Earth's Atmosphere: Historical Overview

The chemical composition of the atmosphere has changed significantly over the history of the Earth. The composition of the atmosphere has been influenced by a number of processes, including interaction with the solar wind; “outgassing” of volatiles (materials that easily vaporize to form gases) originally trapped in the Earth’s interior during its formation; the geochemical cycling of carbon, nitrogen, hydrogen, and oxygen compounds between the surface, the ocean, and the atmosphere; and the origin and evolution of life.

Overview

About 4.5 to 4.6 billion years ago, the primordial solar nebula, a part of a large interstellar cloud of gas and dust, began to contract under the influence of gravity. This contraction led to the formation of the sun and the rest of the solar system including Earth. The primordial solar nebula was composed mostly of hydrogen gas, with a smaller amount of helium, still smaller amounts of carbon, nitrogen, and oxygen, and still smaller amounts of the rest of the elements of the periodic table.

As the solar nebula contracted, its rotational speed increased to conserve angular momentum. Most of its mass contracted to its center, there becoming the protosun, while the remaining matter was spun off into an equatorial disk. Within the disk, matter condensed from the gaseous state into small, solid grains. Only materials with high melting-point temperatures could condense near the developing protosun; materials with lower melting points condensed farther out. The small solid grains collided with each other and stuck together in a process called accretion that led to the growth of larger bodies called planetesimals. Continued accretion resulted in fewer but larger planetesimals that eventually grew into protoplanets and finally planets, such as Earth.

About the time that the newly formed Earth was reaching its approximate present mass, it may have acquired a temporary atmosphere of hydrogen, helium, methane, ammonia, water vapor, and carbon dioxide—gases that were common in the solar nebula. However, even if such an atmosphere had surrounded the very young Earth, it would have been very short-lived. The Earth’s mass was too small to have enough gravity to retain hydrogen and helium for long, and those gases would have quickly escaped into space. Ammonia and methane were chemically unstable in the early Earth’s environment and were readily destroyed by ultraviolet radiation from the young sun. Also, as the young Sun went through its T-Tauri phase of evolution, very strong solar winds (the supersonic flow of protons and electrons from the Sun) would have quickly stripped most of the rest of this primitive atmosphere away.

The early Earth was heated by the intense bombardment of remaining planetesimals and the decay of radioactive elements, to the point where it at least partly melted. The heating and melting released volatiles trapped in its interior by a process called outgassing, forming a gravitationally bound atmosphere. (It is believed that the atmospheres of Mars and Venus also originated in this manner.) The period of extensive volatileoutgassing may have lasted for many tens of millions of years. The outgassed volatiles probably had roughly the same chemical composition as do present-day volcanic gaseous emissions: by volume about 80 percent water vapor, 10 percent carbon dioxide, 5 percent sulfur dioxide, 1 percent nitrogen, and smaller amounts of hydrogen, carbon monoxide, sulfur, chlorine, and argon.

The water vapor that outgassed from the interior soon reached its saturation point, which is controlled by the atmospheric temperature and pressure. Once the saturation point was reached, the atmosphere could not hold any additional gaseous water vapor. Any new outgassed water vapor that entered the atmosphere would have precipitated out of the atmosphere as rain that fell and formed the Earth’s vast oceans. Only small amounts of water vapor remained in the atmosphere—ranging from a fraction of a percentage point to several percent by volume, depending on atmospheric temperature, season, and latitude.

The outgassed atmospheric carbon dioxide, being very water-soluble, readily dissolved into the newly formed oceans and formed carbonic acid. In the oceans, carbonic acid formed ions of hydrogen, bicarbonate, and carbonate. The carbonate ions reacted with ions of calcium and magnesium in the ocean water, forming carbonate rocks, which precipitated out of the ocean and accumulated as seafloor carbonate sediments. Most of the outgassed atmospheric carbon dioxide formed carbonates, leaving only trace amounts of gaseous carbon dioxide in the atmosphere (about 0.035 percent by volume).

Sulfur dioxide, the third most abundant component of volatile outgassing, was chemically transformed into other sulfur compounds and sulfates in the atmosphere. Eventually, the sulfates formed atmospheric aerosols and diffused out of the atmosphere onto the surface.

The fourth most abundant outgassed component, nitrogen, is chemically inert in the atmosphere and thus was not chemically transformed, as was sulfur dioxide. Unlike carbon dioxide, nitrogen is relatively insoluble in water and, unlike water vapor, does not condense out of the atmosphere. For these reasons, nitrogen remained in the atmosphere to become its major constituent (now 78.08 percent by volume). In this way, volatile outgassing led to the formation of the Earth’s atmosphere, oceans, and carbonate rocks.

