Oxygen presence in atmosphere

Today Earth’s atmosphere contains 20.9% oxygen and 78% nitrogen but it wasn’t always the case. 4.65 billion years ago when Earth was formed it was surrounded by gases rich in hydrogen compounds, methane, water, ammonia.. It has been continuing controversy how the Earth got its oxygen, it is now believed that it is not a remnant of the first primitive atmosphere. Scientist now think that water vapor, gases and carbon dioxide were released from Earth’s core at the beginning and then heated up as a result of meteoric collisions and radioactive decay, replacing first primitive atmosphere.

There are two processes that allowed oxygen rich conditions to evolve. First was the breakup of water molecules into hydrogen and oxygen in the upper layer of atmosphere by UV (ultra-violet) radiation from the sun and second was the photosynthesis after green plants evolved. Sediments formed prior to 2.5 billion years ago were deposited under oxygen-poor conditions. Life and the atmosphere evolved together as oxygen molecules increased in number; small amounts high in the atmosphere were split into oxygen atoms by solar ultraviolet radiation, which then combined with oxygen molecules (O2) to form ozone (O3).

The ozone layer itself absorbs UV radiation, which is lethal to life on Earth and was key factor for later evolving of more complex life forms. Oxygen content reached a critical level about 700 million years ago as life began to rapidly evolve into the great variety of life forms characteristic of the our Earth today.

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