About 4.6 billion years ago, the hot globe was surrounded by a thick layer of its original atmosphere consisting primarily of hydrogen and helium. These light gases tended to float upwards and so gradually escaped into space. The cooling earth was subject to violent volcanic eruptions accompanied by the release of vast amounts of other gases, water vapor, carbon dioxide, ammonia, methane, and sulfur dioxide. At the prevailing high temperatures, ammonia and methane reacted with trace amounts of oxygen, resulting in the formation of a secondary atmosphere. It is composed chiefly of water vapor, carbon dioxide, and nitrogen.
About 3.8 billion years ago, when the Earth’s surface cooled down below 100 degrees Celsius the water vapor condensed to form seas and oceans. Nearly 3 billion years ago, the first primitive life-forms developed in the waters of blue-green algae and bacteria. The former began to utilize solar energy to produce nutrients by the process of photosynthesis. As a result, the oxygen content of the atmosphere started to increase.
Simultaneously, the proportion of carbon dioxide began to fall because the plants absorbed the gas. This plant activity led to the formation of the present atmosphere about 200 million years ago. It has not changed much in all the time since. Today four-fifths of the atmosphere is nitrogen, 21 percent of oxygen. And the remaining fraction can of other gases such as carbon dioxide and water vapor.
How did the atmosphere form?
The Sun, the Earth, and other solar system planets were formed when matter coalesced from a rotating nebula approximately 4,567 million years ago. The atmosphere of the earth has been changing regularly since its formation. The Earth’s atmosphere has a thickness of about 100 kilometers. But above 35 kilometers from the Earth’s surface, the air pressure is so low that water cannot exist in liquid form. The habitable zone is within 5 kilometers of sea level. The highest city globally is La Rinconada in Peru, at an altitude of 5,100 meters (16,700 ft). It has an economy based on gold mining. This city is at about the same elevation as Mt. Everest base camp.
- The air at this altitude has only 11% oxygen compared to 21% at sea level. The nebula that produced the solar systems originated from the explosion of older stars containing heavy elements like iron. The accumulation of mass at the center of the rotating nebula was so large. That gravitational compression initiated hydrogen fusion into helium. Thus giving birth to Sun.
The planets are orbiting the Sun, formed by accretion. The heavier elements concentrated in the cores, and the lighter gaseous elements became the atmospheres. The most abundant chemical elements in the Sun are hydrogen, 73.5%, and helium, 24.9%. All other elements are in concentrations of less than one percent. The atmosphere of Jupiter, which is a giant planet. It is mostly hydrogen with about 10% helium and small amounts of other gases like methane, ammonia, hydrogen sulfide, and water.
These compositions indicate that the solar system’s nebula originated mainly was hydrogen, helium, and small amounts of heavier elements. Mercury, Venus, Earth, and Mars lost their hydrogen and helium rapidly because their gravitational pull is not strong enough to retain these light elements. Further away from the Sun, where it is much colder, methane can condense as a liquid. And Saturn’s moon Titan has a predominantly nitrogen atmosphere with pools of liquid methane on its surface.
When the Earth’s material 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 the gases of the condensing planetary nebula.
- During the Hadean Eon, the Earth’s surface consisted of molten rock, a magma ocean, and water existed only as vapor in the atmosphere.
Hydrogen and helium were lost early in the Hadean Eon due to Earth’s weak gravity. The circulation of molten metallic iron-nickel alloy in the core of the Earth established the magnetosphere. It is a region in space where the Earth’s magnetic field controls the motions of gas and fast charged particles. 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. There high-energy collisions with atoms of the atmosphere born an aurora light display.
History of the atmosphere
Around 4.45 billion years ago, the Earth experienced a violent collision with a planetoid called Theia 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. The great collision sent much of the atmosphere into space. But most of it remained within the Earth’s gravitational field. It was recaptured when the debris from the giant impact cooled and was partitioned between the Earth. And the Moon gave them similar crustal composition.
Earth’s Hadean atmosphere had methane, ammonia, water vapor, and small percentages of nitrogen and carbon dioxide. A cataclysmic meteorite bombardment around 3.9 billion years ago kept much of the Earth’s surface in the molten state. And the incoming impactors may have brought additional water, methane, ammonia, hydrogen sulfide, and other gases that supplemented the atmosphere.
- During the Hadean eon, the high surface temperature of the Earth favored the depletion of atmospheric methane through the endothermic reaction of methane with steam in the atmosphere. The resulting carbon monoxide is readily combined with metals to form carbonyl compounds.
The Hadean Eon was too hot for liquid water to condense on the surface of the Earth. But water vapor would have been able to condense at high altitudes in the atmosphere and produce rain. That evaporated as it fell when it approached the ground. Toward the end of the Hadean Eon, volcanic activity started. It was increasing the percentage of carbon dioxide in the atmosphere.
