<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2016) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2016) (Learn how and when to remove this template message) </Td> </Tr> <P> An atmosphere (from Greek ἀτμός (atmos), meaning' vapor', 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 . </P> <P> The atmosphere of Earth is composed of nitrogen (about 78%), oxygen (about 21%), argon (about 0.9%) with carbon dioxide and other gases in trace amounts . Oxygen is used by most organisms for respiration; nitrogen is fixed by bacteria and lightning to produce ammonia used in the construction of nucleotides and amino acids; and carbon dioxide is used by plants, algae and cyanobacteria for photosynthesis . The atmosphere helps to protect living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays . The current composition of the Earth's atmosphere is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms . </P>

What is meant by the earth's atmosphere
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