egzospere

Egzospere, also known as the exosphere, is the outermost layer of a planet’s atmosphere. It marks the transition between the atmosphere and outer space, where particle densities become so low that collisions between molecules are extremely rare and individual particles can follow ballistic or escaping trajectories.

In Earth's atmosphere, the egzospere extends roughly from about 600 kilometers above the surface to tens of thousands of kilometers, gradually fading into the vacuum of space. The lower boundary is defined by the exobase, where the mean free path of particles becomes comparable to the scale height, effectively ending collisional behavior. The composition of the egzospere varies with altitude and solar activity, with lighter species such as hydrogen and helium prevailing at higher altitudes, and heavier species being more common closer to the exobase but still at extremely low densities.

Particle dynamics in the egzospere are dominated by free motion rather than collisions. Some particles remain bound by Earth's gravity, following long, ballistic paths, while others have sufficient energy to escape into space. Escape mechanisms include thermal (Jeans) escape, where high-velocity particles depart the atmosphere, and non-thermal processes driven by solar radiation, chemical reactions, or interactions with the solar wind.

Observationally, the egzospere is challenging to measure directly due to its thinness. It is studied via ultraviolet and infrared spectroscopy, solar wind interactions, and satellite drag measurements that infer upper-atmosphere conditions. The egzospere plays a crucial role in atmospheric loss, space weather interactions, and the long-term evolution of a planet’s atmosphere. The term is commonly used in planetary science and astronomy; in English, it is typically referred to as the exosphere.