Atmosphere’s Evolution

The chemical composition of the atmosphere has changed significantly over the 4.6-billion-year history of Earth. The composition of the atmosphere has been influenced by a number of processes, including the “outgassing” of volatile materials originally trapped in 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 evolution of life.

Volatile Outgassing

About 5 billion years ago, a cloud of interstellar gas and dust, called the primordial solar nebula, began to condense under gravity's influence. This condensation led to the formation of the sun, moon, Earth, the other planets and their satellites, asteroids, meteors, and comets. The primordial solar nebula was composed almost entirely 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. When the newly formed Earth attained its approximate mass, gases released from the planet’s interior could be retained by Earth’s gravity instead of escaping into space, thus forming a gravitationally bound atmosphere. The atmospheres of the other terrestrial planets, Mars and Venus, also formed this way. The release of gases and other volatiles in this manner is called volatile outgassing. The period of extensive volatile outgassing may have lasted for tens of millions of years. The outgassed volatiles or gases had roughly the same chemical composition as present-day volcanic emissions: 80 percent water vapor by volume, 10 percent carbon dioxide by volume, 5 percent sulfur dioxide by volume, 1 percent nitrogen by volume, and smaller amounts of hydrogen, carbon monoxide, sulfur, chlorine, and argon.

The water vapor that outgassed from the interior eventually reached the 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 in the form of liquid water. The equivalent of several cubic kilometers of liquid water released from Earth’s interior in gaseous form precipitated out of the atmosphere and formed the oceans. Only small amounts of water vapor remained in the atmosphere, ranging from a fraction of a percent to several percent by volume, depending on atmospheric temperature, season, and latitude.

The outgassed atmospheric carbon dioxide, being somewhat water soluble, dissolved in the newly formed oceans and subsequently formed carbonic acid through its reaction with water. Once formed, carbonic acid can dissociate into ions of hydrogen, bicarbonate, and carbonate. The carbonate ions reacted with ions of calcium and magnesium in the ocean water, forming first insoluble carbonate salts, which precipitated out of the ocean and accumulated as seafloor carbonate sediments, eventually accumulating in sufficient quantities to form beds of carbonate rock. 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, including sulfuric acid and other sulfates in the atmosphere. Eventually, the sulfates formed atmospheric aerosols and gravitated out of the atmosphere to settle on the surface.

The fourth most abundant outgassed compound, nitrogen, is almost completely chemically inert in the atmosphere and thus was not readily transformed in large quantities, as was sulfur dioxide. Only minor amounts of nitrogen would be converted to various oxides by the action of lightning, to be trapped as nitrogen oxide salts in minerals. Unlike carbon dioxide, nitrogen is relatively insoluble in water and, unlike water vapor, does not condense out of the atmosphere. For these reasons, nitrogen built up in the atmosphere to become its major constituent (78.08 percent by volume). Accordingly, outgassed volatiles led to the formation of Earth’s atmosphere, oceans, and the earliest, prebiotic carbonate rocks.

Chemical Evolution

It has been demonstrated in laboratory experiments that molecular nitrogen, carbon dioxide, and water vapor in the early atmosphere would have been acted upon by solar ultraviolet radiation and atmospheric lightning. In the process, molecules of formaldehyde and hydrogen cyanide were chemically synthesized in the early atmosphere. These molecules were precipitated and diffused out of the atmosphere into the oceans. In the water, formaldehyde and hydrogen cyanide entered into chemical reactions that eventually led to the chemical synthesis of amino acids—the building blocks of proteins in living systems. The synthesis of amino acids and other compounds from nitrogen, carbon dioxide, and water vapor in the atmosphere is called chemical evolution. Chemical evolution preceded and provided the material for biological evolution.

