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The exosphere is a thin, atmosphere-like volume surrounding a
planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
or
natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
where
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of bodies with substantial atmospheres, such as
Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...
, the exosphere is the uppermost layer, where the atmosphere thins out and merges with outer space. It is located directly above the
thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...
. Very little is known about it due to a lack of
research Research is creative and systematic work undertaken to increase the stock of knowledge. It involves the collection, organization, and analysis of evidence to increase understanding of a topic, characterized by a particular attentiveness to ...
. Mercury, the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, Ceres, Europa, and Ganymede have surface boundary exospheres, which are exospheres without a denser atmosphere underneath. The Earth's exosphere is mostly
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
, with some heavier
atoms Atoms are the basic particles of the chemical elements. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other ...
and
molecules A molecule is a group of two or more atoms that are held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemistry ...
near the base.


Surface boundary exosphere

Mercury, Ceres and several large natural satellites, such as the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, Europa, and Ganymede, have exospheres without a denser atmosphere underneath, referred to as a surface boundary exosphere. 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

The most common molecules within Earth's exosphere are those of the lightest atmospheric gases.
Hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
is present throughout the exosphere, with some
helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
,
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
, 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 The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planet A dwarf planet is a small planetary ...
or outer space. Earth's exosphere produces Earth's geocorona.


Lower boundary

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. On Earth, the altitude of the exobase ranges from about depending on solar activity. 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 In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
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 and height equal to the mean free path l, at pressure p and temperature T. For an
ideal gas An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to interparticle interactions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is ...
, the number of molecules contained in it is: : N = \frac where k_B is the
Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative thermal energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin (K) and the ...
. From the requirement that each molecule traveling upward undergoes on average one collision, the pressure is: : p = \frac where m_ is the mean molecular mass of the gas. Solving these two equations gives: : l = \frac 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 The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid. The number is nam ...
is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where \mathrm(h_) \simeq 1. The fluctuation in the height of the exobase is important because this provides atmospheric drag on satellites, eventually causing them to fall from
orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
if no action is taken to maintain the orbit.


Upper boundary

In principle, the exosphere covers distances where particles are still gravitationally bound to
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
, 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 Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of ...
on atomic
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
exceeds that of Earth's gravitational pull. This happens at half the distance to the Moon or somewhere in the neighborhood of . The exosphere, observable from space as the geocorona, is seen to extend to at least from Earth's surface.


Exosphere of other celestial bodies

If the atmosphere of a celestial body is very tenuous, like the
atmosphere of the Moon The atmosphere of the Moon is a very sparse layer of gases surrounding the Moon, consisting only of an exosphere. For most practical purposes, the Moon is considered to be surrounded by vacuum. The elevated presence of atomic and molecular parti ...
or that of Mercury, the whole atmosphere is considered exosphere.


The Exosphere of Mercury

Many hypotheses exist about the formation of the surface boundary exosphere of Mercury, which has been noted to include elements such as
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
(Na),
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
(K), and
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
(Ca). 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. Meteoroids have been reported to commonly impact the surface of Mercury at speeds ranging up to 80 km/s, which are capable of causing vaporization of both the meteor and surface regolith upon contact. 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 O2. 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. Calcium is more likely to be a result of impacts, though its transport is thought to be completed through
photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...
of its former oxides or hydroxides rather than atoms released during the moment of impact such as sodium, potassium, and
iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
(Fe).   Another possible method of the exosphere formation of Mercury is due to its unique
magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
and
solar wind The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
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
sputter In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and c ...
the components of the surface that become possible sources of material in the exosphere. 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.


See also

*
Aeronomy Aeronomy is the scientific study of the upper atmosphere of the Earth and corresponding regions of the atmospheres of other planets. It is a branch of both atmospheric chemistry and atmospheric physics. Scientists specializing in aeronomy, know ...
* Extraterrestrial atmospheres * Extraterrestrial skies *
List of natural satellites Of the Solar System's eight planets and its list of possible dwarf planets, nine most likely dwarf planets, six planets and seven dwarf planets are known to be orbited by at least 430 natural satellites, or moons. At least List of gravitational ...


References


Further reading

* {{Authority control Atmosphere