The Evolution of Earth’s Atmosphere


Earth’s atmosphere has been subject to continuous change since its formation. Earth’s primordial atmosphere started 4 billion years ago when the material forming Earth coalesced and melted; it organized itself into layers with dense materials at the core and less dense compounds closer to the surface.

The average speed of hydrogen molecules and helium atoms is greater than the escape velocity from Earth; these light gases were lost and swept away through photo-evaporation by the solar wind early in the Hadean Eon due to Earth's weak gravity and a violent collision with a planetoid. Earth’s Hadean atmosphere was left with methane, ammonia, water vapor, and small percentages of nitrogen and carbon dioxide. A cataclysmic meteorite bombardment kept much of Earth’s surface in the molten state; the incoming impactors may have brought additional water, methane, ammonia, hydrogen sulfide, and other gases that supplemented the atmosphere.

Earth’s second atmosphere, 2.5 billion years ago, was formed when Earth’s crust started to cool down and the amount of water vapor in the atmosphere decreased, as water started condensing in liquid form. As steam condensed into water, the atmospheric pressure of Earth became lower, and the water started dissolving gases such as ammonia, removing them from the atmosphere and creating ammonium compounds, amines, and other nitrogen-containing substances suitable for the origin of life.


The condensation of water with gases, such as sulfur dioxide, produced acid rain that created new minerals on Earth’s surface. Microfossils of sulfur-metabolizing cells have been found in 3.4-billion-year-old rocks, and it is known that the first aquatic photosynthetic organisms originated around 3.5 billion years ago. Billions of years would pass before the photosynthetic microorganisms could eventually change the composition of the atmosphere.

By then, Earth had cooled enough so that most of the water vapor in the atmosphere had condensed as water, and Earth had its first days without clouds. Ammonia and methane were only minor constituents of the atmosphere; carbon dioxide comprised about 15% of the atmosphere and the percentage of nitrogen was 75%. In essence, most of the original components of the atmosphere had escaped, precipitated as liquids, or reacted chemically to form solid compounds. Volcanic activity and photosynthetic bacteria were now the major factors influencing the Earth’s atmospheric composition.

During the stage of Earth’s third atmosphere, 0.5 billion years ago, methanogens combined hydrogen and carbon dioxide to produce methane and water. Sulfate-reducing bacteria combined methane and sulfate radicals; other organisms capable of photosynthesis used the energy of sunlight to convert the abundant carbon dioxide and water into carbohydrates (C6H12O6) and oxygen. The reaction of oxygen with iron in its reduced state continued to create deposits of iron in its oxidized state; additional oxygen continued to be consumed by oxidation of minerals on the Earth’s crust.

Oxygen molecules migrated into the upper atmosphere and formed an ozone layer; a region in the stratosphere located 15–35 kilometers above the Earth’s surface where oxygen molecules (O2) are converted to ozone (O3) by Sun's ultraviolet rays. The reverse conversion of ozone back to oxygen releases heat; the ozone layer basically absorbs high-energy ultraviolet radiation and converts it to heat.

Formation of Earth’s present atmosphere was marked by an abundance of multicellular life; most of the major groups of animals first appeared at this time. Vegetation covered the surface of Earth, and oxygen accounted for 30% of the atmosphere; air enriched with so much oxygen allowed giant insects to develop and caused frequent forest fires set off by lightning. A great mass-extinction event occurred 251 million years ago, as oxygen levels dropped from 30% to 12%, and carbon dioxide levels reached about 2000 ppm.

This was Earth’s worst mass extinction, and it eliminated 90% of ocean dwellers, and 70% of land plants and animals. The cause of this mass extinction is thought to have been a series of volcanic events in Siberia that lasted for about one million years and released large volumes of carbon dioxide and gases containing sulfur, chlorine, and fluorine.

Around 228 million years ago, oxygen levels had risen to about 15% of the atmosphere, and the first dinosaurs appeared. Oxygen levels continued to increase until 100 million years ago, oxygen had risen to about 23% of the atmosphere. At this time, dinosaurs were well established and modern mammals and birds began to develop. For the last 100 million years, the percentage of oxygen has fluctuated between 18% and 23% to the present level of about 21% of the atmosphere.



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SCIplanet is a bilingual edutainment science magazine published by the Bibliotheca Alexandrina Planetarium Science Center and developed by the Cultural Outreach Publications Unit ...
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