Anatmosphere (fromAncient Greekἀτμός (atmós) 'vapour, steam' andσφαῖρα (sphaîra) 'sphere')^[1] is a layer ofgases that envelop anastronomical object, held in place by thegravity of the object. A planet retains an atmosphere when the gravity is great and thetemperature of the atmosphere is low. Astellar atmosphere is the outer region of a star, which includes the layers above theopaquephotosphere; stars of low temperature might have outer atmospheres containing compoundmolecules.

Theatmosphere of Earth is composed ofnitrogen (78%),oxygen (21%),argon (0.9%),carbon dioxide (0.04%) and trace gases.^[2] Most organisms use oxygen forrespiration; lightning and bacteria performnitrogen fixation which producesammonia that is used to makenucleotides andamino acids;plants,algae, andcyanobacteria use carbon dioxide forphotosynthesis. The layered composition of the atmosphere minimises the harmful effects ofsunlight,ultraviolet radiation,solar wind, andcosmic rays and thus protects the organisms from genetic damage. The current composition of the atmosphere of the Earth is the product of billions of years of biochemical modification of thepaleoatmosphere by living organisms.^[3]

Atmospheres are clouds of gas bound to and engulfing an astronomical focal point ofsufficiently dominating mass, adding to its mass, possibly escaping from it or collapsing into it.Because of the latter, suchplanetary nucleus can develop from interstellarmolecular clouds orprotoplanetary disks intorockyastronomical objects with varyingly thick atmospheres,gas giants orfusors.

Composition and thickness is originally determined by the stellar nebula's chemistry and temperature, but can also by a product processes within the astronomical body outgasing a different atmosphere.

The atmospheres of the planetsVenus andMars are principally composed ofcarbon dioxide andnitrogen,argon andoxygen.^[4]

The composition of Earth's atmosphere is determined by the by-products of the life that it sustains. Dry air (mixture of gases) fromEarth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally a variable amount of water vapor is also present, on average about 1% at sea level.^[5]

The low temperatures and higher gravity of the Solar System'sgiant planets—Jupiter,Saturn,Uranus andNeptune—allow them more readily to retain gases with lowmolecular masses. These planets have hydrogen–helium atmospheres, with trace amounts of more complex compounds.

Two satellites of the outer planets possess significant atmospheres.Titan, a moon of Saturn, andTriton, a moon of Neptune, have atmospheres mainly ofnitrogen.^[6][7] When in the part of its orbit closest to the Sun,Pluto has an atmosphere of nitrogen and methane similar to Triton's, but these gases are frozen when it is farther from the Sun.

Other bodies within the Solar System have extremely thin atmospheres not in equilibrium. These include theMoon (sodium gas),Mercury (sodium gas),Europa (oxygen),Io (sulfur), andEnceladus (water vapor).

The first exoplanet whose atmospheric composition was determined isHD 209458b, a gas giant with a close orbit around a star in theconstellationPegasus. Its atmosphere is heated to temperatures over 1,000 K, and is steadily escaping into space. Hydrogen, oxygen, carbon and sulfur have been detected in the planet's inflated atmosphere.^[8]

• Atmosphere of the Sun
• Atmosphere of Mercury
• Atmosphere of Venus
• Atmosphere of Earth
  • Atmosphere of the Moon
• Atmosphere of Mars
• Atmosphere of Ceres
• Atmosphere of Jupiter
  • Atmosphere of Io
  • Atmosphere of Callisto
  • Atmosphere of Europa
  • Atmosphere of Ganymede
• Atmosphere of Saturn
  • Atmosphere of Titan
  • Atmosphere of Enceladus
• Atmosphere of Uranus
  • Atmosphere of Titania
• Atmosphere of Neptune
  • Atmosphere of Triton
• Atmosphere of Pluto

Theatmosphere of Earth is composed of layers with different properties, such as specific gaseous composition, temperature, and pressure.

Thetroposphere is the lowest layer of the atmosphere. This extends from the planetary surface to the bottom of thestratosphere. The troposphere contains 75–80% of the mass of the atmosphere,^[9] and is the atmospheric layer wherein the weather occurs; the height of the troposphere varies between 17 km at the equator and 7.0 km at the poles.

Thestratosphere extends from the top of the troposphere to the bottom of themesosphere, and contains theozone layer, at an altitude between 15 km and 35 km. It is the atmospheric layer that absorbs most of theultraviolet radiation that Earth receives from the Sun.

Themesosphere ranges from 50 km to 85 km and is the layer wherein mostmeteors are incinerated before reaching the surface.

