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View of Jupiter's active atmosphere, including the Great Red Spot.

An atmosphere (from Greek ἀτμός - atmos "vapor" and σφαῖρα - sphaira "sphere") is a layer of gases that may surround a material body of sufficient mass,[1] 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, but only their outer layer is their atmosphere (see gas giants).

The term stellar atmosphere describes 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.

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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 atmospheric gas 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.[2]

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

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo when seen from space.

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 0.247%, National Center for Atmospheric Research) 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 giantsJupiter, 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.[3]

Structure

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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, thermosphere (which contains the ionosphere and exosphere) and also 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.

In our solar system

Outside our solar system

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.

See also

References

  1. ^ Ontario Science Centre website
  2. ^ 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. doi:10.1126/science.1058913. PMID 11239148. http://www.sciencemag.org/cgi/content/full/291/5510/1939. Retrieved 2007-03-07.  
  3. ^ Weaver, D.; Villard, R. (2007-01-31). "Hubble Probes Layer-cake Structure of Alien World's Atmosphere". Hubble News Center. http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/. Retrieved 2007-03-11.  

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1911 encyclopedia

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From LoveToKnow 1911

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Simple English

File:Atmosphere
Layers of the atmosphere (not to scale)

Earth's atmosphere is the layer of gases around the planet Earth. The atmosphere is held in place by Earth's gravity. It is made up of nitrogen (78.1%) and oxygen (20.9%), with small amounts of argon (0.9%), carbon dioxide (~ 0.035%), water vapor, and other gases. The atmosphere protects life on Earth by absorbing (taking) ultraviolet radiation from the sun, and balancing the temperature on earth between day and night.

Solid particulates, including ash, dust, volcanic ash, etc. are small part of atmosphere. They are important for the formation of clouds and fog.

The atmosphere does not end at a specific place. It just gets thinner when you go higher. There is no clear line between the atmosphere and outer space. 75% of the atmosphere is within 11 km of the Earth's surface.

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Temperature and the atmospheric layers

Some parts of the atmosphere are hot or cold, depending on height. If you were to climb straight up, it would get colder, then, it would get hotter, as you got higher. These changes of temperature are divided into layers. These are like layers of an onion. But you can not see any difference going from one layer to another layer. You can only feel the change in temperature; start getting hotter in the new layer, or start getting colder.

These are the layers of the atmosphere, starting from the ground:

  • Troposphere - Starts at the ground. Ends somewhere between 7 to 14 kilometers. It gets colder as you get higher. This layer affects our daily life.
  • Stratosphere - Starts at 7 to 14 kilometers. Ends at 50 km. It gets hotter as you get higher. There is little water vapor and other substances in this layer. Airplanes fly in this layer because it is usually stable.
  • Mesosphere - Starts at 50 km. Ends at 80 or 85 km. It gets colder again as you get higher. Winds in this layer are strong, so the temperature is not stable.
  • Thermosphere - Starts at 80 or 85 kilometers Ends at 640 km or higher. It gets hotter again as you get higher. This layer is very important in radio communication because it helps to reflect AM radio waves.

Where one layer changes to the next have been named "-pauses." So the tropopause is where the troposphere ends (7 to 14 kilometers high). The stratopause is at the end of the stratosphere. The mesopause is at the end of the mesosphere. These are called boundaries.

The average temperature of the atmosphere at the surface of earth is 14 °C.

Pressure

The atmosphere has pressure. This is because, even though air is a gas, it has weight. The average pressure of the atmosphere at sea level is about 101.4 kilopascals (about 14.7 pounds per square inch).

Density and mass

The density of air at sea level is about 1.2 kilograms per cubic meter. This density becomes less at higher altitudes at the same rate that pressure becomes less. The total mass of the atmosphere is about 5.1 × 1018 kg, which is only a very small part of the Earth's total mass.

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