How high is the oxygen content on earth
The evolution of the earth's atmosphere
4.6 billion years ago
4.6 billion years ago, a cosmic cloud of gas and dust formed a new planet: the earth. Their mass holds the cosmic gases that surround them in place by gravity. This is how a primordial atmosphere is formed from hydrogen and helium.
However, the attraction of the freshly baked earth is not yet sufficient. The primordial atmosphere cannot withstand the onslaught of solar winds from space and is simply blown into the vastness of space.
4.4 billion years ago
The young earth is kept very hot and largely in a viscous state by the processes in its interior as well as by numerous meteorites that constantly hit it. In this way, the individual elements that make up the earth can be separated according to their weight and the current shell structure of the earth arises - from the solid inner core to the outer crust.
From now on, a new atmosphere is formed, the gas components of which this time do not come from outside - from space - but through outgassing - above all through volcanism - from the interior of the earth.
The earth's atmosphere now contains mainly carbon dioxide; in addition there are nitrogen, methane, ammonia, hydrogen sulfide and: water vapor. This condenses on dust and ash particles and so rain showers pelt non-stop on the earth.
Initially, however, it is still too hot for oceans to form. The water evaporates immediately when it hits the surface of the earth.
3.5 billion years ago
The earth has cooled down so much that the water no longer evaporates and can now collect on the earth's surface. Rain that lasts for hundreds of millions of years creates the oceans and washes the carbon dioxide and sulfur from the atmosphere.
What is left is mainly nitrogen. A little oxygen is already being formed: photochemically, by splitting water vapor (H2O) and carbon dioxide (CO2). The ozone layer begins to form in the stratosphere, the second layer of the earth's atmosphere.
In the meantime, amazing things are also happening under water: tiny tiny blue-green cells, cyanobacteria, are photosynthesizing for the first time. They use the energy of sunlight to turn water and carbon dioxide into sugar.
2.5 billion years ago
As a kind of waste product, photosynthesis creates oxygen, which initially accumulates in the water of the oceans. When the water is "saturated" with oxygen, this new "exhaust gas" then finally escapes into the earth's atmosphere. Life can now develop on land that breathes oxygen.
The ozone layer already formed in the stratosphere absorbs harmful ultraviolet (UV) radiation from the sun and protects life on earth so that it can spread unhindered.
The living things that breathe oxygen breathe out carbon dioxide. The carbon dioxide is absorbed by the plants, which in turn release oxygen into the atmosphere.
500 million years ago until today
The oxygen content gradually reaches the volume fraction that it still has in the atmosphere today. The ozone layer in the stratosphere is almost as pronounced as it is today.
About 200 million years ago, the composition of the air is as we know it today: 78 percent nitrogen, 21 percent oxygen, a little less than one percent argon - the rest of them are trace gases, which only make up a tiny proportion of the total volume but some, such as carbon dioxide and methane, are of paramount importance to our climate.
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