The Earth’s surface changed from molten lava to solid rock, and liquid water started to accumulate on the surface. The crust of the Earth started to cool down during the Archean Eon. The amount of water vapor in the atmosphere decreased as water started condensing in liquid form.
- Continuous rainfall for millions of years led to the buildup of the oceans. As steam condensed into water, the atmospheric pressure of the Earth decreased. And liquid water dissolved gases like ammonia and removed them from the atmosphere by creating ammonium compounds. Amines and other nitrogen-containing substances suitable for the origin of life.
Liquid water changed the chemistry of the Earth’s surface. Water combined with sulfur dioxide to produce acid rain 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.
Evolution of Earth’s atmosphere
How has the earth’s atmosphere changed over time? Microfossils of sulfur-metabolizing cells have been found in 3.4-billion-year-old rocks. And it is known that the first aquatic photosynthetic organisms originated around 3.5 billion years ago. During the Archean Eon, the oxygen produced by cyanobacteria (blue-green algae) reacted with metal ions in the anoxic sea. Billions of years would pass before the photosynthetic microorganisms could eventually change the composition of the atmosphere.
In the middle of the Archean Eon, the Earth had cooled enough. So 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%.
Most of the original components of the atmosphere had escaped the Earth, precipitated as liquids, or reacted chemically to form solid compounds. Volcanic activity and photosynthetic bacteria were now the major factors influencing the Earth’s atmospheric composition.
Monocellular life proliferated during the Proterozoic Eon. Anaerobic microbial life thrived during the beginning of the Proterozoic Eon because the Earth had little oxygen. Anaerobic organisms do not require oxygen for growth. They obtain their energy in various ways. Methanogens combine hydrogen and carbon dioxide to produce methane and water. Sulfate-reducing bacteria get their energy by metabolizing methane and sulfate radicals. Organisms were capable of photosynthesis, such as cyanobacteria. They used sunlight to convert the abundant carbon dioxide and water into carbohydrates and oxygen. Oxygen was deadly to the anaerobes. So this gave photosynthetic organisms a competitive advantage.
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 carbon dioxide had been depleted from the atmosphere, leaving nitrogen as the primary atmospheric gas with a small percentage of oxygen.
- Nitrogen gas, 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.
Timeline of Earth’s atmosphere
Photosynthetic organisms had been releasing oxygen since the Archean Eon, but the oxidation immediately depleted the oxygen of metals. An increased period of oxygen production occurred between 2.4 and 2.0 billion years ago and is known as the Great Oxidation Event or the Oxygen Catastrophe. The higher oxygen level created banded iron formations by precipitating dissolved iron as iron(III) oxide. Enough free oxygen accumulated in the atmosphere to kill anaerobes near the Earth’s surface and created an opportunity to develop aerobic life forms.
Around 2.4 billion years ago, oxygen molecules migrated into the upper atmosphere and formed an ozone layer. This is a region in the stratosphere located between 15 to 35 kilometers above the Earth’s surface.
- On this surface, 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 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.
During the last billion years of the Proterozoic Eon, the Earth’s atmospheric composition was steady with approximately 10% oxygen. At this time, soft-bodied multicellular organisms developed. By 850 million years ago, the minerals in the sea and the excess oxygen began to accumulate in the atmosphere.
- With the increased oxygen levels and the protection of the ozone layer, organisms capable of aerobic respiration. They could now increase all over the surface of the Earth.
The abundance of multicellular life marks the Cambrian period at the beginning of the Phanerozoic Eon. Most of the major groups of animals first appeared at this time. Vegetation covered the surface of the Earth, and oxygen comprised 30% of the atmosphere. Air enriched with so much oxygen allowed giant insects to develop and caused frequent forest fires set off by lightning.
A great mass-extinction event occurred 251 million years ago, marking the boundary of the Permian and Triassic periods. Oxygen levels dropped from 30% to 12%, and carbon dioxide levels reached about 2000 parts per million. This was Earth’s worst mass extinction, and it eliminated 90% of ocean dwellers and 70% of land plants and animals. This mass extinction was caused by a series of volcanic events in Siberia that lasted for about one million years. It released large volumes of carbon dioxide and gases containing sulfur, chlorine, and fluorine.
By 228 million years ago, oxygen levels had risen to about 15% of the atmosphere. And the first dinosaurs appeared. Oxygen levels continued to increase, and by the end‐Cretaceous.
About 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%.
- The Earth’s current atmosphere contains about 78% nitrogen, 21% oxygen, 1% argon, and 400 parts per million of carbon dioxide.
The combustion of fossil fuels has generated large quantities of carbon dioxide since the Industrial Revolution. And it continues today at a very high rate. Mammals, including humans, can only live within a skinny layer of the Earth’s atmosphere. We should keep our air clean so that we can live in harmony with nature.