It is a mixture of gases that surround any planet. (Cambridge Online Dictionary) If planet has gravity and atmosphere has low temperature, then this mixture of gases is more likely to stay around.
In relation to our planet Earth, atmosphere is first of all the air that we breathe and also several other gases that cover Earth’s surface up. Earth atmosphere consists mostly from nitrogen (78%), oxygen(21%), argon (0.9%) and few other gases in minor quantities including carbon dioxide (0.04%).
Various creatures depend on all of those gases. Nitrogen is used mostly by bacteria and fungi inside soil. They break down nitrogen and produce ammonia which acts as fertiliser for plants.
Oxygen is vital for all creatures who breathe. And carbon dioxide is useful for plants and certain types of bacteria for photosynthesis.
There are five sub-layers in Earth atmosphere – troposphere, stratosphere, mesosphere, thermosphere and exosphere. They differ in composition, density and pressure. It is important to not that ozone layer, protecting Earth from ultraviolet radiation is located in stratosphere.
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