For many years, it was thought that the early atmosphere was composed of ammonia, methane, and hydrogen rather than of carbon dioxide, nitrogen, and water vapor. Experiments show, however, that ammonia and methane are chemically unstable and are readily destroyed by both solar ultraviolet radiation and chemical reaction with the hydroxyl radical, which is formed from water vapor. In addition, ammonia is very water soluble and is readily removed from the atmosphere by precipitation. Hydrogen, the lightest element, is readily lost from a planet by gravitational escape. Thus, an early atmosphere composed of methane, ammonia, and hydrogen would be very short lived, unless these gases were produced at a rate equal to their destruction or loss rates (an equilibrium state). These gases are also known to be extremely efficient “greenhouse gases,” even more effective than carbon dioxide; their presence in the primordial atmosphere in any substantial amount would have maintained an extraordinarily high atmospheric temperature, by which many of the materials that were formed would thermally decompose. Today, methane and ammonia are very minor components of the atmosphere, at concentrations of 1.7 parts per million by volume and 1 part per billion by volume, respectively. Both gases are produced by microbial activity at the ground surface, and methane is released during coal mining and oil production, and from seafloor accumulations of methane hydrate. Clearly, microbial activity and microbes were nonexistent during the prebiotic phase of the planet. The atmospheres of the outer gas giant planets—Jupiter, Saturn, Uranus, and Neptune—all contain quantities of hydrogen, methane, and ammonia. It is believed that the atmospheres of these planets, unlike the atmospheres of the terrestrial planets—Earth, Venus, and Mars—are captured remnants of the primordial solar nebula resulting from the greater ability of the gravitational fields of those large planets to capture such light materials, preventing them from being drawn toward the sun. Because of the outer planets’ great distance from the sun and their very low temperatures, hydrogen, methane, and ammonia are stable and long-lived constituents of their atmospheres. This is not true of hydrogen, methane, and ammonia in Earth’s atmosphere.

Some have suggested that at the time of its formation, Earth may have also captured a remnant of the primordial solar nebula as its very first atmosphere. Such a captured primordial solar nebula atmosphere would have been composed of mostly hydrogen (about 90 percent) and helium (about 10 percent), the two major elements of the nebula. Even if such an atmosphere had surrounded the very young Earth, it would have been very short-lived. As the young sun went through the T Tauri phase of its evolution, very strong solar winds (the supersonic flow of protons and electrons from the sun) associated with that phase would have quickly dissipated this remnant atmosphere. In addition, there is no geochemical evidence to suggest that early Earth ever possessed a primordial solar nebula remnant atmosphere.

Evolution of Atmospheric Oxygen

There is microfossil evidence for the existence of fairly advanced anaerobic microbial life on Earth by about 3.8 billion years ago. The ability to carry out photosynthesis evolved in one or more of these early microbial species. Through photosynthesis, the organism utilizes water vapor and carbon dioxide in the presence of sunlight and chlorophyll to form glucose and molecular oxygen. The glucose molecules are subsequently used by the organism for food and for biopolymerization into starches and celluloses. The production of oxygen by photosynthesis was a major event on Earth and eventually transformed the composition and chemistry of the early atmosphere as oxygen built up to become the second most abundant constituent of the atmosphere (20.90 percent by volume). It has been estimated that atmospheric oxygen reached only 1 percent of its present atmospheric level 2 billion years ago, 10 percent of its present atmospheric level about 550 million years ago (at the beginning of the Paleozoic), and its present atmospheric level as early as 400 million years ago.

The evolution of atmospheric oxygen had important implications for the evolution of life. Because molecular oxygen is a very effective oxidizing agent and would have been harmful to existing anaerobic life forms, the presence and buildup of oxygen required the evolution of respiration and aerobic organisms. Accompanying and directly controlled by the buildup of atmospheric oxygen were the origin and evolution of atmospheric ozone, which is chemically formed from oxygen. The evolution of atmospheric ozone resulted in the shielding of Earth’s surface from biologically lethal solar ultraviolet rays. The development of the atmospheric ozone layer and its accompanying shielding of Earth’s surface permitted early life to evolve such that it could leave the safety of the oceans and go ashore for the first time in the planet's history. Before the evolution of the atmospheric ozone layer, early life was restricted to a depth of several meters below the ocean surface. At this depth, the ocean water offered shielding from solar ultraviolet radiation. Theoretical computer calculations indicate that atmospheric ozone provided sufficient shielding from biologically lethal ultraviolet radiation for the evolution of non-marine organisms once oxygen reached about one-tenth of its present atmospheric level.