The molecules of nitrogen, carbon dioxide, and water vapor in the early atmosphere were acted upon by Solar ultraviolet radiation and atmospheric lightning. In the process, molecules of formaldehyde and hydrogen cyanide could have been chemically synthesized, which would have precipitated and diffused out of the atmosphere into the oceans. In the oceans, the formaldehyde and hydrogen cyanide may have entered into polymerization reactions that eventually led to the chemical synthesis of amino acids, the building blocks of living systems. The synthesis of amino acids from nitrogen, carbon dioxide, and water vapor in the atmosphere and ocean is called chemical evolution. Chemical evolution preceded and provided the material for biological evolution.

There is chemical trace evidence for the existence of microbial living organisms on the Earth by about 3.8 billion years ago; the oldest known simple fossils are at least 3.5 billion years old. These earliest living organisms were anaerobic since there was no free oxygen in the atmosphere and oceans. Photosynthesis evolved in one or more of these early microbial groups, such as cyanobacteria. In photosynthesis, the organism utilizes water vapor and carbon dioxide in the presence of sunlight and chlorophyll to form carbohydrates, used by the organism for food. In the process of photosynthesis, oxygen is given off as a metabolic by-product. The production of oxygen by photosynthesis was a major event on the Earth and transformed the composition and chemistry of the early atmosphere. As a result of photosynthetic production, oxygen built up to become the second most abundant constituent of the atmosphere (now 20.9 percent by volume).

The evolution of atmospheric oxygen had very important implications for the evolution of life. The presence and buildup of oxygen led to the evolution of respiration, which replaced fermentation as the energy production mechanism in living systems. Accompanying and directly controlled by the buildup of atmospheric oxygen was the origin and evolution of atmospheric ozone, which is chemically formed from oxygen. The production of atmospheric ozone resulted in shielding the Earth’s surface from biologically lethal solar ultraviolet wavelengths between about 200 and 300 nanometers. Prior to the evolution of the atmospheric ozone layer, early life was restricted to a depth of at least several meters below the ocean surface. At this depth, the ocean water offered shielding from solar ultraviolet radiation. The development of the atmospheric ozone layer and its consequent shielding of the Earth’s surface permitted early life to leave the safety of the oceans and go ashore for the first time in the history of the planet. Theoretical computer calculations indicate that atmospheric ozone provided sufficient shielding from biologically lethal ultraviolet radiation for the colonization of the land once oxygen reached about one-tenth of its present atmospheric level.

Mercury, Venus, and Mars formed in a fashion similar to Earth, but they developed very differently because of their masses and distances from the Sun. They all experienced a period of heating, partial melting, and volatile outgassing of the same gases that led to the formation of the Earth’s atmosphere. However, Mercury’s distance from the sun is so close (resulting in high temperatures) that its relatively weak gravity (due to its small mass) was unable to retain more than a thin trace of gases, and thus today it has virtually no atmosphere. In the case of Venus and Mars, the important difference is that the outgassed water vapor never existed in the form of liquid water on the surfaces of those two planets.

Because of Venus’s closer distance to the sun (108 million kilometers versus 150 million kilometers for Earth), its lower atmosphere was too hot to permit the outgassed water vapor to condense out of the atmosphere. Thus, the outgassed water vapor remained in gaseous form in the atmosphere and, over geological time, was broken apart by solar ultraviolet radiation to form hydrogen and oxygen. The very light hydrogen gas quickly escaped from the atmosphere of Venus, and the heavier oxygen combined with surface minerals to form a highly oxidized surface. In the absence of liquid water on the surface of Venus, the outgassed carbon dioxide remained in the atmosphere and built up to become the overwhelming constituent of the atmosphere of Venus (about 96 percent by volume). The outgassed nitrogen accumulated to comprise only about 4 percent by volume of the atmosphere of Venus. This carbon dioxide and nitrogen atmosphere is very massive—it produces an atmospheric pressure at the surface of the Venus about ninety times the surface pressure of Earth’s atmosphere. If the outgassed carbon dioxide in the atmosphere of Earth had not left via dissolution in the oceans and resultant carbonate rock formation, the Earth’s surface atmospheric pressure would be about seventy times greater than at present, with carbon dioxide comprising about 98–99 percent of the atmosphere and nitrogen about 1–2 percent. Thus, the atmosphere of Earth would closely resemble that of Venus. The thick carbon dioxide atmosphere of Venus causes a very significant greenhouse temperature enhancement, giving the lower atmosphere and surface of Venus a temperature of about 750 kelvins (about 477 degrees Celsius), which is hot enough to melt lead. For comparison, the average surface temperature of Earth is only about 288 kelvins (about 15 degrees Celsius).

