An atmosphere () is a layer of

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

es that envelop an

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...

, held in place by the

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

of the object. A planet retains an atmosphere when the gravity is great and the

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

of the atmosphere is low. A

stellar atmosphere The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. Overview The stellar atmosphere is divided into several regions of distinct character: * The photosphere, whi ...

is the outer region of a star, which includes the layers above the

opaque Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shie ...

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

; stars of low temperature might have outer atmospheres containing compound

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s. The

atmosphere of Earth The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weather ...

is composed of

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

(78%),

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

(21%),

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

(0.9%),

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(0.04%) and trace gases. Most organisms use oxygen for

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

; lightning and bacteria perform

nitrogen fixation Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...

which produces

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

that is used to make nucleotides and amino acids; plants, algae, and cyanobacteria use carbon dioxide for photosynthesis. The layered composition of the atmosphere minimises the harmful effects of sunlight, ultraviolet radiation, solar wind, and cosmic 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 the paleoatmosphere by living organisms.

Occurrence and compositions

Origins

Atmospheres are clouds of gas bound to and engulfing an astronomical focal point of Sphere of influence (astronomy), sufficiently dominating mass, adding to its mass, possibly escaping from it or collapsing into it. Because of the latter, such Protoplanet, planetary nucleus can develop from interstellar molecular clouds or protoplanetary disks into Rocky planet, rocky astronomical objects with varyingly thick atmospheres, gas giants or Fusor (astronomy), fusors. 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.

Compositions

Solar system escape velocity vs surface temperature

The atmospheres of the planets Venus and Mars are principally composed of carbon dioxide and

and

. The composition of Earth's atmosphere is determined by the by-products of the life that it sustains. Dry air (mixture of gases) from Atmosphere of Earth, Earth'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. The low temperatures and higher gravity of the Solar System's giant planets—Jupiter, Saturn, Uranus and Neptune—allow them more readily to retain gases with low molecular masses. These planets have hydrogen–helium atmospheres, with trace amounts of more complex compounds. Two satellites of the outer planets possess significant atmospheres. Titan (moon), Titan, a moon of Saturn, and Triton (moon), Triton, a moon of Neptune, have atmospheres mainly of

. 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 the Moon (sodium gas), Mercury (planet), Mercury (sodium gas), Europa (moon), Europa (oxygen), Io (moon), Io (sulfur), and Enceladus (moon), Enceladus (water vapor). The first exoplanet whose atmospheric composition was determined is HD 209458b, a gas giant with a close orbit around a star in the constellation Pegasus (constellation), Pegasus. 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.

Atmospheres in the Solar System

* Atmosphere of the Sun * Atmosphere of Mercury * Atmosphere of Venus * Atmosphere of Earth ** Atmosphere of the Moon * Atmosphere of Mars * Ceres (dwarf planet)#Atmosphere, Atmosphere of Ceres * Atmosphere of Jupiter ** Io (moon)#Atmosphere, Atmosphere of Io ** Callisto (moon)#Atmosphere and ionosphere, Atmosphere of Callisto ** Europa (moon)#Atmosphere, Atmosphere of Europa ** Ganymede (moon)#Atmosphere and ionosphere, Atmosphere of Ganymede * Atmosphere of Saturn ** Atmosphere of Titan ** Enceladus (moon)#South polar plumes, Atmosphere of Enceladus * Atmosphere of Uranus ** Titania (moon)#Atmosphere, Atmosphere of Titania * Neptune#Atmosphere, Atmosphere of Neptune ** Atmosphere of Triton * Atmosphere of Pluto

Structure of atmosphere

Earth

The

is composed of layers with different properties, such as specific gaseous composition, temperature, and pressure. The troposphere is the lowest layer of the atmosphere. This extends from the planetary surface to the bottom of the stratosphere. The troposphere contains 75–80% of the mass of the atmosphere, 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. The stratosphere extends from the top of the troposphere to the bottom of the mesosphere, and contains the ozone layer, at an altitude between 15 km and 35 km. It is the atmospheric layer that absorbs most of the ultraviolet radiation that Earth receives from the Sun. The mesosphere ranges from 50 km to 85 km and is the layer wherein most meteors are incinerated before reaching the surface. The thermosphere extends from an altitude of 85 km to the base of the exosphere at 690 km and contains the ionosphere, 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. The exosphere begins at 690 to 1,000 km from the surface, and extends to roughly 10,000 km, where it interacts with the magnetosphere of Earth.

Pressure

Atmospheric pressure is the force (per unit-area) perpendicular to a unit-area of planetary surface, as determined by the weight of the vertical column of atmospheric gases. In said atmospheric model, the atmospheric pressure, the weight of the mass of the gas, decreases at high altitude because of the diminishing mass of the gas above the point of barometer, barometric measurement. The units of air pressure are based upon the Atmosphere (unit), standard atmosphere (atm), which is 101,325 Pascal (unit), Pa (equivalent to 760 Torr or 14.696 pounds per square inch, psi). The height at which the atmospheric pressure declines by a factor of ''e (mathematical constant), e'' (an irrational number equal to 2.71828) is called the scale 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 mean molecular mass of dry air, and the local acceleration of gravity at the point of barometric measurement.

Escape

Surface gravity differs significantly among the planets. For example, the large gravitational force of the giant planet Jupiter retains 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, distant and cold Titan (moon), Titan, Triton (moon), Triton, and Pluto 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 thermal kinetic energy, and so gases of low molecular mass, molecular weight are lost more rapidly than those of high molecular weight. It is thought that Venus and Mars may have lost much of their water when, after being Photodissociation, photodissociated into hydrogen and oxygen by solar ultraviolet 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. 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. Other mechanisms that can cause atmospheric escape, atmosphere depletion are solar wind-induced sputtering, impact event, impact erosion, weathering, and sequestration—sometimes referred to as "freezing out"—into the regolith and polar ice cap, polar caps.

Terrain

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 impact crater, craters. Without an atmosphere, the planet has no protection from meteoroids, and all of them collide with the surface as meteorites and create craters. For planets with a significant atmosphere, most meteoroids burn up as meteors before hitting a planet's surface. When meteoroids do impact, the effects are often erased by the action of wind. 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 and volcanoes. In addition, since liquids cannot exist without pressure, an atmosphere allows liquid to be present at the surface, resulting in lakes, rivers and oceans. Earth and Titan (moon), Titan are known to have liquids at their surface and terrain on the planet suggests that Mars had liquid on its surface in the past.

Outside the Solar System

* Atmosphere of HD 209458 b

Circulation

The circulation of the atmosphere occurs due to thermal differences when convection becomes a more efficient transporter of heat than thermal 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 for Jupiter, convection in the atmosphere can transport thermal energy from the higher temperature interior up to the surface.

Importance

From the perspective of a planetary geologist, the atmosphere acts to shape a planetary surface. Wind picks up dust and other particles which, when they collide with the terrain, erode the Terrain, relief and leave Deposition (sediment), deposits (Aeolian processes, eolian processes). Frost line, Frost and Precipitation (meteorology), precipitations, 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 a meteorologist, the composition of the Earth's atmosphere is a factor affecting the climate and its variations. For a biologist or paleontologist, the Earth's atmospheric composition is closely dependent on the appearance of life and its evolution.

References

External links

Properties of atmospheric strata – The flight environment of the atmosphereAtmosphere – Everything you need to know
{{Authority control Atmosphere, Gases Planetary science