An atmosphere (from Greek "ατμός" - "atmos", "
vapor" + "σφαίρα" - "sphaira", " sphere") is a layer of gases that may surround a material body of sufficient mass, [ [http://www.ontariosciencecentre.ca/school/clc/visits/glossary.asp Ontario Science Centre website] ] by the gravityof 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).
stellar atmosphereis used for the outer region of a star, and typically includes the portion starting from the opaque photosphereoutwards. Relatively low-temperature stars may form compound molecules in their outer atmosphere. Earth's atmosphere, which contains oxygenused by most organisms for respirationand carbon dioxideused by plants, algaeand cyanobacteriafor 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 paleoatmosphereby living organisms.
Atmospheric pressureis 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 numberwith a value of 2.71828..) is called the scale heightand 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 massof 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.
Surface gravity, the force that holds down an atmosphere, differssignificantly among the planets. For example, the large gravitational force of the giant planet Jupiteris able to retain light gases such as hydrogenand heliumthat 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 motionexceed 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 Plutoare 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 fieldhelps 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. [cite journal | author=Seki, K.; Elphic, R. C.; Hirahara, M.; Terasawa, T.; Mukai, T. | title=On Atmospheric Loss of Oxygen Ions from Earth Through Magnetospheric Processes | journal=Science | year=2001 | volume=291 | issue=5510 | pages=1939–1941 | url=http://www.sciencemag.org/cgi/content/full/291/5510/1939 | accessdate=2007-03-07 | doi=10.1126/science.1058913 | pmid=11239148]
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 regolithand polar caps.
Initial atmospheric makeup is generally related to the chemistry and temperature of the local
solar nebuladuring 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
Venusand Marsare primarily composed of carbon dioxide, with small quantities of nitrogen, argon, oxygenand 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, Uranusand 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 (
sodiumgas), Mercury (sodium gas), Europa (oxygen), Io ( sulfur), and Enceladus (water vapor).
The atmospheric composition of an
extra-solar planetwas 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. [cite news | author=Weaver, D.; Villard, R. | title=Hubble Probes Layer-cake Structure of Alien World's Atmosphere | publisher=Hubble News Center | date=2007-01-31 | url=http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/ | accessdate=2007-03-11 ]
Earth's atmosphereconsists, from the ground up, of the troposphere(which includes the planetary boundary layeror peplosphere as lowest layer), stratosphere, mesosphere, ionosphere(or thermosphere), exosphereand 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 ultravioletenergy 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.
Other astronomical bodies such as these listed have known atmospheres.
The circulation of the atmosphere occurs due to thermal differences when
convectionbecomes 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.
From the perspective of the planetary
geologist, the atmosphere is an evolutionary agent essential to the morphology of a planet. The windtransports dustand other particles which erodes the relief and leaves deposits ( eolianprocesses). 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.
meteorologist, the composition of the atmosphere determines the climateand its variations.
biologist, the composition is closely dependent on the appearance of the life and its evolution.
Edge of space
Table of Global Climate System Components
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