About 21 percent of Earth’s atmosphere consists of oxygen.
Reported from Science Alert, Sunday (16/1/2022), the earliest gas mixtures to form a thick layer around our cooling planet about 4.6 billion years ago are not much different from the types of substances emitted by volcanoes, such as methane, hydrogen sulfide and carbon dioxide. These are tough times – not only is Earth’s bowels churning with volatile matter, the constant bombardment of cold rock from the inner Solar System provides a constant supply of fresh ingredients.
Larger collisions sometimes heat back up to the surface, removing accumulated moisture and ensuring the atmosphere remains warm. But the flow of minerals that descend from the sky also provides a lot of nitrogen in the form of ammonia.