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Carbon dioxide (CO2) is a compound with molecules each consisting of one carbon and two oxygen atoms. It is a constituent of Earth's atmosphere, a major constituent in that of Venus and Mars, and a possibility in that of extra-solar planets. It also occurs in clouds such as protoplanetary disks and molecular clouds and has been a target of intensity mapping.
Like Earth's water cycle, Earth has a carbon cycle that adds and removes carbon dioxide from the atmosphere. In addition to the obvious cycle between animal and plant life, it also encompasses an oceanic carbon cycle that includes a long-term deep CO2 cycle in which ocean carbon dioxide settles and sinks into the mantle to return later, analogous to and related to the deep water cycle. These long-term cycles influence Earth's development of life, and such cycles are of interest in evaluation planets for their life potential and in devising biosignatures.
Like all gases, the transparency of carbon dioxide depends upon wavelength, and the gas's presence in Earth's atmosphere affects the atmosphere's ability to absorb and emit energy via radiation, and the gas acts as a greenhouse gas. It is produced through combustion (e.g., in power stations and vehicles) and through the metabolism of animal life, and the potential that modern human activity may be increasing the amount within Earth atmosphere and by that means increasing its warming greenhouse effect is of high interest.