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Atmospheric composition of the Earth

The atmosphere of the Earth has been changing continuously since its formation. The Sun, the Earth and other planets were formed when matter coalesced from a rotating nebula approximately 4,567 million years ago, according to the most widely accepted theory. The nebula had originated from the explosion of older stars containing heavy elements like iron that had been created by nuclear fusion of lighter atoms. The accumulation of mass at the center of the rotating nebula was so large that gravitational compression initiated the fusion of hydrogen into helium thus giving birth to the Sun which became a new shining star. The planets orbiting the Sun formed by accretion; the heavier elements concentrated in the cores and the lighter gaseous elements became the atmospheres.

Atmospheric evolution of the Earth

Composition of the protoplanetary nebula.
The most abundant chemical elements in the Sun are hydrogen (H 73.46%), helium (He 24.85%), oxygen (O 0.77%), carbon (C 0.29%), iron (Fe 0.16%) and smaller percentages of neon, silicon, magnesium, sulfur, etc.[1]  The atmosphere of Jupiter, which is the largest planet, is mostly hydrogen with about 10% helium and small amounts of other gases. These compositions indicate that the nebula from which our solar system originated was mostly hydrogen, helium and small amounts of heavier elements. Planets like Mercury, Venus, Earth and Mars lost their hydrogen and helium rapidly because their gravitational pull was not strong enough to retain these light elements. The loss of hydrogen and helium increased the concentration of heavier gases in the atmospheres of these terrestrial planets.


Earth's Magnetosphere
The Earth's magnetosphere protects
the atmosphere from the solar wind.
      Aurora Australis
Aurora light display around
the South Magnetic Pole

Earth's primordial atmosphere (Hadean Eon, 4.56 to 3.8 Ga)
When the material forming the Earth coalesced and melted, it organized itself into layers with dense materials at the core and less dense compounds closer to the surface. The gases comprising the atmosphere formed the outermost layer and had a composition similar to that of the gases of the condensing planetary nebula. The circulation of a molten metallic iron-nickel alloy in the core of the Earth established the magnetosphere, a region in space where the motions of gas and fast charged particles are controlled by the magnetic field of the Earth. The magnetosphere deflects most solar wind ions before they penetrate the atmosphere, but the charged particles which are not deflected are directed toward the Earth's magnetic poles where high-energy collisions with atoms of the atmosphere produce an aurora light display. The average speed of hydrogen molecules and helium atoms is greater than the escape velocity from the Earth, and these light gases were lost and swept away through photoevaporation by the solar wind early in the Hadean Eon due to Earth's weak gravity and a violent collision with a planetoid around 4.45 Ga. The planetoid, called Theia, is estimated to have been about the size of Mars. The impact added extra mass to the Earth, but a portion of the impact debris went into orbit and accreted to form the Moon.[10]

After the hydrogen and helium had escaped, Earth's Hadean atmosphere was left with methane, ammonia, water vapor, and small percentages of nitrogen and carbon dioxide. A cataclysmic meteorite bombardment around 3.9 Ga kept the Earth's surface in the molten state, and the incoming comets or meteorites may have brought additional water, methane, ammonia, hydrogen sulfide and other gases that supplemented the atmosphere. Toward the end of the Hadean Eon, volcanic activity started increasing the percentage of carbon dioxide in the atmosphere.

The high surface temperature of the Earth during the Hadean eon favored the depletion of atmospheric methane through the endothermic reaction of methane with the steam in the atmosphere. Reactions such as the following require high temperatures of approximately 700°C to 1100°C which would have been common in the magma oceans and volcanic areas of the Hadean Earth. The resulting carbon monoxide would readily combine with metals to form carbonyl compounds.

CH4 + H2O    CO + 3 H2
Methane reacts with steam to produce carbon monoxide and hydrogen.

Earth's second atmosphere (Archean Eon, 3.8 to 2.5 Ga)
The Earth started to cool down during the Archean Eon. The surface of the Earth changed from molten lava to solid rock. Water started condensing in liquid form and the amount of water vapor in the atmosphere started to decrease. Continuous rainfall for millions of years led to the buildup of the oceans. As steam condensed into water, the atmospheric pressure of the Earth became lower, and the water started dissolving gases like ammonia and removed them from the atmosphere 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 the Earth's surface. Volcanic carbon dioxide peaked during the Archean Eon and started to decrease through the formation of carbonate minerals that resulted from reactions of metals with the carbonic acid generated from carbon dioxide and water.

