|
|
| Line 1: |
Line 1: |
| [[File:EarthAtmosphereBig.jpg|thumb|80px|right|Earth atmosphere diagram showing the exosphere and other layers. The layers are to scale. From Earth's surface to the top of the stratosphere (50km) is just under 12% of Earth's radius.]]
| | My name is Michale and I am studying Computer Science and Occupational Therapy at San Fantino / Italy.<br><br>Also visit my website homes for sale in sacramento ca ([http://www.systems-biology.org/~myukiko/FCSB2008/doku.php?id=profile_odessabilliot http://www.systems-biology.org/~myukiko/FCSB2008/doku.php?id=profile_odessabilliot]) |
| The '''exosphere''' ({{lang-grc|ἔξω}} {{lang|grc-Latn|''éxō''}} "outside, external, beyond", {{lang-grc|σφαῖρα}} {{lang|grc-Latn|''sphaĩra''}} "sphere") is a thin, atmosphere-like volume surrounding a planetary body where [[molecule]]s are gravitationally bound to that body, but where the density is too low for them to behave as a gas by [[particle collision|colliding]] with each other. In the case of bodies with substantial atmospheres, such as the [[Atmosphere of Earth|Earth's atmosphere]], the exosphere is the uppermost layer, where the atmosphere thins out and merges with interplanetary space. It is located directly above the [[thermosphere]].
| |
| | |
| Several moons, such as [[Earth's moon]] and the [[Galilean moons|Galilean satellites]], have exospheres without a denser atmosphere underneath. Here molecules are ejected from surface rocks and follow independent [[parabolic trajectory|parabolic trajectories]] until they collide with the surface. Authors differ as to whether such moons are considered to have atmospheres or not. 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==
| |
| The main gases within the Earth's exosphere are the lightest atmospheric gases, mainly [[hydrogen]], with some [[helium]], [[carbon dioxide]], and [[atomic oxygen]] near the base of the exosphere. Since there is no clear boundary between outer space and the exosphere, the exosphere is sometimes considered a part of [[outer space]].
| |
| | |
| ===Lower boundary===
| |
| {{Main|Thermopause}}
| |
| The lower boundary of the exosphere is known as ''exopause''; it is also called the ''[[exobase]]'', as in Earth's atmosphere the atmospheric temperature becomes nearly a constant above this altitude. Before the term exobase was established the boundary was also called the ''critical altitude'' where [[Barometric formula|barometric conditions]] no longer apply.<ref>Bauer & Lammer, Planetary Aeronomy: Atmosphere Environments in Planetary Systems, [[Springer Publishing|Springer]], 2004.</ref> The altitude of the exobase ranges from about {{convert|500|to|1000|km|lk=on}} depending on solar activity. {{Citation needed|date=January 2009}}
| |
| | |
| 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} {RT} </math> | |
| | |
| where <math>R</math> is the universal [[gas 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{R 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 since 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=== | |
| In principle, the exosphere covers all distances where particles are still [[Gravity|gravitationally]] bound to [[Earth]], i.e. particles still have ballistic orbits that will take them back towards Earth. Theoretically, 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 the Earth’s gravitational pull. This happens at half the distance to the Moon ({{convert|190000|km}}). The exosphere observable from space as the [[geocorona]] is seen to extend to at least {{convert|10,000|km}} from the surface of the Earth. The exosphere is a transitional zone between Earth’s atmosphere and [[interplanetary space]].
| |
| | |
| {{-}}
| |
| | |
| ==References==
| |
| {{portal|Atmospheric sciences}}
| |
| {{reflist}}
| |
| {{-}}
| |
| ==External links==
| |
| * Gerd W. Prolss: ''Physics of the Earth's Space Environment: An Introduction''. ISBN 3-540-21426-7
| |
| | |
| {{Earth's atmosphere}}
| |
| | |
| [[Category:Atmosphere]]
| |
| [[Category:Atmosphere of Earth]]
| |
My name is Michale and I am studying Computer Science and Occupational Therapy at San Fantino / Italy.
Also visit my website homes for sale in sacramento ca (http://www.systems-biology.org/~myukiko/FCSB2008/doku.php?id=profile_odessabilliot)