Thethermosphere extends from an altitude of 85 km to the base of theexosphere at 690 km and contains theionosphere, where solar radiation ionizes the atmosphere. The density of the ionosphere is greater at short distances from the planetary surface in the daytime and decreases as the ionosphere rises at night-time, thereby allowing a greater range of radio frequencies to travel greater distances.

Theexosphere begins at 690 to 1,000 km from the surface, and extends to roughly 10,000 km, where it interacts with themagnetosphere of Earth.

Atmospheric pressure is theforce (per unit-area) perpendicular to a unit-area of planetary surface, as determined by theweight of the vertical column of atmospheric gases. In said atmospheric model, theatmospheric pressure, the weight of the mass of the gas, decreases at high altitude because of the diminishing mass of the gas above the point ofbarometric measurement. The units of air pressure are based upon thestandard atmosphere (atm), which is 101,325 Pa (equivalent to 760 Torr or 14.696 psi). The height at which the atmospheric pressure declines by a factor ofe (anirrational number equal to 2.71828) is called thescale height (H). For an atmosphere of uniform temperature, the scale height is proportional to the atmospheric temperature and is inversely proportional to the product of the meanmolecular mass of dry air, and the local acceleration of gravity at the point of barometric measurement.

Surface gravity differs significantly among the planets. For example, the large gravitational force of the giant planetJupiter retains light gases such ashydrogen andhelium 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'sescape velocity, allowing those to escape a planet's gravitational grasp. Thus, distant and coldTitan,Triton, andPluto are able to retain their atmospheres despite their relatively low gravities.

Since a collection of gas molecules may be moving at a wide range of velocities, there will always be some fast enough to produce a slow leakage of gas into space. Lighter molecules move faster than heavier ones with the same thermalkinetic energy, and so gases of lowmolecular weight are lost more rapidly than those of high molecular weight. It is thought thatVenus andMars may have lost much of their water when, after beingphotodissociated into hydrogen and oxygen by solarultraviolet radiation, the hydrogen escaped.Earth's magnetic field helps to prevent this, as, normally, the solar wind would greatly enhance the escape of hydrogen. However, over the past 3 billion years Earth may have lost gases through the magnetic polar regions due to auroral activity, including a net 2% of its atmospheric oxygen.^[10] The net effect, taking the most important escape processes into account, is that an intrinsic magnetic field does not protect a planet from atmospheric escape and that for some magnetizations the presence of a magnetic field works to increase the escape rate.^[11]

Other mechanisms that can causeatmosphere depletion aresolar wind-induced sputtering,impact erosion,weathering, and sequestration—sometimes referred to as "freezing out"—into theregolith andpolar caps.

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 incraters. Without an atmosphere, the planet has no protection frommeteoroids, and all of them collide with the surface asmeteorites and create craters.

For planets with a significant atmosphere, mostmeteoroids burn up asmeteors before hitting a planet's surface. When meteoroids do impact, the effects are often erased by the action of wind.^[12]

Wind erosion is a significant factor in shaping the terrain of rocky planets with atmospheres, and over time can erase the effects of both craters andvolcanoes. In addition, sinceliquids cannot exist without pressure, an atmosphere allows liquid to be present at the surface, resulting inlakes,rivers andoceans.Earth andTitan are known to have liquids at their surface and terrain on the planet suggests thatMars had liquid on its surface in the past.

• Atmosphere ofHD 209458 b

The circulation of the atmosphere occurs due to thermal differences whenconvection becomes a more efficient transporter of heat thanthermal radiation. On planets where the primary heat source is solar radiation, excess heat in the tropics is transported to higher latitudes. When a planet generates a significant amount of heat internally, such as is the case forJupiter, convection in the atmosphere can transport thermal energy from the higher temperature interior up to the surface.

From the perspective of a planetarygeologist, the atmosphere acts to shape a planetary surface.Wind picks updust and other particles which, when they collide with the terrain, erode therelief and leavedeposits (eolian processes).Frost andprecipitations, which depend on the atmospheric composition, also influence the relief. Climate changes can influence a planet's geological history. Conversely, studying the surface of the Earth leads to an understanding of the atmosphere and climate of other planets.

For ameteorologist, the composition of the Earth's atmosphere is a factor affecting theclimate and its variations.

For abiologist orpaleontologist, the Earth's atmospheric composition is closely dependent on the appearance of life and itsevolution.

• Atmometer (evaporimeter)
• Atmospheric pressure
• International Standard Atmosphere
• Kármán line
• Sky

• Sanchez-Lavega, Agustin (2010).An Introduction to Planetary Atmospheres.Taylor & Francis.ISBN 978-1420067323.

• Properties of atmospheric strata – The flight environment of the atmosphere
• Atmosphere – Everything you need to know