Methane, also marsh gas, gas composed of carbon and hydrogen with formula CH₄, the first member of the paraffin or alkane series of hydrocarbons. It is lighter than air, colorless, odorless, and flammable. It occurs in natural gas, as firedamp in coal mines, as a by-product of petroleum refining, and as a product of decomposition of matter in swamps. It is a major component in the atmosphere of the outer planets, Jupiter, Saturn, Uranus, and Neptune. In the earth's atmosphere it apparently plays a role in the formation of noctilucent clouds (see Cloud). Methane can be produced by the hydrogenation of carbon or carbon oxide, by the action of water on aluminum carbide, or by heating sodium acetate with alkali. Methane is valuable as a fuel and in the production of hydrogen, hydrogen cyanide, ammonia, acetylene, and formaldehyde.

Methane melts at -182.5° C (-296.5° F) and boils at -161.5° C (-258.7° F).