exosphere

The exosphere is the outermost layer of a planet’s atmosphere, forming the transition region between the atmosphere and outer space. It begins near the exobase, the altitude where particle collisions become rare, and extends outward until the atmosphere blends into the solar wind or magnetospheric environment. In the exosphere, densities are extremely low and particles can travel long distances without colliding.

On Earth, the exosphere starts at roughly several hundred kilometers above the surface and can extend up to about 10,000 kilometers or more. Its composition is dominated by light species such as hydrogen and helium, though heavier elements can be present in trace amounts. The boundary is not fixed; it is defined by the onset of collisionless behavior rather than a sharp altitude.

The exosphere is governed by weak interactions and various escape processes. Thermal escape, or Jeans escape, occurs when particles in the tail of the velocity distribution exceed the planet’s escape velocity. Non-thermal processes, including charge exchange with solar wind, photochemical reactions, and sputtering, also contribute to atmospheric loss. The exosphere gradually thins with altitude and can be influenced by solar activity, gravity, magnetic fields, and interactions with the solar wind.

Observations of exospheres rely on ultraviolet and spectroscopic techniques, revealing escaping hydrogen and other species through emissions such as Lyman-alpha. In addition to Earth, exospheres exist around other planets with atmospheres, and exoplanet studies use transit spectroscopy to detect extended exospheres around distant worlds. The exosphere plays a key role in the long-term evolution of planetary atmospheres.