The gaseous mantle surrounding a planet, star or other astronomical body.

It is thought that the atmosphere of the Earth is a secondary atmosphere. The theory is that the original (primary) atmosphere was lost during the T-Tauri stage of the Sun’s evolution. Volcanoes gradually replaced this with an atmosphere of methane, carbon dioxide and water vapour. The current atmosphere evolved from this. The oceans were formed as the water vapour condensed as Earth cooled down. When plants containing chlorophyll evolved, they used the carbon dioxide for photosynthesis and introduced oxygen into our atmosphere. Ultraviolet radiation converted some oxygen to ozone in the upper atmosphere and was therefore absorbed and with the development of the ozone layer, life on earth could colonise the land.

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