Methane molecule. Computer model of a methane molecule (CH4). Atoms are represented as spheres and are colour-coded: carbon (green) and hydrogen (white). Methane, an alkane, is a colourless, odourless gas. It is the chief constituent of natural gas, which is used as a fuel. It can be formed by the decay of vegetable matter. When released into the atmosphere, methane helps to trap the heat of the Sun, acting as a greenhouse gas and contributing to global warming.