The exosphere (Ancient Greek: ἔξωéxō "outside, external, beyond", Ancient Greek: σφαῖραsphaĩra "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less.[1] In the case of bodies with substantial atmospheres, such as 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, the Moon, Ceres, Europa, and 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.[2]
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.
Mercury, Ceres and several large natural satellites, such as the Moon, Europa, and Ganymede, have exospheres without a denser atmosphere underneath,[3] referred to as a surface boundary exosphere.[4] Here, molecules are ejected on 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 exosphereEdit
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.
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 conditions no longer apply. Atmospheric temperature becomes nearly a constant above this altitude.[5] On Earth, the altitude of the exobase ranges from about 500 to 1,000 kilometres (310 to 620 mi) depending on solar activity.[6]
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 and height equal to the mean free path , at pressure and temperature . For an ideal gas, the number of molecules contained in it is:
where is the Boltzmann constant. From the requirement that each molecule traveling upward undergoes on average one collision, the pressure is:
where is the mean molecular mass of the gas. Solving these two equations gives:
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 .
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 boundaryEdit
In principle, the exosphere covers distances where particles are still 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 200,000 kilometres (120,000 mi). The exosphere, observable from space as the geocorona, is seen to extend to at least 100,000 kilometres (62,000 mi) from Earth's surface.[7] Other scientists consider the exosphere to end at around 10,000 kilometres (6,200 mi).[8]
^Showman A. P., Dowling T. E. (2014). "Earth as a Planet: Atmosphere and Oceans". Encyclopedia of the Solar System (3 ed.). Elsevier. T. Spohn, D. Breuer, T. Johnson. p. 427. ISBN9780124160347.
Further readingEdit
Prölss, Gerd W. (2012) [2004]. Physics of the Earth's Space Environment: An Introduction. Translated by Michael Keith Bird. Springer. ISBN978364297123-5. OCLC 942901197.
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For the racehorse see Exosphere horse See also Extraterrestrial atmospheres and Extraterrestrial skies The exosphere Ancient Greek ἔ3w exō outside external beyond Ancient Greek sfaῖra sphaĩra sphere is a thin atmosphere like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body but where the density is so low that the molecules are essentially collision less 1 In the case of bodies with substantial atmospheres such as 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 the Moon Ceres Europa and 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 2 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 Contents 1 Surface boundary exosphere 2 Earth s exosphere 2 1 Lower boundary 2 2 Upper boundary 3 Exosphere of other celestial bodies 4 See also 5 References 6 Further readingSurface boundary exosphere EditMercury Ceres and several large natural satellites such as the Moon Europa and Ganymede have exospheres without a denser atmosphere underneath 3 referred to as a surface boundary exosphere 4 Here molecules are ejected on 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 EditThe 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 Lower boundary Edit Main article 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 conditions no longer apply Atmospheric temperature becomes nearly a constant above this altitude 5 On Earth the altitude of the exobase ranges from about 500 to 1 000 kilometres 310 to 620 mi depending on solar activity 6 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 A displaystyle A nbsp and height equal to the mean free path l displaystyle l nbsp at pressure p displaystyle p nbsp and temperature T displaystyle T nbsp For an ideal gas the number of molecules contained in it is N p A l k B T displaystyle N frac pAl k B T nbsp where k B displaystyle k B nbsp is the Boltzmann constant From the requirement that each molecule traveling upward undergoes on average one collision the pressure is p m A N g A displaystyle p frac m A Ng A nbsp where m A displaystyle m A nbsp is the mean molecular mass of the gas Solving these two equations gives l k B T m A g displaystyle l frac k B T m A g nbsp 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 K n h E B 1 displaystyle mathrm Kn h EB simeq 1 nbsp 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 Edit In principle the exosphere covers distances where particles are still 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 200 000 kilometres 120 000 mi The exosphere observable from space as the geocorona is seen to extend to at least 100 000 kilometres 62 000 mi from Earth s surface 7 Other scientists consider the exosphere to end at around 10 000 kilometres 6 200 mi 8 Exosphere of other celestial bodies EditIf the atmosphere of a celestial body is very tenuous like the atmosphere of the Moon or that of Mercury the whole atmosphere is considered exosphere 9 See also EditAeronomy List of natural satellitesReferences Edit Steigerwald William 17 August 2015 NASA s LADEE Spacecraft Finds Neon in Lunar Atmosphere NASA Retrieved 18 August 2015 Exosphere an overview ScienceDirect Topics Day Brian 20 August 2013 Why LADEE Matters NASA Ames Research Center Retrieved 19 April 2015 Is There an Atmosphere on the Moon NASA 30 January 2014 Retrieved 4 August 2016 Bauer Siegfried Lammer Helmut Planetary Aeronomy Atmosphere Environments in Planetary Systems Springer Publishing 2004 Exosphere overview UCAR 2011 Archived from the original on 17 May 2017 Retrieved 19 April 2015 The Exosphere University Corporation for Atmospheric Research Retrieved 5 October 2022 Earth s Atmospheric Layers 22 January 2013 Showman A P Dowling T E 2014 Earth as a Planet Atmosphere and Oceans Encyclopedia of the Solar System 3 ed Elsevier T Spohn D Breuer T Johnson p 427 ISBN 9780124160347 Further reading EditProlss Gerd W 2012 2004 Physics of the Earth s Space Environment An Introduction Translated by Michael Keith Bird Springer ISBN 978364297123 5 OCLC 942901197 Portals nbsp Earth sciences nbsp Weather nbsp Astronomy nbsp Stars nbsp Outer space nbsp Science Retrieved from https en wikipedia org w index php title Exosphere amp oldid 1174089224, wikipedia, wiki, book, books, library,
