atmosphere

atmosphere [Gr.,=sphere of air], the mixture of gases surrounding a celestial body with sufficient gravity to maintain it. Although some details about the atmospheres of other planets and satellites are known, only the earth's atmosphere has been well studied, the science of which is called meteorology.

Components and Characteristics of the Earth's Atmosphere

The first 40 to 50 mi (64-80 km) above the earth contains 99% of the total mass of the earth's atmosphere and is generally of a uniform composition, except for a high concentration of ozone, known as the ozone layer, at 12-30 mi (19-50 km). Calculated according to their relative volumes, the gaseous constituents of the atmosphere are nitrogen, 78.09%; oxygen, 20.95%; argon, 0.93%; carbon dioxide, 0.03%; and minute traces of neon, helium, methane, krypton, hydrogen, xenon, and ozone. The lower atmosphere contains varying amounts of water vapor, which determine its humidity. Condensation and sublimation within the atmosphere cause clouds or fog, and the resulting liquid water droplets or ice crystals may precipitate to the ground as rain, sleet, snow, hail, dew, or frost. The air also carries many kinds of dust, of meteoric as well as terrestrial origin, and microorganisms, pollen, salt particles, and various gaseous and solid impurities resulting from human activity (see pollution). Because of the pull of gravity the density of the atmosphere and the pressure exerted by air molecules are greatest near the earth's surface (about 1 gram per 10³ cc and about 10⁶ dynes per sq cm, respectively). The instrument used to measure air pressure is called a barometer. Air pressure decreases quickly with altitude, reaching one half of its sea-level value at about 18,000 ft (5,500 m).

Layers of the Earth's Atmosphere

The earth's atmosphere is composed of distinct layers. The troposphere extends upward from the earth to a height of about 5 mi (8.1 km) at the poles, to about 7 mi (11.3 km) in mid-latitudes, and to about 10 mi (16.1 km) at the equator. The air in the troposphere is in constant motion, with both horizontal and vertical air currents (see wind). Throughout the troposphere temperature decreases with altitude at an average rate of about 3.6°F; per 1,000 ft (2°C; per 305 m), reaching about -70°F; (-57°C;) at its apex, the tropopause. Above the troposphere is an atmospheric ozone layer, which is also the lower layer of the stratosphere. Temperature changes little with altitude in the stratosphere, which extends upward to about 30 mi (50 km). Above this layer is the mesosphere which extends to about 50 mi (80 km above the earth); the temperature sharply decreases from around 20°F; (10°C;) at the base of the mesosphere to -166°F; (-110°C;) before it begins to rise at the top of the mesosphere. The next layer is the thermosphere, which extends upward from the mesosphere to about 400 mi (640 km); its temperature increases rapidly with altitude because of the absorption of shortwave radiation by ionization processes, although, because of the thinness of the air, little heat energy is available. The final layer is the exosphere, which gradually gets thinner as it reaches into the vacuum of space at around 435 mi (700 km) above the earth's surface; the atmosphere is so attenuated at this altitude that the average distance air molecules travel without colliding is equal to the radius of the earth. Although some gas molecules and particles out to about 40,000 mi (64,400 km) are trapped by the earth's gravitational and magnetic fields, the density of the atmosphere at an altitude of about 6,000 mi (9,700 km) is comparable to that of interplanetary space.

Certain layers of the atmosphere within the main regions exhibit characteristic properties. Aurorae (see aurora borealis), or northern and southern lights, appear in the thermosphere. The ionosphere is in the range (50-400 mi/80-640 km) that contains a high concentration of electrically charged particles (ions); these particles are responsible for reflecting radio signals important to telecommunications.

Role of the Earth's Atmosphere

The earth's atmosphere is the environment for most of its biological activity and exerts a considerable influence on the ocean and lake environment (see biosphere). Weather consists of the day-to-day fluctuations of environmental variables and includes the motion of wind and formation of weather systems such as hurricanes. Climate is the normal or long-term average state of the atmospheric environment (as determined in spans of about 50 years). The atmosphere protects earth's life forms from harmful radiation and cosmic debris. The ozone layer also protects the earth from the sun's harmful ultraviolet rays; seasonal "holes" in the ozone layer, the first detected above Antarctica and the Arctic in the 1980s, have caused considerable alarm about the consequences of air pollution. Meteors strike the thermosphere and mesosphere and burn from the heat generated by air friction.

See also Van Allen radiation belts; global warming.

Gaseous envelope that surrounds the Earth. Near the surface it has a well-defined chemical composition (see air). In addition to gases, the atmosphere contains solid and liquid particles in suspension. Scientists divide the atmosphere into five main layers: in ascending order, the troposphere (surface to 6–8 mi, or 10–13 km); the stratosphere (4–11 mi, or 6–17 km, to about 30 mi, or 50 km); the mesosphere (31–50 mi, or 50–80 km); the thermosphere (50–300 mi, or 80–480 km); and the exosphere (from 300 mi and gradually dissipating). Most of the atmosphere consists of neutral atoms and molecules, but in the ionosphere a significant fraction is electrically charged. The ionosphere begins near the top of the stratosphere but is most distinct in the thermosphere. Seealso ozone layer.