NH3 + H2O    NH4+ + OH-
CO2 + H2O    H2CO3
Reactions of ammonia and carbon dioxide with water

Calcite
Calcite crystal consisting of calcium carbonate (CaCO3)

Microfossils of sulfur-metabolizing cells have been found in 3.4-billion-year-old rocks[6], and it is known that the first aquatic photosynthetic organisms originated around 3.5 Ga. The oxygen-producing cyanobacteria (blue-green algae) would eventually change the composition of the atmosphere. By the middle of the Archean Eon, the Earth had cooled enough so that most of the water vapor in the atmosphere had condensed as water, and the 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%.[5] In essence, most of the original components of the atmosphere had escaped, precipitated as liquids or reacted chemically to form solid compounds. The volcanic activity and the photosynthetic bacteria were now the major factors influencing the Earth's atmospheric composition.

Earth's third atmosphere (Proterozoic Eon, 2.5 to 0.54 Ga)
Monocellular life proliferated during the Proterozoic Eon. Anaerobic microbial life thrived in a planet with little oxygen. Anaerobic bacteria obtained their energy by reducing sulfate (SO4--) to hydrogen sulfide (H2S), but other organisms capable of photosynthesis were using the energy of sunlight to convert the abundant carbon dioxide and water into carbohydrates (C6H12O6) and oxygen which was deadly to the anaerobes.

6 CO2 + 6 H2O    C6H12O6 + 6 O2
Production of oxygen through photosynthesis

By the first quarter of the Proterozoic Eon, the Sun had become brighter and its luminosity had increased to 85% of the present level. By this time, most of the carbon dioxide had been depleted from the atmosphere, leaving nitrogen as the main atmospheric gas with a small percentage of oxygen. Nitrogen gas (N2), which is quite inert chemically, had been a small percentage of the Earth's atmosphere during the Hadean Eon, but it became the major component of the atmosphere during the Proterozoic Eon once all the other gases were gone.

Banded Iron Formation
Banded Iron Formation (BIF) from Australia

The Earth's surface and seas contained great quantities of iron that readily combined with oxygen to produce iron oxides. Although photosynthetic organisms had been releasing oxygen since Archean times, the oxygen levels could not build up in the atmosphere because the oxygen was being depleted by the oxidation of metals and by the oxidation of methane to yield carbon dioxide and water in the presence of ultraviolet (UV) radiation.

4 Fe + 3 O2    2 Fe2O3
Oxidation of metallic iron to form iron(III)oxide

CH4 + 2 O2    CO2 + 2 H2O
Oxidation of methane

The cooling of the Earth during the Proterozoic stabilized the land masses and reduced the volcanic outgassing of carbon dioxide. Methane and carbon dioxide are greenhouse gases; their decrease in the atmosphere may have contributed to the Huronian glaciation that lasted from 2.4 Ga to 2.1 Ga. The cold temperature sequestered additional methane from the atmosphere by forming methane clathrate, a crystal structure of water similar to ice that traps a large amount of methane. The Great Oxidation Event or Oxygen Catastrophe also occurred around 2.4 Ga, and it created banded iron formations (BIF) by precipitating dissolved iron.[7] The reaction of oxygen with iron in its reduced state (Fe2+) continued to create BIF deposits of iron in its oxidized state (Fe3+) until about 1.9 Ga whenever volcanic activity or oceanic currents exposed unoxidized iron. Additional oxygen continued to be consumed by oxidation of mantle minerals, but enough free oxygen accumulated in the atmosphere to kill anaerobes near the Earth's surface thus creating an opportunity for the development of aerobic life forms.

Starting around 2.4 Ga, some oxygen molecules migrated into the upper atmosphere and formed an ozone layer. This is a region in the stratosphere where oxygen molecules (O2) are converted to ozone (O3) by the 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. The high energy UV light is dangerous for life because it can cause mutations in DNA sequences. By the middle of the Proterozoic Eon, around 1.6 Ga, the atmosphere of the Earth had more than 10% oxygen. With the increased oxygen levels and the protection of the ozone layer, organisms capable of aerobic respiration could now proliferate all over the surface of the Earth.

