Where did oxygen come from?

Science in dialogue

How did oxygen get into the air?

What we colloquially refer to as “air” is a mixture of different gases. Oxygen is represented with about 21 percent. Nitrogen has the largest share at 78 percent. The noble gas argon is around one percent contained. In addition, traces of other gases occur, including the carbon dioxide that plants need for photosynthesis. The air as we know it today has only been around for about 350 million years.

After the formation of the earth around 4.5 billion years ago, oxygen was initially bound in minerals (mainly silicates) and carbon dioxide and therefore did not occur in the primordial atmosphere. It was not until 3.5 billion years ago that blue-green algae (which are actually not algae but bacteria) appeared in the water, releasing oxygen from the water through photosynthesis, that gaseous oxygen emerged for the first time on earth. Initially, however, this had little effect on the composition of the atmosphere, but instead reacted immediately with iron and sulfur compounds dissolved in the oceans. Only when this process came to a standstill about 2.3 billion years ago did the gas from the oceans enter the atmosphere. The oxygen concentration gradually increased, but massive iron sulfide deposits also had to be oxidized on land. This process was essentially complete after a few hundred million years.

1.5 billion years ago, the first creatures arose in water that could use oxygen to generate energy through a biochemical process known as breathing. The ozone layer began to form in higher layers of the atmosphere around 700 million years ago. Life on land was now able to develop, as it was better protected from the earth's harmful ultraviolet radiation. The oxygen content in the atmosphere has probably been at today's level for 350 million years. At that time, a balance was established between oxygen production through photosynthesis and oxygen consumption through breathing.

In space, oxygen is the third most abundant element after hydrogen and helium. It is formed inside stars by nuclear fusion. At temperatures of over 15 million degrees Celsius, it is created from the fusion of helium atomic nuclei. When stars explode in a supernova at the end of their "life" or become a so-called red giant and gradually release the matter of their shell into space, the oxygen is released together with the other formed elements and forms gas and dust clouds like those, from which our solar system was formed.

The question was answered by Dr. Mario Trieloff from the Mineralogical Institute of Heidelberg University.