Jeans escape is a process by which atoms and molecules escape a planetary atmosphere.

Atoms and molecules in an atmosphere in thermodynamic equilibrium have velocities which follow the Maxwell-Boltzmann distribution law. Some of these particles will necessarily be at the high-velocity end of the distribution, and some of these may have velocities in excess of the escape velocity. If these particles are low in the atmosphere, chances are they will encounter another particle before escaping the atmosphere and be deflected, because the mean free path for collisions in a dense atmosphere is much shorter than the depth of the atmosphere itself.

However, if these particles are higher in the atmosphere, the density is lower and the mean free path is larger, perhaps to the point where it is larger than the depth of the atmosphere above the particle. If that is the case, a particle with a velocity higher than the escape velocity will escape the atmosphere. Jeans escape is a slow process, with the rate of escape depending upon the velocity distribution and mass of the particles in the atmosphere. The rate is higher for less massive particles and hotter atmospheres.

One interesting consequence of Jeans escape is the ratio of deuterium (D) to hydrogen (H) observed in the atmosphere of the planet Venus. The D/H ratio on Earth is about 100 times less than that seen on Venus. It is believed that when hydrated molecules (like water or sulphuric acid) are photodissociated in the Venusian atmosphere, the hydrogen atoms are preferentially lost because they have half the mass of deuterium atoms. Deuterium atoms with the same kinetic energy as hydrogen atoms will have velocities lower by a factor of (sqrt(2)). Thus, hydrogen atoms will have an easier time escaping the atmosphere. This acts to raise the D/H ratio over time.

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