Some of Earth’s atmosphere was probably gas captured from the solar nebula four and a half billion years ago, when our planet was forming. It is thought that most of Earth’s atmosphere was trapped beneath Earth’s surface, escaping through volcanic eruptions and other crustal cracks and fissures. Water vapor was the most plentiful gas to spew out, and it condensed to form the oceans, lakes, and other surface water. Carbon dioxide was probably the next most plentiful gas, and much of it dissolved in the water or combined chemically with rocks on the surface. Nitrogen came out in smaller amounts, but did not undergo significant condensation or chemical reactions. This is why scientists think it is the most abundant gas in our atmosphere.

The high concentration of oxygen in our atmosphere is very unusual for planets, because oxygen is highly reactive and combines easily with other elements. In order to maintain oxygen in gaseous form, it must constantly be replenished. On Earth, this is accomplished by plants and algae that conduct photosynthesis, removing carbon dioxide from the atmosphere and adding oxygen into it. It was during the Carboniferous Period about 300 million years ago that plant growth dramatically changed the atmosphere, increasing the amount of oxygen to 35 percent. Today, the oxygen content is not quite as rich (21 percent).