Carbon dioxide (CO2) is a compound with molecules each consisting of one carbon and two oxygen atoms. It is a constituent of the atmosphere of Earth, a major constituent in that of Venus and Mars, thus a possibility in that of extra-solar planets.
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 are of interest in evaluation planets for their life potential and in devising biosignatures.