com/books?id=0bEMAwAAQBAJ&dq=exosphere&pg=PA427 | title=Encyclopedia of the Solar System | editor1-first= T. | editor1-last=Spohn | editor2-first=D. | editor2-last=Breuer | editor3-first=T. | editor3-last=Johnson | edition=3 | date=2014 | publisher=Elsevier | pages=427 | isbn=9780124160347}}</ref>
}}

== Further reading ==
* {{cite book |doi=10.1007/978-3-642-97123-5 |title=Physics of the Earth's Space Environment |date=2004 |last1=Prölss |first1=Gerd W. |isbn=978-3-642-05979-7 }}

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