The Earth’s atmosphere began forming when volcanoes released gases just as they do now. These gases were held close to the Earth’s surface by gravity. In addition, as the Earth formed by accretion, the energy of impact of each body released some of the volatiles including water and gaseous compounds of carbon and nitrogen. On heating and venting these chemical species made their way to the Earth’s surface.
The Earth is the only one of the inner planets with a single large moon. The moon may have been formed by the major collision with the moon-forming body, which is about the size of Mars, soon after accretion and core formation changed all that. The volatiles at the surface and much at depth were lost by the energy of the collision. The atmosphere of the ocean and the atmosphere has been the result of degassing since then.
Other evidence for outgassing today is found in the ocean ridge systems where a unique isotope of helium, He-3, characteristic of the primitive Earth forming material, has been discovered venting into the ocean. Outgassing is also seen in the presence of radiogenic Ar-40, an isotope of argon which is the third most abundant gas in the atmosphere.
Because of the higher heat production in the Earth early in its history compared to the present, mantle convection has been slowing down. The rate of volatiles remain in the Earth’s interior and it is getting harder and harder for them to get out. It is believed that the atmosphere and oceans must have been in place about 2.5 billion years ago. Some of the more reactive volatiles like carbon, water or the oxygen produced from the dissolutions of water and nitrogen may be returned to the mantle by the subjection process at plate boundaries.
The growing ocean and atmosphere at first was composed of methane, water and nitrogen either ammonia or molecular nitrogen. The methane in the presence of water and sunlight would be transformed into carbon dioxide; water would be dissociated to hydrogen gas and oxygen gas. As hydrogen was lost from the planet, only hydrogen and helium are lost from the Earth’s gravity field, the oxygen remaining was quickly used up to oxidize iron and sulfide as well as the methane. Thus, the oxygen level was always maintained low.
But the Earth’s atmosphere, as we know, is oxygen and nitrogen, not carbon dioxide. The change in the Earth’s atmosphere is that life caused it to change. The first living organisms probably developed in the oceans while the Earth’s early atmosphere was changing from poisonous gases into carbon dioxide. These living things later developed into forms similar to today’s algae, which are lower forms of plant life. These forms of life used the energy of visible light to break down water and carbon dioxide to produce food, by a process called photosynthesis releasing free oxygen as a waste product. Other organisms took nitrogen into the air.
The Earth’s oxygen-nitrogen atmosphere, thus, was built by countless billions of tiny living organisms over a very long period in the Earth’s history. The arrival of the first photosynthetic organism, possibly around 3.9 billion years ago, resulted in the accelerated formation of oxygen. When the rate of production exceeded the rate of oxidation of iron and sulfide the oxygen began to accumulate earnestly in the atmosphere. This resulted in the development of an ozone shield around the world which diminished the ultraviolet rays reaching the Earth’s surface. In this new oxygen-rich atmosphere protected from deadly high energy rays of the sun the eukaryote cell developed and the future of nonbacterial life on Earth was assured. This seems to have occurred about 1.4 billion years ago.
The atmosphere since then has been maintained more or less, with its present mix of gases controlled by the life processes on the Earth’s surface. See figure 4.
(figure available in print form)