Atmospheric escape

Atmospheric escape removes gas from a planet through thermal motion, hydrodynamic outflow, photochemistry, and interactions with stellar radiation and plasma.

For particles of mass $m$ at radius $r$ and temperature $T$, the Jeans escape parameter isLarge $\lambda$ gives weak tail escape, while order-unity $\lambda$ indicates hydrodynamic blow-off.

For exobase number density $n_e$ and $v_{\mathrm{th}}=\sqrt{2k_{B}T/m}$, the collisionless Jeans number flux is

Hydrodynamic atmospheric escape is a collisional transonic outflow in which bulk gas carries multiple species away. It replaces the hydrostatic Jeans-tail description when gravity is weak compared with thermal energy.