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For other uses, see Atmosphere (disambiguation).
"Atmospheric environment" redirects here. For the scientific journal, see Atmospheric Environment.
Mars's thin atmosphere
The layers of Earth's atmosphere

An atmosphere (from Greek ἀτμός (atmos), meaning "vapour", and σφαῖρα (sphaira), meaning "sphere"[1][2]) is a layer of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if its gravity is high and the atmosphere's temperature is low.

The atmosphere of Earth is mostly composed of nitrogen. It also contains oxygen used by most organisms for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis. The atmosphere helps protect living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays. Its current composition is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.

The term stellar atmosphere describes the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards. Stars with sufficiently low temperatures may form compound molecules in their outer atmosphere.


Main article: Atmospheric pressure

Atmospheric pressure is the force per unit area that is applied perpendicularly to a surface by the surrounding gas. It is determined by a planet's gravitational force in combination with the total mass of a column of gas above a location. On Earth, units of air pressure are based on the internationally recognized standard atmosphere (atm), which is defined as 101,325 kPa (760 Torr or 14.696 psi).

The pressure of an atmospheric gas decreases with altitude due to the diminishing mass of gas above. The height at which the pressure from an atmosphere declines by a factor of e (an irrational number with a value of 2.71828..) is called the scale height and is denoted by H. For an atmosphere with a uniform temperature, the scale height is proportional to the temperature and inversely proportional to the product of the mean molecular mass of dry air and the local acceleration of gravity at that location. For such a model atmosphere, the pressure declines exponentially with increasing altitude. However, atmospheres are not uniform in temperature, so the exact determination of the atmospheric pressure at any particular altitude is more complex.

Atmospheric escape[edit]

Main article: Atmospheric escape

Surface gravity, the force that holds down an atmosphere, differs significantly among the planets. For example, the large gravitational force of the giant planet Jupiter is able to retain light gases such as hydrogen and helium that escape from objects with lower gravity. Secondly, the distance from the Sun determines the energy available to heat atmospheric gas to the point where some fraction of its molecules' thermal motion exceed the planet's escape velocity, allowing those to escape a planet's gravitational grasp. Thus, the distant and cold Titan, Triton, and Pluto are able to retain their atmospheres despite their relatively low gravities. Interstellar planets, theoretically, may also retain thick atmospheres.




Atmospheres have dramatic effects on the surfaces of rocky bodies. Objects that have no atmosphere, or that have only an exosphere, have terrain that is covered in craters. Without an atmosphere, the planet has no protection from meteors, and all of them collide with the surface and create craters.

A rocky body with a thick atmosphere does not have significant craters on its surface. The friction generated when a meteor enters an atmosphere causes the vast majority of it to burn up before hitting the surface. When meteors do impact, the effects are often erased by the action of wind. As a result, craters are rare on objects with atmospheres.




Initial atmospheric composition is generally related to the chemistry and temperature of the local solar nebula during planetary formation and the subsequent escape of interior gases. The original atmospheres started with the radially local rotating gases that collapsed to the spaced rings that formed the planets.[clarification needed] They were then modified over time by various complex factors, resulting in quite different outcomes.


The atmospheric composition on Earth is largely governed by the by-products of the life that it sustains. Dry air from Earth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally a variable amount of water vapour is also present, on average about 1% at sea level.







Main article: Atmosphere of Earth



Other astronomical bodies such as these listed have known atmospheres.

In the Solar System[edit]


Outside the Solar System[edit]

Main article: Extrasolar atmosphere




From the perspective of a planetary geologist, the atmosphere is an evolutionary agent essential to the shaping of a planet. The wind picks up dust and other particles which when they collide with the terrain erodes the relief and leaves deposits (eolian processes). Frost and precipitations, which depend on the atmospheric composition, also influence the relief. Climate changes can influence a planet's geological history. Conversely, studying surface of Earth leads to an understanding of the atmosphere and climate of a planet — both its present state and its past.

For a meteorologist, the composition of the atmosphere determines the climate and its variations.

For a biologist or paleontologist, the atmospheric composition is closely dependent on the appearance of the life and its evolution.

See also[edit]


  1. ^ ἀτμός, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
  3. ^ Seki, K.; Elphic, R. C.; Hirahara, M.; Terasawa, T.; Mukai, T. (2001). "On Atmospheric Loss of Oxygen Ions from Earth Through Magnetospheric Processes". Science 291 (5510): 1939–1941. Bibcode:2001Sci...291.1939S. doi:10.1126/science.1058913. PMID 11239148. Retrieved 2007-03-07. 
  4. ^ Weaver, D.; Villard, R. (2007-01-31). "Hubble Probes Layer-cake Structure of Alien World's Atmosphere". Hubble News Center. Retrieved 2007-03-11. 

Further reading[edit]

  • Sanchez-Lavega,, Agustin (2010). An Introduction to Planetary Atmospheres. Taylor & Francis. ISBN 978-1-4200-6732-3. 

External links[edit]