Exosphere

The Earth's exosphere is mostly hydrogen and helium, with some heavier atoms and molecules near the base.

Smaller bodies such as asteroids, in which the molecules emitted from the surface escape to space, are not considered to have exospheres.

Hydrogen is present throughout the exosphere, with some helium, carbon dioxide, and atomic oxygen near its base.

[5] On Earth, the altitude of the exobase ranges from about 500 to 1,000 kilometres (310 to 620 mi) depending on solar activity.

[6] The exobase can be defined in one of two ways: If we define the exobase as the height at which upward-traveling molecules experience one collision on average, then at this position the mean free path of a molecule is equal to one pressure scale height.

From the requirement that each molecule traveling upward undergoes on average one collision, the pressure is: where

The upper boundary of the exosphere can be defined as the distance at which the influence of solar radiation pressure on atomic hydrogen exceeds that of Earth's gravitational pull.

The exosphere, observable from space as the geocorona, is seen to extend to at least 100,000 kilometres (62,000 mi) from Earth's surface.

[8] Many hypotheses exist about the formation of the surface boundary exosphere of Mercury, which has been noted to include elements such as sodium (Na), potassium (K), and calcium (Ca).

[9] Each material has been suggested as a result of processes such as impacts, solar wind, and degassing from the terrestrial body that cause the atoms or molecules to form the planet's exosphere.

During the impact, the former elements of the colliding bodies are mostly devolved into atoms rather than molecules that can then be reformed during a cooling, quenching process.

[10] Another possible method of the exosphere formation of Mercury is due to its unique magnetosphere and solar wind relationship.

The magnetosphere of this celestial body is hypothesized to be an incomplete shield from the weathering of solar wind.