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The atmosphere of Earth is composed of nitrogen (78 %), oxygen (21 %), argon (0.9 %), carbon dioxide (0.04 %) and trace gases.[2] Most organisms use oxygen for respiration; lightning and bacteria perform nitrogen fixation to produce ammonia 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 to protect 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.[3]

The initial gaseous composition of an atmosphere is determined by the chemistry and temperature of the local solar nebula from which a planet is formed, and the subsequent escape of some gases from the interior of the atmosphere proper. The original atmosphere of the planets originated from a rotating disc of gases, which collapsed onto itself and then divided into a series of spaced rings of gas and matter that, which later condensed to form the planets of the Solar System. The atmospheres of the planets Venus and Mars are principally composed of carbon dioxide and nitrogen, argon and oxygen.[6]

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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.[7]

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), Mercury (sodium gas), Europa (oxygen), Io (sulfur), and 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. 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.[8]

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.

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 barometric measurement. The units of air pressure are based upon the standard atmosphere (atm), which is 101,325 Pa (equivalent to 760 Torr or 14.696 psi). The height at which the atmospheric pressure declines by a factor of 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.

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, Triton, and Pluto are able to retain their atmospheres despite their relatively low gravities.

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 craters. Without an atmosphere, the planet has no protection from meteoroids, and all of them collide with the surface as meteorites and create craters.

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 can not exist without pressure, an atmosphere allows liquid to be present at the surface, resulting in lakes, rivers and oceans. Earth and 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.

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.

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 relief and leave deposits (eolian processes). Frost and 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.

The atmosphere is a mixture of gases that surrounds the Earth. It helps make life possible by providing us with air to breathe, shielding us from harmful ultraviolet (UV) radiation coming from the Sun, trapping heat to warm the planet, and preventing extreme temperature differences between day and night. Without the atmosphere, temperatures would be well below freezing everywhere on Earth's surface. Instead, the heat absorbed and trapped by our atmosphere keeps our planet's average surface temperature at a balmy 15C (59F). Some of the atmosphere's gases, like carbon dioxide, are particularly good at absorbing and trapping radiation. Changes in the amounts of these gases directly affect our climate.

Nitrogen and oxygen are by far the most common gases in our atmosphere. Dry air is composed of about 78% nitrogen (N2) and about 21% oxygen (O2). The remaining less than 1% of the atmosphere is a mixture of gases, including argon (Ar) and carbon dioxide (CO2). The atmosphere also contains varying amounts of water vapor, on average about 1%. There are also many, tiny, solid or liquid particles, called aerosols, in the atmosphere. Aerosols can be made of dust, spores and pollen, salt from sea spray, volcanic ash, smoke, and pollutants introduced through human activity.

The atmosphere becomes thinner (less dense and lower in air pressure) the further it extends from the Earth's surface. It gradually gives way to the vacuum of space. There is no precise top of the atmosphere, but the area between 100-120 km (62-75 miles) above the Earth's surface is often considered the boundary between the atmosphere and space because the air is so thin here. However, there are measurable traces of atmospheric gases beyond this boundary, detectable for hundreds of kilometers/miles from Earth's surface.

There are several unique layers in Earth's atmosphere. Each has characteristic temperatures, pressures, and phenomena. We live in the troposphere, the layer closest to Earth's surface, where most clouds are found and almost all weather occurs. Some jet aircraft fly in the next layer, the stratosphere, which contains the jet streams and a region called the ozone layer. The next layer, the mesosphere, is the coldest because the there are almost no air molecules there to absorb heat energy. There are so few molecules for light to refract off of that the sky also changes from blue to black in this layer. And farthest from the surface we have the thermosphere, which absorbs much of the harmful radiation that reaches Earth from the Sun, causing this layer to reach extremely high temperatures. Beyond the thermosphere is the exosphere, which represents the transition from Earth's atmosphere to space.

Earth is not the only world with an atmosphere. Each of the planets - and even a few moons - in our solar system have an atmosphere. Some planets have active atmospheres with clouds, wind, rain and powerful storms. Scientists use light spectroscopy to observe the atmospheres of planets and moons in other solar systems .

Each of the planets in our solar system has a uniquely structured atmosphere. The atmosphere of Mercury is extremely thin and is not very different from the vacuum of space. The gas giant planets in our solar system - Jupiter, Saturn, Uranus and Neptune - each have a thick, deep atmosphere. The smaller, rocky planets - Earth, Venus and Mars - each have thinner atmospheres, hovering above their solid surfaces. The moons in our solar system typically have thin atmospheres, with the exception of Saturn's moon, Titan. Air pressure at the surface of Titan is higher than on Earth! Of the five officially recognized dwarf planets, Pluto has a thin atmosphere that expands and collapses seasonally, and Ceres has an extremely thin and transient atmosphere made of water vapor. e24fc04721