The classical thermal escape mechanism is when individual molecules from the high velocity tail of the Maxwell–Boltzmann distribution reaches the escape velocity and overcome the gravity field. This is known as Jeans escape and depend 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).
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". The heaviest species of atoms 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 significant hydrodynamic escape due to the heat input from planetary accretion processes. For comparison, Pluto may have lost 0.5% of its total mass over the age of the Solar System due to hydrodynamic escape.
Exoplanets that are extremely close to their star, such as hot Jupiters can experience significant hydrodynamic escape to the point that they cease to be gas giants and are left with just the core, at which point they would be called Chthonian planets.
- 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)
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