What made the atmosphere?
The Solar System first formed from a nebula, a cloud of dust and gas. Gravitation attraction between the dust particles drew then together then the cloud began to spin and differentiate. Imagine water circling a plughole. Closer to the hold the water increases in speed and the circling ripples become closer together and steeper sided. In the embryonic solar system there was a high accumulation of matter in the centre of the cloud. This eventually became the sun. As it spun, it became denser and hotter until, about 5 billion years ago, it ignited. The remaining matter separated into bands, the heavier material was closer to the new sun. With time the inner bands accumulated into the rocky planets like Earth and Mars, the lighter material, which was mostly gases, formed the outer gassy giant planets like Jupiter and Saturn.
By 4.6 billion years ago, the Earth was a very hot inhospitable place. If seen from above, it would have looked like a huge pulsating volcano. Everywhere lava would have been erupting into a black sky. Momentarily, rafts of lava would have cooled into black rock. Almost immediately these would have been fractured by more eruptions from beneath and peppered by falling meteorites from above. Eventually the planet cooled enough for a crust to form and become permanent. By 3.7 billion years ago Earth had a permanent skin. Examples of these very early rocks, although vastly altered, can be found on Earth today. Some of these occur near Kalgoorlie in Western Australia.
The earliest atmosphere on earth was formed from gasses left over when the earth first formed. After the sun ignited and first warmed the earth with its rays, the atmosphere of Hydrogen and Helium mostly burnt off into space. It was replaced by gasses that belched out, during continuous volcanic eruptions associated with the Earth’s cooling. These gasses were mostly water vapour, carbon dioxide, sulphur dioxide (rotten egg gas) and nitrogen dioxide. Today these gases are regarded as Green House gases. The most prominent gas in the atmosphere was water vapour.
As the Earth and its atmosphere cooled, the water vapour condensed into clouds. Eventually it began to rain. It continued to rain for 700 million years. When it stopped raining, the Earth had an ocean. Many scientists believe that not all the ocean’s water came from the earth. Some of it also came from the many comets that slammed into to planet during its early years. No matter how the ocean was formed, what is important for our discussion about the human impact on climate change, is that when the atmosphere first occurred it did not contain any oxygen.
Like on Venus today, the green house gases in the early atmosphere kept the earth hot. These gases in are readily soluble in water. Thus the early ocean was a warm rich soup, full of the elements needed for life. At some point of time, life began in the oceans. Maybe amino-acids arrived on earth via meteorites, maybe they formed here, but, what we know for sure is by about 3.6 billion years ago simple singular celled organisms were living in our oceans. These organisms thrived in the absence of oxygen, but, sometime during the next 20 million years, organisms evolved that used the sun’s heat to convert water and carbon dioxide into hydrocarbon compound that formed their bodies, and a waste product, oxygen. This process is called photosynthesis and takes place in all plants today. The organisms that made this breakthrough were bacteria (cyanobacteria). These little organisms formed Stromatolites, the earliest life forms found in the fossil record. Although rare today, Stromatiolites have survived the eons of time. Good examples can be found at Shark Bay in Western Australia.
For millions of years the oxygen produced by bacteria and later by green algae, was absorbed by the ocean. Oxygen is an element that likes to react easily with other elements. Commonly it reacts with iron to form rust. This is what happened in the ancient oceans.
It was not only atmospheric gases that dissolved in the early oceans. Other elements from the Earth’s early crust, such as iron, were also dissolved. As the ocean waters became saturated with oxygen, it began to react with the iron dissolved in the seawater. The sea began to turn red as iron rusted and iron oxide, (haematite), formed. This drifted down to the seafloor and accumulated in vast layers. Today these layers are mined in places such as the Pilbara of Western Australia.
By 2.5 billion years ago oxygen had reacted with all the metals in the ocean capable of being rusted. The oceans waters were saturated with as much oxygen as they could hold and approximately 2.45 billion years ago, oxygen began to bubble out of the ocean and accumulate in the Earth’s atmosphere.