Exosfeer

Exosfeer, or the exosphere, is the outermost layer of a planet’s atmosphere, distinguished by extremely low densities and by the fact that collisions between particles are rare. In Earth’s atmosphere, the exosphere begins at around 500–1,000 kilometers above the surface at the exobase and extends upward to about 10,000 kilometers, where it smoothly transitions into interplanetary space. The exact boundaries vary with solar activity and atmospheric temperature.

The exosphere contains mostly light particles, such as atomic hydrogen and helium, with heavier atoms and molecules present only in trace amounts. Because particle collisions are infrequent, particles follow ballistic or escaping trajectories rather than behaving as a tightly bound gas. The concept of a single temperature is not very meaningful here; temperatures can be high due to solar heating, but this reflects particle velocities in a sparse distribution rather than a conventional thermodynamic state.

Escape processes dominate the dynamics of the exosphere. Light species can reach escape velocity through thermal (Jeans) escape and other non-thermal processes, leading to gradual loss of atmospheric material to space. The layer is also a boundary region where the planet’s atmosphere interacts with the solar wind and the magnetosphere, influencing space weather conditions and the composition of near-space environments.

Because of its extreme rarity, the exosphere is difficult to observe directly; its presence is inferred from ultraviolet observations, satellite measurements, and models of atmospheric loss. It serves as a transitional zone between the atmosphere and space, linking planetary climate to long-term atmospheric evolution.