An atmosphere is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. The name originates from Ancient Greek ἀτμός (atmós) 'vapour, steam' and σφαῖρα (sphaîra) 'sphere'. An object acquires most of its atmosphere during its primordial epoch, either by accretion of matter or by outgassing of volatiles. The chemical interaction of the atmosphere with the solid surface can change its fundamental composition, as can photochemical interaction with the Sun. A planet retains an atmosphere for longer durations when the gravity is high and the temperature is low. The solar wind works to strip away a planet's outer atmosphere, although this process is slowed by a magnetosphere. The further a body is from the Sun, the lower the rate of atmospheric stripping. Aside from Mercury, all Solar System planets have substantial atmospheres, as does the dwarf planet Pluto and the moon Titan. The high gravity and low temperature of Jupiter and the other gas giant planets allow them to retain massive atmospheres of mostly hydrogen and helium. Lower mass terrestrial planets orbit closer to the Sun, and so mainly retain higher molar mass atmospheres made of carbon, nitrogen, and oxygen, with trace amounts of inert gas. Atmospheres have been detected around exoplanets such as HD 209458 b and Kepler-7b. A stellar atmosphere is the outer region of a star, which includes the layers above the opaque photosphere; stars of low temperature might have outer atmospheres containing compound molecules. Other objects with atmospheres are brown dwarfs and active comets.
Occurrence and compositions
Origins
In the nebular hypothesis, stars form during the gravitational collapse of a mass of gas and dust within an interstellar molecular cloud. This material forms a pancake-like rotating disk with the mass concentrated at the center. The protostar is created at the central mass concentration, while the planets and satellites are formed in the disk through a process of accretion. Dust settles into the median disk plane, forming materials that can collide and accrete to create planetesimals. Close to the star, these bodies grow and accumulate to form protoplanets consisting primarily of refractory materials with few volatiles. Further from the star, planetary embryos are created from accumulation of volatiles up to around ten times the mass of the Earth or more. Masses of gas are then acquired from the surrounding disk nebula, forming a gas giant around the embryo. Planetary satellites form in a similar fashion from the disk of material around the planets. The primary atmosphere of a planet is produced when the gravity is sufficient to retain accreted gas against escape processes. The latter can include collisions with other bodies that impart sufficient energy for the gasses to escape. For the terrestrial planets, the high temperatures generated by their initial bombardment results in the outgassing of volatiles, creating the secondary atmosphere. The original composition and thickness of the atmosphere is thus determined by the stellar nebula's chemistry and temperature, but can be modified by processes within the astronomical body that release different atmospheric components. The circumstellar disk will finally dissipate on time scales of about 107 years, and the star will complete its contraction then ignite hydrogen fusion at its core in a time frame determined by its mass. (For example, a star with the mass of the Sun will spend 3×107 years contracting.)
Compositions
The atmospheres of the planets Venus and Mars are principally composed of carbon dioxide, nitrogen, and argon. Because Venus has no oceans or rain to dissolve the carbon dioxide, large amounts of this greenhouse gas has remained in the atmosphere. The result is a dense atmosphere about 80 times the pressure of Earth's atmosphere. The planet's lack of a magnetic field and closer proximity to the Sun resulted in the loss of its hydrogen (in the form of water) after two billion years. Because Mars is small, cold, and lacks a magnetic field, it has retained only a sparse atmosphere. The surface air pressure of 0.6 kPa for Mars is only 0.6% of Earth's 101.3 kPa. The planet has probably lost at least 80–85% of its original water supply to space. However, the planet has retained significant deposits of frozen water and carbon dioxide. If all of the frozen CO2 were to sublimate, the air pressure could climb to 30 kPa. This is comparable to the air pressure on the top of Mount Everest. The composition of Earth's atmosphere is determined by the by-products of the life that it sustains. Dry air (mixture of gases) from 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. Earth's persistent magnetosphere acts as a shield against atmospheric scavenging by the solar wind, as it fends off the incoming plasma at a distance of about 10 Earth radii. The low temperatures and higher escape velocities 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 reducing atmospheres of hydrogen and helium, with trace amounts of other elements and more complex compounds. Unlike the terrestrial planets, the gas giants lack a well-defined surface. Instead the atmosphere is maintained in hydrostatic equilibrium by intense pressure deep in the body. The dynamic weather on these bodies only occurs in a relatively thin surface layer. Two satellites of the outer planets possess significant atmospheres. Titan, a moon of Saturn, and Triton, a moon of Neptune, have atmospheres mainly of nitrogen. 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, noble gases, hydrogen), Mercury (sodium gas), Callisto (carbon dioxide and oxygen), Europa (oxygen), Io (sulfur dioxide), and Enceladus (water vapor).
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