EARTH’S ATMOSPHERE IS unique in the solar system: of all the heavenly bodies, Earth alone has an atmosphere capable of supporting life. An appreciation of this highly specialized nature of the contemporary atmosphere is vital to understanding its evolution. The atmosphere has not always been the way it is today, and the present atmosphere is almost certainly not directly related to Earth’s primordial atmosphere. The contemporary atmosphere arose from thermal and geological processes occurring some time after Earth’s formation, and its evolution to a life-supporting state is intimately linked to the evolution of life itself.
The present atmosphere
The Earth’s atmosphere is extraordinary; comparing the Earth to other planets in the Solar System illustrates just how extraordinary. The inner planets are often referred to as the terrestrial planets because they consist of rocky masses surrounded by gaseous atmospheres (with the exception of Mercury, whose atmosphere has long since been lost to space because of its proximity to the Sun). Venus and Mars both have substantial atmospheres and make excellent comparisons for the atmospheric evolution of Earth.
The atmospheres of Venus and Mars have evolved to consist primarily of carbon dioxide (CO2), with some nitrogen and hardly any oxygen. Although Venus and Mars have important differences in their atmospheric chemistry, their atmospheres are essentially similar. Roughly speaking, both atmospheres consist of 95 percent CO2 and 3 percent nitrogen. The amount of free oxygen on both planets is less than 1 percent. Earth’s atmosphere may have evolved in a similar way: it was formed at roughly the same time from roughly the same material. Instead, Earth’s atmosphere is markedly different. It contains hardly any CO2 (although CO2 plays a crucial role in regulating the planet’s atmosphere and climate), consisting mainly of nitrogen, with a comparatively high oxygen content of around 21 percent. These differences result in an important chemical distinction between the atmospheres. The atmospheres of Venus and Mars are highly oxidized, containing a lot of chemically combined oxygen (mainly CO2). In contrast, Earth’s atmosphere is highly oxidizing, containing large amounts of free oxygen.
That one fifth of the Earth’s atmosphere is comprised of oxygen is curious. Oxygen is a highly reactive gas, and the atmosphere and surface of the Earth contain many materials with which oxygen can react. Furthermore, the amount of time it would take for all the oxygen in the air to undergo these reactions is much shorter than the time the oxygen has been there. In short, there should not be anywhere near as much oxygen as there is in Earth’s atmosphere; the atmosphere is a long way from chemical equilibrium.
From a chemical perspective, the Earth’s atmosphere is like a bucket that, despite having a hole in the bottom, remains half full. It is impossible, unless water is flowing into the bucket at the same rate that it is leaking through the hole. In the same way, the composition of Earth’s atmosphere is impossible, unless something is constantly adding oxygen to counteract the rate at which it is being lost through chemical reactions. The factor acting on the atmosphere to keep it in its state of chemical disequilibrium is life.
Biological activity on the planet uses energy from the Sun, during photosynthesis, to produce vast quantities of free oxygen. In the absence of life, there is no way for oxygen to have built up to the levels observed today. The evolution of the Earth’s atmosphere has taken a very different path from that of Venus and Mars, and this path has led to an atmosphere that is outstanding among all the other known atmospheres. In this evolution, life has played a critical role.
Formation of the earth and atmosphere
The evolution of the atmosphere begins 4-5 billion years ago with the formation of the Earth, but there is good evidence that the present atmosphere is not directly related to the Earth’s original atmosphere by a continuous line of evolution.
As the Earth formed from the stellar nebula from which the Solar System was created, its rocky core acquired an atmosphere by gravitationally attracting nebula gases. This was the Earth’s first, or primary, atmosphere. However, the hydrogen and helium thus trapped are likely to have escaped into space as the young Sun sprang to life, so these lightweight, primordial-nebula gases are not found in today’s atmosphere. However, heavy, chemically-inert primordial gases such as neon and argon can be found. Being heavy, these gases would not have been lost to space, and their inertness would prevent them being removed from the atmosphere by chemical reactions. Therefore, their presence in the contemporary atmosphere would signal a direct line of descent from that primordial nebula to the present day atmosphere.
