Exosphere

The exosphere is the outermost and least dense region of a planetary atmosphere, forming a transitional zone between an atmosphere and interplanetary space. It represents a layer in which gas molecules are gravitationally bound to a planetary body yet move so far apart that collisions between them become extremely rare. This near-collisionless condition causes the exosphere to behave differently from the lower atmospheric layers, with properties resembling those of outer space. On Earth, the exosphere lies above the thermosphere and marks the interface where atmospheric gases gradually disperse into the vacuum beyond.

Characteristics and Composition

The exosphere is characterised by extremely low densities, with particles travelling on long ballistic trajectories rather than behaving collectively as a fluid. Molecules and atoms move freely and may reach sufficient velocities to escape a planet’s gravitational influence altogether. For bodies with substantial atmospheres, such as Earth, the exosphere forms the thinning upper boundary of the atmosphere. In contrast, some smaller bodies such as Mercury, the Moon, Ceres, Europa and Ganymede possess surface boundary exospheres in which no denser atmospheric layers lie beneath.
Earth’s exosphere is predominantly composed of hydrogen and helium, the lightest atmospheric gases. Heavier constituents such as carbon dioxide and atomic oxygen occur closer to the exobase, where densities are greater. Because the boundaries of the exosphere are diffuse, it is often considered part of the interplanetary medium. It also generates Earth’s geocorona, a faint cloud of hydrogen visible in ultraviolet light.

Surface Boundary Exospheres

Surface boundary exospheres occur on bodies with extremely tenuous atmospheres. Molecules released from the surface follow ballistic paths until they either escape into space or return to collide with the ground. This differs fundamentally from an atmosphere that becomes progressively thinner with altitude. Moons and small terrestrial bodies with weak gravity commonly exhibit this type of exosphere. Asteroids, however, do not generally maintain exospheres because emitted particles typically escape and are not gravitationally retained.

Earth’s Exosphere and Its Boundaries

The lower boundary, known as the thermopause or exobase, marks the altitude at which gas molecules cease interacting regularly through collisions. Above this height, the barometric formula becomes invalid because the mean free path of particles becomes roughly equal to the pressure scale height. The exobase typically lies between several hundred and more than a thousand kilometres above Earth’s surface, with the exact altitude varying according to solar activity. Here, the Knudsen number approaches unity, indicating that the conditions favour free molecular motion.
Fluctuations in the height of the exobase are significant due to their influence on atmospheric drag experienced by satellites. Increased drag can lower satellite orbits and eventually cause re-entry unless orbital corrections are made.
The upper boundary of the exosphere is less clearly defined. It is commonly taken to be the altitude at which the pressure exerted by solar radiation on atomic hydrogen exceeds Earth’s gravitational attraction. This occurs at roughly half the distance to the Moon. Observations of the geocorona suggest that Earth’s exosphere extends to at least several hundred thousand kilometres into space.

Exospheres of Other Celestial Bodies

Bodies whose atmospheres are extremely thin may be composed entirely of exosphere-like gases. For planets such as Mercury, the exosphere exists without any underlying layers. Mercury’s exosphere includes sodium, potassium and calcium. Several mechanisms have been proposed to explain its formation.
Meteor impacts are a major source of vapour production on Mercury. Meteoroids travelling at speeds of up to 80 km/s can vaporise both themselves and Mercury’s surface materials, producing plumes of atoms and molecules that rise to form part of the exosphere. Compounds such as sodium hydroxide and molecular oxygen have been detected, reforming from ejected atomic species during cooling. Although meteoritic impacts release sodium and potassium, they are thought to be minor contributors to the overall concentration of these elements.
Calcium, in contrast, is likely to originate primarily from impact processes. Its transport into the exosphere is believed to rely on photodissociation of oxides and hydroxides. This creates reactive fragments capable of entering ballistic trajectories above the surface.
Another major mechanism is solar wind sputtering, resulting from Mercury’s relatively weak magnetosphere. Unlike Earth’s strong magnetic field, Mercury’s magnetosphere does not fully shield the surface. Openings allow solar wind particles to strike the crust, knocking atoms into the exosphere. Sputtering is intermittent and cannot fully account for all observed exospheric species, but it remains an important component of the system.

Theoretical Description of the Exobase

The height of the exobase can be mathematically defined by comparing the mean free path of particles with the atmospheric scale height. At the exobase, particles travelling upward experience, on average, one collision. Using basic thermodynamic relations for an ideal gas, the mean free path can be shown to equal the pressure scale height under these conditions. This correlation indicates that the exobase occurs in the region where the density gradient becomes insufficient to sustain collisional interactions. The Knudsen number, representing the ratio of mean free path to typical density scale height, approaches unity at this point, signalling the transition to free molecular flow.

Importance and Observational Features

The exosphere plays a crucial role in satellite operations, atmospheric escape and planetary evolution. It influences long-term atmospheric loss, especially for light gases such as hydrogen and helium, which gradually diffuse outward. For Earth, the exosphere also forms part of the magnetospheric environment, interacting with energetic particles and solar radiation.
The visibility of Earth’s geocorona from space illustrates that exospheric gases scatter ultraviolet solar radiation. This makes the exosphere an observable feature despite its low density.