An Etymological Dictionary of Astronomy and Astrophysics
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A process by which a planet loses its atmospheric gases to space. There are three main types: 1) → thermal escape, 2) → suprathermal escape (or → nonthermal escape), and 3) → impact erosion. According to models, the two mechanisms that can most efficiently cause substantial atmospheric loss are hydrodynamic escape and impact erosion (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ atmospheric; → escape.

1) To get away; to get free of.
2) An act or instance of escaping.

From M.E. escapen; O.Fr. eschaper, from V.L. *excappare, literally "to get out of one's cape, leave a pursuer with just one's cape," from L. → ex- "out" + L.L. cappa "mantle."

Gorixtan, goriz- "to escape; to flee, run away;" Mid.Pers. virextan; Proto-Iranian *vi-raik, from vi- "apart, asunder" + *raik; Av. raek- "to leave, set free, let off;" Mid./Mod.Pers. reg/rig (in mordé-rig "inheritance"); Skt. ric- "to leave," rinakti "gives up, evacuates;" Gk. leipein "to leave;" L. linquere "to leave;" from PIE *linkw-, from *leikw- "to leave behind" (cf. Goth. leihvan; O.E. lænan "to lend;" O.H.G. lihan "to borrow;" O.N. lan "loan").

The speed an object must attain in order to free itself from the gravitational influence of an astronomical body. It is the minimum velocity for the object to enter a parabolic trajectory. The escape velocity is given by: V_e = (2GM/r)^1/2, where G is the → gravitational constant, M is the mass of the astronomical body, and r is its radius. The escape velocity of the Earth is about 11.2 km s^-1 that of the Moon is 2.4 km s^-1. The escape velocity from the Sun is about 618 km s^-1, and the solar escape velocity from Earth's orbit is about 42.1 km s^-1.

→ escape; → velocity.

A → thermal escape process by which the atmosphere of a planet loses gases to outer space. This form of thermal escape occurs because some molecules, especially low mass ones, are within the higher-velocity end of the → Maxwell-Boltzmann distribution. The possibility for the gases to escape occurs when the thermal energy of air molecules becomes greater than the → gravitational potential energy of the planet: (3/2)kT = (1/2)mv²  >  GmM/R where v is upward velocity of a molecule of mass m, M is the mass of the planet, and R is the radius of the planet at which thermal escape occurs. The minimum velocity for which this can work is called the → escape velocity is: v_e = (2MG/R)^1/2. Hydrogen molecules (H₂) and helium, or their ions tend to have velocities high enough so that they are not bound by Earth's gravitational field and are lost to space from the top of the atmosphere. This process is important for the loss of hydrogen, a low-mass species that more easily attains escape speed at a given temperature, because v ~ (2kT/m)^1/2. As such, Jeans' escape was likely influential in the atmospheric evolution of all the early terrestrial planets. Jeans' escape currently accounts for a non-negligible fraction of hydrogen escaping from Earth, Mars, and Titan, but it is negligible for Venus because of a cold upper atmosphere combined with relatively high gravity (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ Jeans; → escape.

The process whereby → Lyman continuum photons produced by → massive stars escape from a galaxy without being absorbed by interstellar material. Some observations indicate that the Lyman continuum escape fraction evolves with → redshift.

→ Lyman; → continuum; → escape.

Same as → suprathermal escape.

→ non-; → thermal; → escape.

The time required for a photon created in the Sun's core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks before arriving at the surface by chance. The typical time is approximately 5 x 10⁴ years for a constant density Sun.

→ photon; → escape; → time.

An → atmospheric escape mechanism that occurs where individual atoms or molecules in the atmosphere are raised to → escape velocity because of chemical reactions or ionic interactions. Same as → nonthermal escape (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ supra-; → thermal; → escape.

An → atmospheric escape that occurs when irradiation from a parent star (or a very high heat flux from a planet interior) heats a planetary atmosphere, causing its molecules to escape to space. In basic models, the theory assumes neutral species with a → Maxwell-Boltzmann distribution of velocities, which occurs when collisions between molecules are frequent. Thermal escape has two types: → Jeans' escape and → hydrodynamic escape (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ thermal; → escape.