Atmosphere Atmosphere An atmosphere (from Modern Greek ἀτμός "(atmos)", meaning 'vapour', and σφαῖρα "(sphaira)", meaning 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in place by the gravity of that body. An atmosphere is more likely to be retained if the gravity it is subject to is high and the temperature of the atmosphere is low. The atmosphere of Earth is composed of nitrogen (about 78%), oxygen (about 21%), argon (about 0.9%) , carbon dioxide (0.03%) and other gases in trace amounts. Oxygen is used by most organisms
Carbon dioxide in Earth's atmosphere may have been higher, with estimated partial pressure as large as , because there was no bacterial photosynthesis to reduce the gas to carbon compounds and oxygen. Methane, a very active greenhouse gas which reacts with oxygen to produce and water vapor, may have been more prevalent as well, with a mixing ratio of 10 (100 parts per million by volume). Though water is responsible for most (about 36-70%) of the total greenhouse effect, the role of water vapor as a greenhouse gas depends on temperature. On Earth, carbon dioxide is the most relevant, direct anthropologically influenced greenhouse gas. Carbon