The atmosphere is composed of elements and molecules that change in relative composition across a 10,000 km range extending from the surface of the planet. Within this layer, we have the ozone layer and the auroras. 

Close to Earth, the atmosphere is composed of nitrogen (78%), oxygen (20.9%), argon (0.93%), and green houses gases such as carbon dioxide, methane, nitrous oxide, and ozone (0.04%).

This delicate mixture is produced by the very organisms that it sustains! Oxygen, for example, is a byproduct of photosynthesis, the process whereby sugars are produced from sunlight by green plants.