Hydrodynamic escape

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Hydrodynamic escape refers to a thermal atmospheric escape mechanism that can lead to the escape of heavier atoms of a planetary atmosphere through numerous collisions with lighter atoms.

The classical thermal escape mechanism is when individual molecules from the high velocity tail of the Maxwell–Boltzmann distribution reach the escape velocity and overcome the gravity field. This is known as Jeans escape and depends on the temperature of the planet's exosphere and the strength of its gravity field. It can be shown that for cold giant gas planets such as Jupiter and Saturn there is no thermal driven atmospheric escape of significance while for smaller and warmer planets such as Earth only light atoms may escape in this manner (heavier atoms stay).[1]

Hydrodynamic escape occurs if there is a strong thermal driven atmospheric escape of light atoms which through drag effects (collisions) also drive off heavier atoms—a bulk flow type of escape of the upper atmosphere, a so-called "blowoff".[2] The heaviest species of atom that can be removed in this manner is called the cross-over mass.

It requires a large source of energy at a certain altitude to maintain a significant hydrodynamic escape. Solar radiation is seldom enough for known present day atmospheres in the Solar System. It is speculated that early atmospheres of Earth, Venus and Mars have experienced periods of hydrodynamic escape due to the heat input from planetary accretion processes.[3]

Exoplanets that are extremely close to their parent star, such as hot Jupiters can experience significant hydrodynamic escape[4][5] to the point where the star "burns off" their atmosphere upon which they cease to be gas giants and are left with just the core, at which point they would be called Chthonian planets.


  1. ^ Irwin, Patrick G. J. (2006). Giant planets of our solar system: an introduction. Birkhäuser. p. 56. ISBN 3-540-31317-6. Retrieved 22 Dec 2009. (table 3.1)
  2. ^ Irwin, Patrick G. J. (2006). Giant planets of our solar system: an introduction. Birkhäuser. p. 58. ISBN 3-540-31317-6. Retrieved 22 Dec 2009.
  3. ^ Pater, Imke De; Jack Jonathan Lissauer (2001). Planetary sciences. Cambridge University Press. p. 129. ISBN 0-521-48219-4.
  4. ^ Tian, Feng; Toon, Owen B.; Pavlov, Alexander A.; de Sterck, H. (March 10, 2005). "TRANSONIC HYDRODYNAMIC ESCAPE OF HYDROGEN FROM EXTRASOLAR PLANETARY ATMOSPHERES". The Astrophysical Journal. 621: 1049–1060. Bibcode:2005ApJ...621.1049T. CiteSeerX doi:10.1086/427204.
  5. ^ Swift, Damian C.; Eggert, Jon; Hicks, Damien G.; Hamel, Sebastien; Caspersen, Kyle; Schwegler, Eric; Collins, Gilbert W. "Mass-radius relationships for exoplanets". arXiv:1001.4851. Bibcode:2012ApJ...744...59S. doi:10.1088/0004-637X/744/1/59.

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