• Any gaseous layer surrounding a solid object such as a planet, or a star. Atmospheres in the solar system include the Earth’s, which is the only one to include free oxygen and has been shaped by living matter, as well as that of Venus, which is massively corrosive and acidic, and those of the gas giants, whose atmospheres make up a large percentage of their total volume. The atmosphere of the Sun is highly active and energetic, with strong magnetic fields shaping its component parts.
  • The gaseous envelope surrounding the earth, consisting of the troposphere, stratosphere, mesosphere, and thermosphere, although there are other criteria utilized to describe its various layers. The mixture of gases composing it is called air, and at sea level dry air is approximately 78% nitrogen, 21% oxygen, 0.9% argon, and some other components in lesser amounts.
  • A unit of pressure intended to equal the pressure of the earth's atmosphere at sea level. One atmosphere equals 101.325 kilopascals, or 760 torr. Its abbreviation is atm. Also called standard atmosphere.

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