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The Earth was formed approximately 4.6 billion years ago, with extensive bombardment by meteorites for almost the first billion years of its existence (until at least 3.8 billion years ago). The atmosphere of early Earth, which was very different from that of the present day, was lost during this bombardment process. The current atmosphere formed as the result of degassing from the Earth’s interior. Much of this degassing occurred (and continues to occur) in volcanic eruptions.

The present-day atmosphere is made up of a few highly concentrated gases (primarily molecular nitrogen and molecular oxygen) and a large number of trace gases (such as water vapor, carbon dioxide, and ozone). The atmosphere also contains water in the liquid and solid states in the form of clouds, as well as aerosols (solid, liquid, or mixed-phase particles suspended in air). The thermodynamic state of the atmosphere is described by pressure, density, and temperature, which are related by the equation of state