exobase

The exobase is the altitude in a planetary atmosphere where the atmosphere transitions from a collisional, fluid-like layer to a collisionless, free-molecule regime. At and above this boundary, the mean free path of atmospheric particles becomes comparable to the vertical scale height, and collisions become rare. In many treatments the exobase is defined when the Knudsen number (the ratio of mean free path to a characteristic length) is of order one, or when the atmosphere ceases to behave as a continuum.

In practice, the exobase marks the lower boundary of the exosphere, the outermost atmospheric layer where particles can follow ballistic, or nearly collisionless, trajectories. Above the exobase, molecules may escape to space or be gravitationally trapped by the planet, depending on their velocity and the body's gravity and radiation environment. Thermal escape (Jeans escape) and various non-thermal processes contribute to mass loss across this boundary.

Altitude of the exobase varies with planetary gravity, atmospheric composition, and solar activity. For Earth, the exobase lies several hundred to about a thousand kilometers above the surface, fluctuating with temperature and solar input. Other planets and moons exhibit different exobase heights. Observationally, the exobase is inferred from models of atmospheric structure, along with in situ measurements and remote sensing data.

Understanding the exobase is important for estimating atmospheric escape rates, long-term evolution of atmospheres, and the coupling between the lower atmosphere and space environments.