Sunset from Earth orbit

Earth's atmosphere is a layered mixture of gases, mainly nitrogen (78%) and oxygen (21%). Argon, water vapour, carbon dioxide and methane are among the other gases present in small amounts. The atmosphere helps to protect our planet from asteroid impacts and solar radiation.

The innermost layer, the troposphere, contains most of the planet's weather and extends out to 10–15km above the surface.

The next layer out, the stratosphere, is drier and less dense and extends out to about 50km. The Sun's UV light breaks down oxygen in the stratosphere to form the Earth's protective ozone layer.

The mesosphere, thermosphere, and ionosphere make up the remaining outer layers that extend out to about 100km.

Image: The rising Sun highlights Earth's atmosphere in this photo taken from the International Space Station (credit: NASA/SPL)


Sunset from Earth orbit Atmosphere

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The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.

The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

The study of Earth's atmosphere and its processes is called atmospheric science (aerology). Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann.

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