Earth has had three different atmospheres. The first was mostly hydrogen and helium, but it dissipated due to the heat of the molten crust and solar wind. The second was mainly carbon dioxide and water vapor, similar to Venus, and was 100 times denser than today. The third atmosphere was formed by cyanobacteria, which converted the Earth’s atmosphere from oxygen-free to oxygen-containing. This event was toxic to most life forms, but it led to the atmosphere we need to survive today.
The Earth has had three atmospheres, each different in chemical composition. The first of Earth’s atmospheres, formed when the planet was still very young, was mostly hydrogen and helium. This atmosphere is about 4.57 billion years old and was short-lived: the heat of the molten crust and the solar wind dissipated this layer. Hydrogen and helium aren’t heavy enough to create a stable atmosphere unless the planet is very massive—these elements are more likely to gain escape velocities during random thermal fluctuations. This is one of the reasons why hydrogen and helium are very rare in the earth’s atmosphere today.
About 4.4 billion years ago, the crust solidified and numerous volcanoes formed, spewing steam, carbon dioxide and ammonia into the skies. Over time, the density of these volcanic gases has become sufficient to form a second atmosphere of the earth, mainly of carbon dioxide and water vapor. There was some nitrogen present, but hardly any free oxygen. This is similar to the current atmosphere of Venus, which is 96.5% carbon dioxide and 3.5% nitrogen. Earth’s atmosphere at this point had something else in common with Venus: It was about 100 times denser than today. The pressure at the surface would have been similar to the pressure under 1 km (0.62 mi) of water.
Earth’s atmosphere slowly began to thin as carbon dioxide dissolved in the oceans and precipitated out as carbonates. This covered much of the Earth in a layer of carbon and set the stage for the first few years of life.
By 3.5 billion years ago, life had emerged, in the form of archaea. About 2.7 billion years ago they were joined by microbes called cyanobacteria. Cyanobacteria were the first phototropic, oxygen-producing organisms and slowly began to absorb carbon dioxide from the atmosphere and release oxygen. It took a long time for cyanobacteria to really get started, but between 2.7 and 2.2 billion years, during the early Paleoproterozoic, these microbes converted the Earth’s atmosphere from an anoxic (oxygen-free) atmosphere to an oxic (oxygen-containing) state. ). . This is called the Great Oxidation or, more commonly, the Oxygen Catastrophe.
The oxidation event is sometimes considered a catastrophe because, to most life forms alive at the time, oxygen was toxic. Thus the oxygen catastrophe was one of the first major mass extinctions. However, as a benefit, we got another of the Earth’s atmospheres, the one we need to survive today.
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