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evolution of the atmosphere
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A scientific graph shows the estimated abundance of oxygen in Earth's atmosphere over time on a logarithmic scale, with key evolutionary and geological events such as the first appearance of oxygenic photosynthesis, the rise of land plants, and the emergence of animals labeled along the curve. Two diagrams compare Earth's prebiotic and modern atmospheres. The top shows a barren landscape with gases like hydrogen, nitrogen, carbon dioxide, and ammonia labeled above, indicating a weakly reducing atmosphere. The bottom shows a lush landscape with plants and animals, labeling oxygen, argon, and other gases, illustrating an oxidizing modern atmosphere. Orion Nebula (M42) Mount Pinatubo Illustrated diagram of the carbon cycle showing how carbon moves as carbon dioxide through the atmosphere, water, living organisms, rocks, and soil. It depicts processes like photosynthesis, respiration, sedimentation, volcanic activity, fossil fuel use, and carbon sequestration, with arrows indicating the flow between air, water, plants, animals, rocks, and industrial sources. banded-iron formation (BIF)
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abundance of oxygen
abundance of oxygen A “best guess” reconstruction of the abundance of O2 in Earth's atmosphere as a function of time. The O2 abundance axis is logarithmic.

evolution of the atmosphere

Written by
J.M. Hayes
Distinguished Professor of Biogeochemistry, Indiana University, Bloomington. Coauthor of Chemical Separations and Measurements.
J.M. Hayes
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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History

evolution of the atmosphere, the development of Earth’s atmosphere across geologic time. The process by which the current atmosphere arose from earlier conditions is complex; however, evidence related to the evolution of Earth’s atmosphere, though indirect, is abundant. Ancient sediments and rocks record past changes in atmospheric composition due to chemical reactions with Earth’s crust and, in particular, to biochemical processes associated with life.

Earth's early and modern atmospheres compared
Earth's early and modern atmospheres comparedComparison of Earth's prebiotic and modern atmospheres. Before life began on the planet, Earth's atmosphere was largely made up of nitrogen and carbon dioxide gases. After photosynthesizing organisms multiplied on Earth's surface and in the oceans, much of the carbon dioxide was replaced with oxygen.

Earth’s original atmosphere was rich in methane, ammonia, water vapour, and the noble gas neon, but it lacked free oxygen. It is likely that hundreds of millions of years separated the first biological production of oxygen by unicellular organisms and its eventual accumulation in the atmosphere.

The composition of the atmosphere encodes a great deal of information bearing on its origin. Furthermore, the nature and variations of the minor components reveal extensive interactions between the atmosphere, terrestrial environment, and biota.

The development of the atmosphere and such interactions are discussed in this article, with particular attention given to the rise of biologically produced molecular oxygen, O2, as a major component of air. For modern atmospheric chemistry and physics, see atmosphere.

Concepts related to atmospheric development

A complete reconstruction of the origin and development of the atmosphere would include details of its size and composition at all times during the 4.5 billion years since Earth’s formation. This goal could not be achieved without knowledge of the pathways and rates of supply and consumption of all atmospheric constituents at all times. Information regarding these particular processes, however, is incomplete even for the present atmosphere, and there is almost no direct evidence regarding atmospheric constituents and their rates of supply and consumption in the past.

The contrast with related fields of Earth’s history is notable. Fossils and other structural and chemical details of ancient rocks provide information useful to evolutionary biologists and historical geologists, but ancient atmospheres, “mere vapours,” have not left such substantial remnants. These vapours are, however, the stuff of stars and the moving force of storms and erosion.

The atmosphere as part of the crust

To the Earth scientist, the crust includes not only the top layer of solid material (soil and rocks to a depth of 6 to 70 km [4 to 44 miles], separated from the underlying mantle by differences in density and by susceptibility to surficial geologic processes) but also the hydrosphere (oceans, surface waters on land, and groundwater beneath the land surface) and the atmosphere. Interactions among these solid, liquid, and gaseous portions of the crust are so frequent and thorough that considering them separately introduces more complexities than it eliminates. As a result, a description of the history of the atmosphere must concern itself with all volatile components of the crust.

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Materials

Volatile compounds as well as elements important in present and past atmospheres or in interactions between the atmosphere, biosphere, and other portions of the crust include the following:

  1. Present major components: molecular nitrogen (N2) and molecular oxygen (O2)
  2. Noble gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe)
  3. Abundant variable components: water vapour (H2O) and carbon dioxide (CO2)
  4. Other components: molecular hydrogen (H2), methane (CH4), carbon monoxide (CO), ammonia (NH3), nitrous oxide (N2O), nitrogen dioxide (NO2), hydrogen sulfide (H2S), dimethyl sulfide [(CH3)2S], sulfur dioxide (SO2), and hydrogen chloride (HCl).

Some elements appear in multiple form—for example, carbon as carbon dioxide, methane, or dimethyl sulfide. It is useful to consider the occurrence of the elements before focusing on the more specific aspects of atmospheric chemistry (the forms in which the elements are present). One can speak of Earth’s “inventory of volatiles,” recognizing that the components of the inventory may be reorganized from time to time, but also that it is always composed primarily of the compounds of hydrogen, carbon, nitrogen, and oxygen, along with the noble gases.

Processes

A process that delivers a gas to the atmosphere is termed a source for the gas. Depending on the question under consideration, it can make sense to speak in terms of either an ultimate source—the process that delivered a component of the volatile inventory to Earth—or an immediate source—the process that sustains the abundance of a component of the present atmosphere. Any process that removes gas either chemically, as in the consumption of oxygen during the process of combustion, or physically, as in the loss of hydrogen to space at the top of the atmosphere, is called a sink.

Throughout the history of the atmosphere, sources and sinks have often been simultaneously present. While one process consumes a particular component, another produces it, and the concentration of that component in the atmosphere will rise or fall depending on the relative strengths of the sources and sinks. If those strengths are balanced (or nearly so), the composition of the atmosphere will not change (or will change only very slowly, perhaps imperceptibly); however, the molecules of the gas in question are passing through the atmosphere and are not permanently resident. The rate of the resulting turnover of molecules in the atmosphere is expressed in terms of the residence time, the average time spent by a molecule in the atmosphere after it leaves a source and before it encounters a sink.