Venus and Mars

Calculations indicate that the atmospheres of Venus and Mars also formed due to the volatile outgassing of the same gases that led to the formation of Earth’s atmosphere—water vapor, carbon dioxide, and nitrogen. In the case of Venus and Mars, however, the outgassed water vapor may never have existed in the form of liquid water in quantities comparable to those on Earth. 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 remained in gaseous form in the atmosphere and, over geological time, was decomposed by solar ultraviolet radiation into molecular 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 Venus’s atmosphere, about 96 percent by volume. The outgassed nitrogen accumulated to make up about 4 percent by volume of the atmosphere of Venus. The present-day carbon dioxide and nitrogen atmosphere of Venus is massive—its atmospheric surface pressure is about 90 atmospheres (compared to the surface pressure of Earth’s atmosphere of only one atmosphere). If the outgassed carbon dioxide in Earth’s atmosphere had not been dissipated via dissolution in the oceans and carbonate formation, the planet’s surface atmospheric pressure would presumably be about 70 atmospheres, with carbon dioxide accounting for about 98 to 99 percent of the atmosphere and nitrogen about 1 to 2 percent. Thus, the atmosphere of Earth would closely resemble that of Venus. The carbon dioxide-rich atmosphere of Venus causes a very significant greenhouse temperature enhancement, giving the surface of Venus a temperature of about 750 kelvins, which is hot enough to melt lead. Earth's surface temperature is only about 288 kelvins, a range at which water can exist in equilibrium as a solid, liquid, or gas.

Like Venus, Mars has an extremely thin atmosphere composed primarily of carbon dioxide (about 95 percent by volume) and nitrogen (about 3 percent by volume). Mars's total atmospheric surface pressure is only about seven millibars (one atmosphere is equivalent to 1,013 millibars). There may be large quantities of outgassed water in the form of ice or frost below the surface of Mars, but in the absence of liquid water, the outgassed carbon dioxide has remained in the atmosphere. The smaller mass of the atmosphere of Mars compared to the atmospheres of Venus and Earth may be attributable to the smaller mass of the planet and, accordingly, the smaller mass of gases that could have been trapped in the interior of Mars during its formation. In addition, the amounts 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 amounts of gases and was the most volatile-rich planet. Earth trapped the next greatest amounts, and Mars trapped the smallest amounts.

Study of Earth’s Atmosphere

Information about the origin, early history, and evolution of Earth’s atmosphere comes from a variety of sources. Information on the origin of Earth and other planets is based on theoretical computer simulations, with ever-increasing empirical data input from celestial observation. These computer models simulate the collapse of the primordial solar nebula and the formation of the planets. Astronomical observations of what appears to be the collapse of interstellar gas clouds and the possible formation of planetary systems 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, the geochemical cycling of the outgassed volatiles, and the 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 which mixtures of gases simulating Earth’s hypothetical early atmosphere are energized by solar ultraviolet radiation and atmospheric lightning. The resulting products are analyzed by chemical techniques. A key parameter affecting atmospheric photochemical reactions, chemical evolution, and the origin of life was the flux of solar ultraviolet radiation on the early Earth. Astronomical measurements of the ultraviolet emissions from young sun-like stars have provided important information about the probable ultraviolet emissions from the sun during the 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-rich atmosphere. Studies of the biogeochemical cycling of the elements have provided essential 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 science involving astronomy, geology, geochemistry, geophysics, biology, and atmospheric chemistry.

Significance

Studies of the origin and evolution of Earth’s atmosphere have provided new insights into the processes and parameters responsible for global change. Understanding the atmosphere's history provides a sound basis for better understanding its future. Today, several global environmental changes are of national and international concern, including the preservation of the ozone and the increasing global temperatures caused by the buildup of greenhouse gases in the atmosphere. 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 cycle is key to understanding environmental problems. 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 Earth.

Principal Terms

chemical evolution: the synthesis of amino acids and other complex organic molecules—the precursors of living systems—by the action of atmospheric lightning and solar ultraviolet radiation on atmospheric gases

photosynthesis: the biochemical synthesis of glucose and molecular oxygen from carbon dioxide and water by chlorophyll-containing organisms in the presence of sunlight

prebiotic: relating to the period of time before the appearance of life on Earth

primordial solar nebula: an interstellar cloud of gases and dust that condensed by the action of gravitational forces to form the bodies of the solar system about five billion years ago

solar ultraviolet radiation: biologically lethal solar radiation in the spectral interval between approximately 0.1 and 0.3 micron (1 micron = 0.0001 centimeter)

T Tauri stars: a class of stars that exhibits rapid and erratic changes in brightness

volatile outgassing: the release of the gases and liquids, such as argon, water vapor, carbon dioxide, and nitrogen sulfur, trapped within Earth’s interior during its formation

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