Earth's atmosphere

Earth's atmosphere is the layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly 78% nitrogen and 21% oxygen, with trace amounts of other gases. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.

The evolution of the Earth's atmosphere

The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research.

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The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current chemical composition from two notably different previous compositions. The original atmosphere was primarily helium and hydrogen; heat (from the still molten crust, and the sun) dissipated this atmosphere.

Related Topics:
Chemical - Helium - Hydrogen - Heat

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About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the "second atmosphere"; which was, primarily, carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. (Though very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggested that it may have had up to 40% hydrogen.http://newsrelease.uwaterloo.ca/news.php?id=4348) This second atmosphere had ~100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.

Related Topics:
Crust - Volcano - Steam - Carbon dioxide - Ammonia - Water vapor - Nitrogen - Oxygen - Times - Gas - Greenhouse effect - Freezing

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During the next few billion years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria are the cyanobacteria. Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen producing evolving phototropic organisms. They are responsible for the initial conversion of the earth’s atmosphere from an anoxic (state without oxygen) to an oxic (with oxygen) state. Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen playing a major role in oxygenating the atmosphere.

Related Topics:
Condensed - Rain - Ocean - Cyanobacteria

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Photosynthesizing plants would evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen.

Related Topics:
Photosynthesizing - Evolve - Fossil fuels - Sedimentary rock - Limestone - Animal shell - Bacteria

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As more plants appeared, the levels of oxygen increased significantly (while carbon dioxide levels dropped). At first it combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere — resulting in mass extinctions and further evolution. With the appearance of an ozone layer (ozone is an allotrope of oxygen) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".

Related Topics:
Element - Iron - Mass extinction - Ozone layer - Allotrope - Life - Ultraviolet - Radiation

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