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Atmospheric escape
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There are several different processes that can lead to the escape of a planetary atmosphere. In some cases this can be a very important process; for example, both Venus and Mars have probably lost much of their water due to atmospheric escape since they have weaker gravity than Earth.
Thermal escape mechanisms
One classical thermal escape mechanism is Jeans escape. It is the escape of individual molecules from the high tail of the Maxwell distribution, at a level in the atmosphere where the mean free path is comparable to the scale height.
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Encyclopedia
There are several different processes that can lead to the escape of a planetary atmosphere. In some cases this can be a very important process; for example, both Venus and Mars have probably lost much of their water due to atmospheric escape since they have weaker gravity than Earth.
Thermal escape mechanisms
One classical thermal escape mechanism is Jeans escape. It is the escape of individual molecules from the high tail of the Maxwell distribution, at a level in the atmosphere where the mean free path is comparable to the scale height. Maxwell's distribution prescribes the kinetic energy distribution of the molecules, which depends on the mass and the velocity according to .
From this dependence, we see that the more massive a gas molecule is, the lower its average speed at a given temperature, meaning it is less likely to escape. This is why hydrogen escapes from a given atmosphere more easily than carbon dioxide. Also, if the planet has a higher mass, the escape velocity is greater, and fewer particles will escape. This is why the gas giant planets are able to have significant amounts of hydrogen and helium, while they escape on Earth. The distance to the Sun also plays a part; a close planet has a hotter atmosphere, which generally leads to a faster range of velocities, and more chance of escape. This helps Titan, which is small compared to Earth but further from the Sun, 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 than 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. Most people erroneously think that the primary non-thermal escape mechanism is atmospheric stripping by a solar wind in the absence of a magnetic field. 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 ions, 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.
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, Venus has an atmosphere two order 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, 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 velocities . Nearer to the surface, solar wind dynamic pressure balances with pressure from the ionosphere, at a region called the ionopause. This interaction typically prevents solar wind stripping from being the dominant loss process of atmosphere.
On planets without a magnetosphere, some combination of solar wind mechanisms very often dominate atmospheric escape. Both Venus and Mars are currently losing their water this way. First, the water is dissociated into hydrogen and oxygen by ultraviolet light from the Sun, and then the light hydrogen is pulled away in the solar wind.
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 bodies without magnetic fields, are dissimilar (table 2). 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 reactions, driven by proximity to the sun. Notably, oxygen atoms are too heavy to escape Venus by this process. Photo-chemical 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 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 and pick-up. The shape of the bow-shock, however, allows for some moons, such as Titan, to pass through the bow-shock when its orbit takes it between the sun and Saturn. 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 around Saturn. Io, in its transit around Jupiter, encounters a plasma cloud . Interaction with the plasma cloud induces sputtering, kicking off sodium particles. The interaction produces a stationary banana-shaped charged sodium cloud along a part of the orbit of Io.
Impact erosion
The impact of a large meteoroid 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 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 perhaps more of a loss than an escape, because this is when molecules solidify out of the atmosphere onto the surface. This happens on Earth in glaciers or when carbon is lost to sediments. The dry ice caps 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 at Mars. Oxygen can be sequestered by oxidation of rocks, for example, by increasing the oxidation states of ferric rocks from Fe2+ to Fe3+. Gases can also be sequestered by adsorption, where fine particles in the regolith capture gas which adheres to the surface of grains.
Dominant Atmospheric Escape and Loss Processes on Earth
Earth is too large to efficiently lose particles through Jeans Escape. Through Jeans escape calculations, using a temperature of 1800 degrees at Earth’s exosphere (the exosphere is a region of high altitude and sparse atmospheric density where Jean’s Escape occurs, and the modeled temperature of 1800 degrees is greater than the observed exosphere temperature on Earth), we find that it takes nearly a billion years for one e-folding depletion of O+ ions for Earth. The average exosphere temperature of Earth will not allow depletion of these ions on a trillion year timescale. Moreover, most oxygen on Earth is bound as O2, which cannot escape Earth by Jeans Escape.
Earth’s magnetic field protects it from solar winds and prevents escape of ions, except at open field lines in the poles. Earth’s mass, increasing gravitational attraction, prevents other non-thermal loss processes from appreciably depleting the atmosphere. Yet Earth’s atmosphere is two order 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 and 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 rocks. Some estimates indicate that carbon is trapped 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.
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