Atmospheric escape

Atmospheric escape

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Atmospheric escape is the loss of planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

ary atmospheric
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 gases to outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

.

Thermal escape mechanisms


One classical thermal escape mechanism is Jeans escape. In a quantity of gas, the average velocity of a molecule is determined by temperature, but the velocity of individual molecules varies continuously as they collide with one another, gaining and losing kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

. The variation in kinetic energy among the molecules is described by the Maxwell distribution. The kinetic energy and mass of a molecule determine its velocity by .

Individual molecules in the high tail
Long tail
Long tail may refer to:*The Long Tail, a consumer demographic in business*Power law's long tail, a statistics term describing certain kinds of distribution*Long-tail boat, a type of watercraft native to Southeast Asia...

 of the distribution may reach escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

, at a level in the atmosphere where the mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...

 is comparable to the scale height
Scale height
In various scientific contexts, a scale height is a distance over which a quantity decreases by a factor of e...

, and leave the atmosphere.

The more massive the molecule of a gas is, the lower the average velocity of molecules of that gas at a given temperature, and the less likely it is that any of them reach escape velocity.

This is why hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 escapes from a given atmosphere more easily than carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

. Also, if the planet has a higher mass, the escape velocity is greater, and fewer particles will escape. This is why the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

 planets have significant amounts of hydrogen and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

, which escape on Earth. Distance from the star also plays a part; a close planet has a hotter atmosphere, with a faster range of velocities, and more chance of escape. A distant body has a cooler atmosphere, with a slower range of velocities, and less chance of escape. This helps Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, which is small compared to Earth but further from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, keep its atmosphere.

However, while it has not been observed, it is theorized that an atmosphere with a high enough pressure and temperature can undergo a 'blow-off'. In this situation molecules basically just flow off into space. Here it is possible to lose heavier molecules that would not normally be lost.

Significance of solar winds


The relative importance of each loss process is a function of planet mass, atmosphere composition, and distance from a star. A common erroneous belief is that the primary non-thermal escape mechanism is atmospheric stripping by a solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 in the absence of a magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

. Excess kinetic energy from solar winds can impart sufficient energy into atmospheric particles to reach escape velocity, causing atmospheric escape. The solar wind, composed of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s, is deflected by magnetic fields because the charged particles within the wind flow along magnetic field lines. The presence of a magnetic field thus deflects solar winds, preventing atmospheric loss to solar winds. On Earth, for instance, the interaction between the solar wind and magnetic field deflects the solar wind around the planet, with near total deflection around 10 Earth radii away. This region of deflection is called a bow shock
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....

.

Depending on planet size and atmospheric composition, however, a lack of magnetic field does not determine the fate of a planet's atmosphere. Venus, for instance, has no powerful magnetic field. Its close proximity to the sun also increases the speed and number of particles, and would presumably cause the atmosphere to be stripped almost entirely, much like that of Mars. Despite this, the atmosphere of Venus is two orders of magnitudes denser than Earth’s. Recent models indicate that stripping by solar wind accounts for less than 1/3 of total non-thermal loss processes.

While Venus and Mars have no magnetosphere to protect the atmosphere from solar winds, photoionizing radiation (sunlight) and the interaction of the solar wind with the atmosphere of the planets causes ionization of the uppermost part of the atmosphere. This ionized region of atmosphere, in turn, induces magnetic moments that deflect solar winds much like a magnetic field, limiting solar wind effects to the uppermost altitudes of atmosphere, roughly 1.2-1.5 planetary radii away from the planet, or an order of magnitude closer to the surface than Earth's magnetic field creates. Past this region, also called a bow shock, the solar wind is slowed to subsonic
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

 velocities. Nearer to the surface, solar wind dynamic pressure balances with pressure from the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

, at a region called the ionopause. This interaction typically prevents solar wind stripping from being the dominant loss process of the atmosphere.

Comparison of non-thermal loss processes based on planet and particle mass


Dominant non-thermal loss processes differ based on the planetary body in discussion. The varying relative significance of each process is based on planetary mass, atmospheric composition, and distance from the sun. The dominant nonthermal loss processes for Venus and Mars, two terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

s without magnetic fields, are dissimilar. The dominant nonthermal loss process on Mars is pick-up from solar winds, because the atmosphere is not dense enough to shield itself from the winds during peak solar activity. Venus is somewhat shielded from solar winds by merit of a denser atmosphere, and solar pick-up is not the dominant nonthermal loss process on Venus. Smaller bodies without magnetic fields are more likely to suffer from solar winds, because the planet is too small to hold sufficient atmosphere to stop solar winds.

The dominant loss process for Venus is loss through electric force field acceleration. Because electrons are more mobile than other particles, they are more likely to escape from the top of the ionosphere of Venus. As a result, a minor net positive charge can develop. The net positive charge, in turn, creates an electric field that can accelerate other positive charges out of the system. Through this, H+ ions are accelerated beyond escape velocity, causing atmospheric escape through this process. Other important loss processes on Venus are photochemical
Photochemistry
Photochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...

 reactions, driven by proximity to the Sun. Photochemical reactions rely on splitting the molecules into constituent atoms, often with a significant portion of kinetic energy maintained in the less massive particle. This particle is of sufficiently low mass and high kinetic energy to escape from Venus. Oxygen, relative to hydrogen, is not of sufficiently low mass to escape through this mechanism on Venus.

