Heterosphere
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The heterosphere is the layer of an
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
where gases are separated out by molecular diffusion with increasing altitude such that lighter species become more abundant relative to heavier species. The heavier molecules and atoms tend to be present in the lower layers of the heterosphere while the lighter ones are present higher up. The exact boundaries between the different molecules vary according to temperature and solar activity. The heterosphere extends from the
turbopause The turbopause, also called the homopause, marks the altitude in an atmosphere below which turbulent mixing dominates. Mathematically, it is defined as the point where the coefficient of Eddy diffusion is equal to the coefficient of molecular dif ...
to the edge of a planet's atmosphere and lies directly above the homosphere.


Earth's heterosphere

The heterosphere of
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
begins at an altitude of about and extends to the outer reaches of its atmosphere. It incorporates most of the
thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...
and all of the
exosphere The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of ...
. The major constituents of Earth's heterosphere are nitrogen, oxygen, helium, and hydrogen. Nitrogen and oxygen compose the lower portion of the heterosphere. In the higher levels of the heterosphere, above about helium and hydrogen are the dominant species present. The heterosphere also incorporates the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
with ions present in the heterosphere's lower levels. These include O+, NO+, O2+, and N2+. Due to the diffused nature of the heterosphere's gases, its density at any given height is not entirely dependent on the temperature. Other factors contributing to density variations in the heterosphere include day and night cycles, solar activity, geomagnetic activity, and seasonal cycles. The heterosphere contains less than 0.001% of the Earth's atmosphere's total mass.


References

{{Reflist Atmosphere of Earth