Learn more about atmosphere with a free trial on Britannica.com.

An atmosphere (from Greek ατμός - atmos, "vapor" + σφαίρα - sphaira, "sphere") is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low. Some planets consist mainly of various gases, and therefore have very deep atmospheres (see gas giants).

The term stellar atmosphere is used for the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards. Relatively low-temperature stars may form compound molecules in their outer atmosphere. Earth's atmosphere, which contains oxygen used by most organisms for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis, also protects living organisms from genetic damage by solar ultraviolet radiation. Its current composition is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.

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 air above a location. Units of air pressure are based on the internationally-recognized standard atmosphere (atm), which is defined as 101,325 Pa (or 1,013,250 dynes per cm²).

The pressure of an atmosphere decreases with altitude due to the diminishing mass of gas above each location. 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 mean molecular mass of dry air times the planet's gravitational acceleration. 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.

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 lower gravity objects. Second, the distance from the sun determines the energy available to heat atmospheric gas to the point where its molecules' thermal motion exceed the planet's escape velocity, the speed at which gas molecules overcome a planet's gravitational grasp. Thus, the distant and cold Titan, Triton, and Pluto are able to retain their atmospheres despite relatively low gravities. Interstellar planets, theoretically, may also retain thick atmospheres.

Since a gas at any particular temperature will have molecules moving at a wide range of velocities, there will almost always be some 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 weight are lost more rapidly than those of high molecular weight. It is thought that Venus and Mars may have both lost much of their water when, after being photodissociated into hydrogen and oxygen by solar ultraviolet, 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 the Earth may have lost gases through the magnetic polar regions due to auroral activity, including a net 2% of its atmospheric oxygen.

Other mechanisms that can cause atmosphere depletion are solar wind-induced sputtering, impact erosion, weathering, and sequestration — sometimes referred to as "freezing out" — into the regolith and polar caps.

Composition

Initial atmospheric makeup is generally related to the chemistry and temperature of the local solar nebula during planetary formation and the subsequent escape of interior gases. These original atmospheres underwent much evolution over time, with the varying properties of each planet resulting in very different outcomes.

The atmospheres of the planets Venus and Mars are primarily composed of carbon dioxide, with small quantities of nitrogen, argon, oxygen and traces of other gases.

The atmospheric composition on Earth is largely governed by the by-products of the very life that it sustains. Earth's atmosphere contains roughly (by molar content/volume) 78.08% nitrogen, 20.95% oxygen, a variable amount (average around 1%) water vapor, 0.93% argon, 0.038% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (and of volatile pollutants).

The low temperatures and higher gravity of the gas giants — Jupiter, Saturn, Uranus and Neptune — allows them to more readily 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 non-negligible atmospheres: Titan, a moon of Saturn, and Triton, a moon of Neptune, which are mainly nitrogen. Pluto, in the nearer part of its orbit, has an atmosphere of nitrogen and methane similar to Triton's, but these gases are frozen when farther from the Sun.

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

The atmospheric composition of an extra-solar planet was first determined using the Hubble Space Telescope. Planet HD 209458b is a gas giant with a close orbit around a star in the constellation Pegasus. The 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.

Structure

Earth

The Earth's atmosphere consists, from the ground up, of the troposphere (which includes the planetary boundary layer or peplosphere as lowest layer), stratosphere, mesosphere, ionosphere (or thermosphere), exosphere and the magnetosphere. Each of the layers has a different lapse rate, defining the rate of change in temperature with height.

Three quarters of the atmosphere lies within the troposphere, and the depth of this layer varies between 17 km at the equator and 7 km at the poles. The ozone layer, which absorbs ultraviolet energy from the Sun, is located primarily in the stratosphere, at altitudes of 15 to 35 km. The Kármán line, located within the thermosphere at an altitude of 100 km, is commonly used to define the boundary between the Earth's atmosphere and outer space. However, the exosphere can extend from 500 up to 10,000 km above the surface, where it interacts with the planet's magnetosphere.

Others

Other astronomical bodies such as these listed have known atmospheres.

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 the planetary geologist, the atmosphere is an evolutionary agent essential to the morphology of a planet. The wind transports dust and other particles which erodes the relief and leaves deposits (eolian processes). Frost and precipitations, which depend on the 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, the composition is closely dependent on the appearance of the life and its evolution.

References

See also

• Atmometer (evaporimeter)
• Edge of space
• Ionosphere
• Stellar atmosphere
• Table of Global Climate System Components