However, isotopic analysis shows that the neon and argon in today’s atmosphere are predominantly the result of nuclear decay, rather than the remnants of that early atmosphere. It seems that at some stage soon after formation, the Earth lost its primordial atmosphere of nebula gases, and acquired a secondary atmosphere, from which the present atmosphere evolved.
The young Earth was a violent place, with radioactive decay, heavy meteor bombardment, and frictional and gravitational forces, all acting together to heat up the rocky mass of the planet. Much of the rock was molten. Under these conditions, gases were released from the planet’s hot rocky core. These gases, predominantly nitrogen, CO2, and water, had either been physically trapped within the solid Earth as it formed, or else were released by thermal decomposition of rocks and minerals. These are the gases that formed Earth’s secondary atmosphere, the starting point for evolution towards the contemporary atmosphere.
In this earliest of epochs of geological history, the atmospheric CO2 probably reacted again to form carbonate rocks, and the water eventually condensed to form the oceans. This left an atmosphere consisting predominantly of nitrogen. Trace amounts of free oxygen would be able to form in this atmosphere, as sunlight split (photolysed) molecules of water and CO2 in the air. However, photolysis cannot result in particularly high concentrations of oxygen; a limit exists because the oxygen produced by photolysis absorbs light at the same frequency that CO2 and water photolyse. As the concentration of oxygen builds up, it blocks the light needed to further increase its concentration. Without photosynthesis (that is, without a biological process for oxygen production), there is a natural limit on the amount of oxygen in the atmosphere. The exact concentration depends on CO2 and water concentrations, and sunlight intensity, but no more than one-billionth of the present atmospheric level of oxygen existed before the emergence of life.
This limit to the pre-biological free oxygen concentration turns out to be critically important to the evolution of life, and, hence, to the evolution of the atmosphere. Had the atmosphere been appreciably oxidizing (containing significant levels of free oxygen), it would not have been possible for life’s chemical precursors to accumulate on the planet’s surface. The complex and fragile molecules of life, slowly forming and beginning to organize into proto-living systems in stagnant ponds and lakes, would have rapidly oxidized, and thus, destroyed, had the pre-biotic atmosphere been too oxidizing.
The emergence of life and oxygen
The emergence of life elegantly illustrates the intimacy of the connection between atmospheric evolution and biological evolution. The pre-biotic atmosphere, with its low concentration of free oxygen, not only provides favorable conditions for the development of biologically important molecules; the atmosphere also contains the chemicals from which life’s precursor molecules themselves can be synthesized.
The famous Miller-Urey experiment in the 1950s demonstrated that a gas mixture containing methane, ammonia, hydrogen, and water subjected to simulated lightening discharges is capable of producing amino acids (essential biochemicals) within a very short time. Subsequent experiments have shown that gas mixtures more closely resembling the actual composition of the pre-biological atmosphere produce essential biomolecules under the influence of electrical discharges. Hence, laboratory experiments demonstrate that the atmospheric conditions on the early Earth were sufficient to give birth to the molecules of life.
It is unknown how life arose from this primordial mixture of chemicals, though plausible mechanisms can be postulated. There is certainly plenty of time in which rare events leading to unusually stable states could repeatedly occur; about a billion years separate Earth’s formation and the dawn of life, sometime around four to five billion years ago.
The number of places on the planet’s surface where life could form, however, must have been somewhat limited. The organic molecules needed for living systems are susceptible to destruction in the presence of free oxygen, and sunlight is a catalyst for this oxidation. Even though the concentration of oxygen in the early atmosphere was low, its presence would have confined the initial emergence of life to the water. Water acts as a filter, preventing the damaging ultraviolet wavelengths of light penetrating much below the surface. However, the oceans would have been unsuitable environments for early life because the turbulent vertical mixing of ocean waters would bring biomolecules and early organisms to the surface where sunlight could initiate oxidation. The first living things were, therefore, confined to the sub-surface waters of stagnant pools.
These early life forms were different from the majority of living things alive today; there was so little oxygen available that they evolved to survive without it. Relatives still survive on Earth today as the anaerobic bacteria that live in stagnant water. These early bacteria were photosynthetic, using visible wavelengths of light to make food and biomolecules from CO2 and water. An important by-product of this activity of the first living things was oxygen.
