It emerged in a process that lasted at least 4 billion years and was formed from the same matter that makes up the ground we walk on. That’s right: all the gases that make up the air were once underground, stuck to the rocks. It was volcanoes that spat out these elements. In addition to lava and toxic compounds, volcanic smoke released nitrogen, water vapor and carbon dioxide into the air. In excess, this last gas can kill, but at that time it served as an excellent food for the first living organisms on Earth: bacteria that carried out photosynthesis, using carbon dioxide, light and water to produce their energy. Lucky for us, the main residue of this activity is the most important gas for human life, oxygen. This means that without the precious “poop” of these bacteria, complex beings like you, reader, would never have appeared! But of course the planet didn’t fill up with breathable air overnight.
In this primordial atmosphere, the Earth’s crust was full of oxygen-sucking elements, such as sulfur and iron compounds. In order for these minerals to become saturated, the gas had to be produced for 2 billion years before it started to concentrate in the air. Today, it occupies 21% of the atmosphere. Nitrogen dominates 78% of the air. Finally, the old carbon dioxide was swallowed up by the processes of photosynthesis and by the soil itself. Currently, it only makes up the remaining 1% of the atmosphere, along with dozens of other gases. Fortunately. Otherwise, the Earth could be an uninhabitable furnace like Venus, where the gas traps the sun’s rays and generates a heat of 400ºC. “Actually, the two planets have the same amount of carbon dioxide. The difference is that on Venus the gas is all in the atmosphere; here, it is dissolved in the subsoil and in the body of living beings”, says meteorologist Fábio Gonçalves, from the University of São Paulo (USP).
The oxygen we breathe took billions of years to reach today’s levels.
1a – The first gases in the atmosphere probably appeared during the formation of the Earth, 4.5 billion years ago. The planet’s low gravity would have attracted heavy elements such as methane, ammonia and water vapor from space. Other lighter ones, such as oxygen and hydrogen, only appear much later.
1b – At that time, the solid part of the Earth is in full formation. Our planet is little more than a disk of gas and dust that revolves around the Sun. As these solar system disks cool, larger and larger blocks of rock form, which become planets. Most of the molecules that will later form the atmosphere are trapped in these rocks.
2a – 3.8 billion years ago, the Sun’s action transformed the atmosphere, disintegrating methane, ammonia and water vapor molecules, forming a thin layer of oxygen at the top of the atmosphere that helps protect the planet against the ultraviolet rays. The most present gases become those that come off the crust
2b – Impacts from comets and asteroids ignite the planet, turning it into a sea of lava. The intense action of volcanoes makes the volatile elements trapped in the rocks gush out in the form of gases. The most important ones that appear are carbon dioxide and nitrogen, in addition to a lot of water vapour, which also appears adhered to solid elements trapped in these rocks.
3a – With the protection of the oxygen layer against solar disintegration, a good part of the water vapor starts to condense in the form of liquid water. Afterwards, carbon dioxide begins to be absorbed by the soil and by bacteria that carry out photosynthesis. During this period, between 3.8 billion and 1.7 billion years ago, nitrogen began to dominate the atmosphere.
3b – On the surface, liquid water allows the emergence of life. Because of photosynthesis, bacteria start releasing oxygen. At first, the gas doesn’t last long in the air because it’s sucked in by certain minerals, most notably iron. When one layer of the crust becomes saturated with oxygen, another layer comes to the surface. And the cycle repeats itself for millions of years, removing almost all the oxygen from the air.
4a – 1.7 billion years ago, oxygen got rid of sucking minerals. When the level of this gas (O2) reaches 10% of the current level, a reasonable layer of ozone (O3) is formed — from the extra oxygen released by chemical reactions in the upper layers of the atmosphere in formation. 400 million years ago, the atmosphere is already ready: the amount of oxygen and nitrogen in the air is similar to today, around 99%
4b – As most of the iron is saturated, the oxygen produced by photosynthesis of bacteria is ready to concentrate in the air. The ozone layer, protecting the surface from ultraviolet rays, allowed the emergence of land plants, 400 million years ago. This boosts photosynthesis and raises the oxygen level in the air to its current level, around 21%.