Why the composition of the atmosphere is so specific and steady with 21% oxygen and 78% nitrogen?

The Earth’s atmosphere is composed of mixtures of gases that envelop the planet. Among these, two gases, Nitrogen, and Oxygen make up the majority of the atmosphere in terms of volume. They are crucially important as they propel an array of processes on Earth and maintain life on the planet. But, we cannot disregard the other gases regarded as minor or variable gases as they also play significant roles in managing heat and ensuring the availability of moisture on Earth.

Nitrogen and oxygen are deemed as constant gases. That is because their volume has not changed virtually for most of the planet’s recent history.

Nitrogen stays at 78%. It is an inert gas, mainly produced through volcanic activities. Under normal circumstances, it is a colorless, tasteless, and odorless gas. Nevertheless, it is present in all life forms, including the human body and plants. Moreover, Nitrogen is a vital component of protein found in eggs, milk, eggs, and vegetative tissues, mostly in grains and peas. It is also used in food storage, keeping bulk and packaged food as fresh as possible.

Meanwhile, oxygen is vital for human, plant, and animal respiratory processes. Without it, they would all die. Oxygen is also crucial in chemical reactions that break down rock materials through chemical weathering. No oxygen would also imply that there would be no fire or combustion process that can occur. Oxygen in the atmosphere is the main product of plant photosynthesis, wherein products absorb carbon dioxide and release oxygen in the process.

We’ve now established the importance, and we are fortunate that they have remained constant or steady at their rate. Such a drastic change in the air mixture may also cause dramatic effects in the natural processes and affect life on Earth. But, why isn’t the composition is specific and isn’t changing?

Truth to be told, various processes natural processes took a long time in the past before it has settled in its current makeup. For instance, oxygen concentrations in the Earth’s atmosphere were actually negligible during the Archaean era before rising to the constant 21% composition we have today. The increase didn’t go in one outburst but said to have occurred in various steps many years ago.

Oxygen levels started to go up, thanks to a new type of organism called the cyanobacteria, that took over and started to spread throughout the oceans. Not like the life forms before them, these bacteria evolved to produce oxygen through the process of photosynthesis. Soon, oxygen started accumulating in the planet’s atmosphere, giving birth to new life forms. Oxygen then has been present in the air ever since.

But, that brings us to newer questions. What made allowed cyanobacteria to take over? What were the actual oxygen levels in the said era? And, coming from zero, how did oxygen remain unwaveringly at 21% of the Earth’s atmosphere.

Well, no one actually knows. Scientists are baffled why it is at 21 percent and not 25, 30, or 40 percent. We don’t know how the modern oxygen system functions on the planet. But, one thing is for sure, the present equilibrium in the atmosphere is a miracle that continues to sustain life on Earth.

More Readings:

Atmosphere of Earth (Wikipedia)

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