Phenomena of non-thermal loss processes on moons with atmospheres


Several moons within our solar system have atmospheres and are subject to atmospheric loss processes. They typically have no magnetic fields of their own, but orbit planets with powerful magnetic fields. Many of these moons lie within the magnetic fields generated by the planets and are less likely to undergo sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...

 and pick-up. The shape of the bow shock, however, allows for some moons, such as Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, to pass through the bow shock when their orbits takes them between the sun and their primary. Titan spends roughly half of its transit time outside of the bow-shock and being subjected to unimpeded solar winds. The kinetic energy gained from pick-up and sputtering associated with the solar winds increases thermal escape throughout the transit of Titan, causing neutral hydrogen to escape from the moon. The escaped hydrogen maintains an orbit following in the wake of Titan, creating a neutral hydrogen torus
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

 around Saturn. Io, in its transit around Jupiter, encounters a plasma cloud. Interaction with the plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 cloud induces sputtering, kicking off sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 particles. The interaction produces a stationary banana
Banana
Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....

-shaped charged sodium cloud along a part of the orbit of Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

.

Impact erosion


The impact
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

 of a large meteoroid
Meteoroid
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...

 can lead to the loss of atmosphere. If a collision is energetic enough, it is possible for ejecta, including atmospheric molecules, to reach escape velocity. Just one impact such as the Chicxulub event
Chicxulub Crater
The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...

 does not lead to a significant loss, but the terrestrial planets went through enough impacts when they were forming for this to matter.

Sequestration


This is a loss, not an escape; it is when molecules solidify out of the atmosphere onto the surface. This happens on Earth, when water vapor forms glacial ice
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

 or when carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 is sequestered in sediments
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

. The dry ice cap
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....

s on Mars are also an example of this process.

One mechanism for sequestration is chemical; for example, most of the carbon dioxide of the Earth's original atmosphere has been chemically sequestered into carbonate rock. Very likely a similar process has occurred on Mars. Oxygen can be sequestered by oxidation of rocks; for example, by increasing the oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

s of ferric rocks from Fe2+ to Fe3+. Gases can also be sequestered by adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...

, where fine particles in the regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 capture gas which adheres to the surface of particles.

Dominant atmospheric escape and loss processes on Earth


Earth is too large to lose particles efficiently through Jeans Escape. The exosphere
Exosphere
The exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...

 is the high altitude region where atmospheric density is sparse and Jeans Escape occurs. Jeans escape calculations assuming an exosphere temperature of 1,800 degrees show that to deplete O+ ions by a factor of e
E (mathematical constant)
The mathematical constant ' is the unique real number such that the value of the derivative of the function at the point is equal to 1. The function so defined is called the exponential function, and its inverse is the natural logarithm, or logarithm to base...

 (2.78...) would take nearly a billion years. 1,800 degrees is higher than the actual observed exosphere temperature; at the actual average exosphere temperature, depletion of O+ ions would not occur even over a trillion years. Furthermore, most oxygen on Earth is bound as O2, which is too massive to escape Earth by Jeans Escape.

Earth’s magnetic field protects it from solar winds and prevents escape of ions, except along open field lines at the magnetic pole
Magnetic pole
Magnetic pole may refer to:* One of the two ends of a magnet* The magnetic poles of astronomical bodies, a special case of magnets, two special cases of which are the Geomagnetic poles:...

s. The gravitational attraction of Earth’s mass prevents other non-thermal loss processes from appreciably depleting the atmosphere. Yet Earth’s atmosphere is two orders of magnitude less dense than that of Venus at the surface. Because of the temperature regime of Earth, CO2 and H2O are sequestered in the hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

 and lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

. H2O vapor is sequestered as liquid H2O in oceans, greatly decreasing the atmospheric density. With liquid water running over the surface of Earth, CO2 can be drawn down from the atmosphere and sequestered in sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s. Some estimates indicate that nearly all carbon on Earth is contained in sedimentary rocks, with the atmospheric portion being approximately 1/250,000 of Earth’s CO2 reservoir. If both of the reservoirs were released to the atmosphere, Earth’s atmosphere would be denser than even Venus’s atmosphere. Therefore, the dominant “loss” mechanism of Earth’s atmosphere is not escape to space, but sequestration.

Sources

  • Hunten, D.M., 1993, "Atmospheric evolution of the terrestrial planets", Science, v. 259, no. 5097, p. 915-920.
  • Lammer, H., and Bauer, S.J., 1993, "Atmospheric mass-loss from Titan by sputtering", Planetary and Space Science, v. 41, no. 9, p. 657-663.
  • Lammer, H., Lichtenegger, H.I.M., Biernat, H.K., Erkaev, N.V., Arshukova, I.L., Kolb, C., Gunell, H., Lukyanov, A., Holmstrom, M., Barabash, S., Zhang, T.L., and Baumjohann, W., 2006, "Loss of hydrogen and oxygen from the upper atmosphere of Venus", Planetary and Space Science, v. 54, no. 13-14, p. 1445-1456.
  • Lammer, H., Stumptner, W., and Bauer, S.J., 1998, "Dynamic escape of H from Titan as consequence of sputtering induced heating" Planetary and Space Science', v. 46, no. 9-10, p. 1207-1213.
  • Shizgal, B.D., and Arkos, G.G., 1996, "Nonthermal escape of the atmospheres of Venus, Earth, and Mars", Reviews of Geophysics, v. 34, no. 4, p. 483-505.
  • Wilson, J.K., Mendillo, M., Baumgardner, J., Schneider, N.M., Trauger, J.T., and Flynn, B., 2002, "The dual sources of Io's sodium clouds" Icarus, v. 157, no. 2, p. 476-489.
  • Kevin J. Zahnle and David C. Catling, May 2009, "Our Planet's Leaky Atmosphere" Scientific American.