Over the next billion years or so, these simple cells carried on a chain of life that steadily increased the oxygen concentration of the atmosphere, from its low point of less than one-billionth of today’s value, to somewhere around one percent of the present concentration.
This increasing oxygen concentration was an important development, for with it the came an increase in the complexity of biochemistry available to living systems. Most importantly, the development of structural proteins that can be formed only in the presence of oxygen allowed the evolution of the cell nucleus.
With this change, biological complexity could increase rapidly by means of sexual reproduction; genetic material could be shared among members of a species. The concomitant increase in variability gave rise to simple animals and plants, as well as to more varied bacteria. Aerobic respiration became dominant, as it is today, and life was able to explore many more configurations and exist in many more niches.
All the while, photosynthesis was generating more and more free oxygen.
An important atmospheric change was occurring alongside all this biological activity; the action of sunlight on the increasing amount of oxygen in the air was generating ever-higher concentrations of ozone. As the ozone concentration increased, the intensity of damaging ultraviolet light at the surface of the waters was rapidly decreasing. Then, as now, the ozone layer served to protect life from harmful radiation from the Sun. The depth of water required to screen the Sun began to be reduced, and life could finally enter the open ocean.
This led to an explosion in the numbers of living creatures on the Earth, as life expanded into the newly available space. More life meant more photosynthesis and rapid increases in the atmospheric oxygen concentration. With increasing availability of free oxygen, even more complex biomolecules became possible, and multicellular life arose. Fossil evidence of jellyfish-like creatures from 670 million years ago gives valuable evidence of the oxygen concentration. These creatures had no lungs, and must have relied upon the ability of oxygen to diffuse across their skin. It is estimated that to support their existence, the atmospheric oxygen concentration must have been around seven percent of its present value at that time.
About 550 million years ago, fossils with strong and impervious exoskeletons suggest that the oxygen concentration had risen to 10 percent of present values (around two percent of total atmospheric composition), and the ozone concentration was approaching levels that would enable life to exist on the land, unmolested by ultraviolet radiation. Land-based life emerges in the fossil record at 420 million years ago, and by 380 million years ago the complexity of land-based life has multiplied remarkably leading to the appearance of the Carboniferous period’s Great Forests (from which fossil fuels derive). The rapidly increasing oxygen and ozone levels allow ever more complex land-based life, and the forests soon find themselves home to amphibious animals, mammals, and eventually the flowering plants. With this blossoming of life, the atmosphere finally reaches present oxygen and ozone levels by 300 million years ago.
Recent changes in the atmosphere
The evolution of the atmosphere has been intimately linked to the development of life, and there is a constant feedback between the atmosphere and the biosphere. Both CO2 and water are greenhouse gases, and have played an important role in regulating the Earth’s surface temperature during the atmosphere’s evolution. Water vapor is by far the more abundant, with an average (though highly variable) concentration of one percent by volume. The equilibrium between liquid water and water vapor is crucial in controlling Earth’s temperature. On Venus, the high temperature prevented liquid water from forming, which is thought to have led to that planet’s runaway greenhouse effect.
Current ice-core data for atmospheric CO2 concentration goes back 650,000 years. These data show that during this time the present ecosystem of land mammals and flowering plants have never experienced CO2 concentrations above 300 ppmv (parts per million by volume) until very recent history. The concentration of CO2 has risen rapidly in the last 300 years, since the beginning of the Industrial Revolution and the dramatic increase in fossil fuel use. Isotopic analysis shows that the rise in CO2 concentration to its present value of more than 370 ppmv is largely due to human activities, chiefly fossil fuel burning.
Without biological activity, the Earth’s state of chemical disequilibrium, including oxygen levels, would be impossible.
The widely held scientific consensus is that this rise will lead to significant perturbations in the surface temperature and large scale global climate change, significantly altering the biosphere-atmosphere feedbacks. The unprecedented release of fossil carbon laid down in geological deposits during the life of the ancient Carboniferous Great Forests is perhaps the unfolding storyline in the atmosphere’s continuing evolution.