Earth's present atmosphere (Phanerozoic Eon, 0.542 Ga to present)
The beginning of the Phanerozoic Eon, the Cambrian period, is marked by an abundance of multicellular life. Most of the major groups of animals first appeared at this time. Vegetation covered the surface of the 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.

Meganeura
Meganeura, a dragonfly from the Carboniferous period had a wingspan of 65 cm (2.1 ft)

A great mass-extinction event occurred 251 million years ago (0.251 Ga) marking the boundary of the Permian and Triassic periods. 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.[8,12] By 228 million years ago, oxygen levels had risen to about 15% of the atmosphere, and the first dinosaurs appeared.[11] Oxygen levels continued to increase, and by the end‐Cretaceous, 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.[13] The following table shows the percentages of the five most common gases in today's atmosphere.[9]

Main components of Earth's atmosphere by volume
Nitrogen (N2) 780,840 ppm (78.084%)
Oxygen (O2) 209,460 ppm (20.946%)
Argon (Ar) 9,340 ppm (0.9340%)
Carbon dioxide (CO2 394.45 ppm (0.039445%)
Neon (Ne) 18.18 ppm (0.001818%)

Earth's future atmosphere.
Since the beginning of the industrial revolution in 1750, humans have been burning coal and petroleum products to provide the energy to power machinery. The combustion of fossil fuels has been generating large quantities of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The chart below shows the levels of atmospheric carbon dioxide during the last millennium and the sharp rise during the twentieth century.[2] The concentration of atmospheric carbon dioxide today is 394 parts per million (ppm) and the North Pole's mean annual temperature is -20°C. When the carbon dioxide concentration was 2,000 ppm 55 million years ago during the Paleocene–Eocene Thermal Maximum (PETM), the North Pole's temperature averaged 23°C (73.4°F).[3] It is thought that elevated levels of greenhouse gases will cause global warming and influence weather patterns. Many cities bordering the coastal areas will be permanently flooded if the ice deposits in Greenland and Antarctica melt.

Carbon Dioxide
Geologic and Biological Timeline of the Earth
Take a trip on a Time Machine
References.
  1. A New Sun: The Solar Results From Skylab, by John Eddy, NASA SP-402, 1979, page 37.
  2. Intergovernmental Panel on Climate Change (IPCC) report, Climate Change 2001: The Scientific Basis.
  3. Pagani, M., K. Caldeira, D. Archer and J.C. Zachos, An ancient carbon mystery. Science 314, 1556-1557, 2006.
  4. At the Heart of Hartley-2, a New Breed of Comet? [link]
  5. Rye R, Kuo PH, Holland HD, Atmospheric carbon dioxide concentrations before 2.2 billion years ago, Nature, 1995 Dec 7; 378(6557):603-5.
  6. Wacey, D., Kilburn, M. R., Saunders, M., Cliff, J., Brasier, M. D., Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia, Nature Geosci. http://dx.doi.org/10.1038/NGEO1238 (2011).
  7. Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. p. 323. ISBN 0-7167-2882-6.
  8. Ward, Peter D., Impact from the Deep, Scientific American, Vol. 295, No. 4, October 2006, pp. 64-71.
  9. Atmosphere of Earth, Wikipedia [link]
  10. Cameron, A. G. W.; Ward, W. R. (March 1976). "The Origin of the Moon". Abstracts of the Lunar and Planetary Science Conference 7: 120–122.
  11. Kristina A. Curry Rogers and Michael D. D'Emic, Triumph of the Titans: How Sauropods Flourished, Scientific American, May 2012, Vol 306, Number 5, p. 52.
  12. Benjamin A. Black, Linda T. Elkins-Tanton, Michael C. Rowe, Ingrid U. Peate, Magnitude and consequences of volatile release from the Siberian Traps, Earth and Planetary Science Letters, Vol. 317-318 (February 2012), pp. 363-373, doi:10.1016/j.epsl.2011.12.001
  13. Thomas J. Algeo, Ellery Ingall, Sedimentary Corg:P ratios, paleocean ventilation, and Phanerozoic atmospheric pO2, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 256, Issues 3–4, 6 December 2007, Pages 130–155


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