Reference Work Entry

Encyclopedia of Astrobiology

pp 879-880

Jeans Escape

  • Daniel RouanAffiliated withLESIA, Observatoire Paris-Site de Meudon Email author 


Thermal escape


The Jeans escape phenomenon is one of the mechanisms by which a planet can loose gradually some constituents of its atmosphere. It is named after James Hopwood Jeans (1877–1946), an English astrophysicist. The process corresponds to the probability for a molecule to travel a distance larger than the atmospheric scale height without colliding with another molecule, at a speed larger than the planet’s escape velocity. The average thermal velocity of a molecule in a gas is proportional to the square root of the temperature and inversely proportional to the square root of its mass, while the escape velocity increases with the mass of the planet. One concludes that a planet will more rapidly lose its atmosphere if it is of a low mass and if its atmosphere is hot and made of light elements such as hydrogen and helium. In the solar system, giant planets which are massive and rather cold do not lose their hydrogen atmosphere, while small planet ...

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