Like Venus, Mars has an atmosphere composed primarily of carbon dioxide (about 95 percent by volume) and nitrogen (about 3 percent by volume). Because of Mars’s greater distance from the Sun (228 million kilometers versus 150 million kilometers for Earth), the temperature of the surface of Mars was too low to support the presence of liquid water. There may be very large quantities of outgassed water in the form of ice or permafrost below the surface of Mars. In the absence of liquid water, the outgassed carbon dioxide remained in the atmosphere. The atmospheric pressure at the surface of Mars, however, is only about 7 millibars (the average surface atmospheric pressure on Earth is 1,013 millibars). The smaller mass of the atmosphere of Mars compared to the atmosphere of Venus and Earth may be attributable to the smaller mass of Mars and, therefore, the smaller mass of volatiles trapped in the interior of Mars during its formation. In addition, it appears that the amount of gases trapped in the interiors of Venus, Earth, and Mars during their formation decreased with increasing distance from the sun. Venus appears to have trapped the greatest amount of gases and was the most volatile-rich planet, Earth trapped the next greatest amount, and Mars trapped the smallest amount.

The atmospheres of the outer planets—Jupiter, Saturn, Uranus, and Neptune—all contain appreciable quantities of hydrogen and helium, along with methane and ammonia. It is believed that the atmospheres of these planets, unlike the atmospheres of the terrestrial planets Venus, Earth, and Mars, are captured remnants of the primordial solar nebula. Because of the outer planets’ large masses and their great distance from the Sun resulting in their very low temperatures, hydrogen, helium, methane, and ammonia are stable and long-lived constituents of their atmospheres.

Methods of Study

Information about the origin, early history, and evolution of the Earth’s atmosphere comes from a variety of sources. Information on the origin of Earth and other planets is based on theoretical computer simulations. These computer models simulate the collapse of the primordial solar nebula and the formation of the planets. Astronomical observations of what appear to be equatorial disks and the possible formation of planetary systems around young stars have provided new insights into the computer modeling of this phenomenon. Information about the origin, early history, and evolution of the atmosphere is based on theoretical computer models of volatile outgassing and the geochemical cycling and photochemistry of the outgassed volatiles. The process of chemical evolution—which led to the synthesis of organic molecules of increasing complexity, the precursors of the first living systems on the early Earth—is studied in laboratory experiments. In these experiments, mixtures of gases simulating the Earth’s early atmosphere are energized by ultraviolet radiation, electrical discharges, or heated rocks, simulations of energy sources available on the early Earth. The resulting products are analyzed by chemical techniques.

One of the parameters affecting atmospheric photochemical reactions, chemical evolution, and the origin of life was the flux of solar ultraviolet radiation incident on the early Earth. Astronomical measurements of the ultraviolet emissions from young, sunlike stars have provided important information about ultraviolet emissions from the young sun during the very early history of the atmosphere.

Geological and paleontological studies of the oldest rocks and the earliest fossil records have provided important information on the evolution of the atmosphere and the transition from an oxygen-deficient to an oxygen-sufficient atmosphere. Studies of the biogeochemical cycling of the elements have provided important insights into the later evolution of the atmosphere. Thus, studies of the origin and evolution of the atmosphere are based on a broad cross-section of the sciences, involving astronomy, geology, geochemistry, geophysics, and biology as well as atmospheric chemistry.

Context

Studies of the origin and evolution of the atmosphere have provided new insights into the processes and parameters responsible for global change. Understanding the history of the atmosphere provides insight into its future. Today, atmospheric changes being studied for their long-term effects include the buildup of greenhouse gases like carbon dioxide, nitrous oxide, methane, and water vapor, as well as the depletion of ozone in the stratosphere. These atmospheric changes, primarily induced by the side effects of human industry and activity, have caused the earth's overall temperature to warm and had other drastic effects such as the rising of sea levels and the melting of glaciers. The study of the evolution of the atmosphere has provided new insights into the biogeochemical cycling of elements between the atmosphere, biosphere, land, and ocean. Understanding this cycling is a key to understanding environmental problems and possible remedies. Studies of the origin and evolution of the atmosphere have also provided new insights into the origin of life and the possibility of life outside the Earth.

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