Evolution of the Atmosphere

 Of all our planetary resources, the atmosphere is the one most taken for granted. The air we breathe and that supports plant and animal life is crucial to the existence of all life. The present-day atmosphere is primarily made up of three major gases: nitrogen (N2) at 78%, oxygen (O2) at 20.9%, and argon (Ar) at 0.9%, with a number of other trace gases making up the remaining 0.2%. These other trace gases include water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), the chlorofluorocarbons (CFCs), ozone (O3), and many others. Water vapor is the one atmospheric gas that is highly variable. It is also the one that is most mobile, since it can condense to form clouds and precipitate back to the ground in the form of solid or liquid water. In this chapter we will examine the current composition of our atmosphere and how it evolved from the primordial to the present state. We will also discuss the circulation patterns of the atmosphere and how they relate to planetary physics and rotation, as well as how they can be affected by variations in terrain. Finally, we will discuss how atmospheric circulation relates to the radiative balance of the atmosphere and, in turn, how this relates to climate forcing.

The concepts covered in this review will be important to the chemistry that we will examine in the chapters that follow, since the chemistry is affected by atmospheric transport and lifetime as well as by the available solar energy. We will also find that the composition and chemistry of the air are not only important in the atmosphere, but have important effects on the hydrosphere and soil environments. The changing physics and chemistry of the atmosphere affect the chemistry and biochemistry in both fresh and salt waters, as well as in soils, and therefore have important impacts on the biosphere.

The Earth’s atmosphere is unique in our solar system as it contains significant amounts of the reactive gas oxygen. The observation of oxygen in the atmosphere is an indication of how important plant life is on the planet, since the source of this oxygen is photosynthesis. Without life, that oxygen would rapidly be depleted over time, forming oxidized forms of carbon and creating an atmosphere that would be more like our neighboring planet, Venus. Our atmosphere was not always oxygen rich. Indeed, it originally had little or no oxygen needed for life as we know it, and evolved over time to be what it is today.

Early Earth | Modern Earth

Our Earth is approximately 4.5 billion years old, based on radionuclide dating of meteorites found on the planet. Meteorites are believed to be formed from the earliest materials in our solar system, at the time when the planets were initially formed, and the meteoritic data has been used to establish the start of our planetary geological clock. The early Earth is thought to have formed from a mixture of hot gases and solids that became a molten surface. These hot gases and solids gradually cooled with little or no atmosphere at all. Once formed, the planet began to outgas materials from the mantle and crust into the atmosphere, with the lighter gases being lost to space over time and the heavier gases being held in the atmosphere by the planet’s gravity. After approximately half a billion years, the earth cooled sufficiently to allow water to condense on the surface. The water would evaporate, returning to the atmosphere, and recondense in the form of precipitation. This hydrological cycle led to dissolution of water-soluble salts in the rocks, resulting in the formation of our oceans.

The atmosphere is thought to have evolved with time as gases were emitted from the planet’s volcanic activity. These gases were likely similar to those emitted from volcanos today and primarily contained hydrogen sulfide (H2S), carbon dioxide (CO2), methane (CH4), and water (H2O), as shown in Figure 2.1. Argon was also emitted into the atmosphere from the Earth’s surface, from the radioactive decay of potassium-40, which has a radioactive half-life of 1.25 billion years.

Argon has threestableisotopes,36Ar, 38Ar, and 40Ar. While the argon in our atmosphere is 99.6% 40Ar, the argon in the Sun is mostly 36Ar with about 15% 38Ar, and this is believed to be the isotopic composition of the original argon produced when the solar system was formed. The isotopic signature of the Earth’s atmospheric argon (40Ar) clearly shows that its source is the radioactive decay of 40K and its half-life of over a billion years is consistent with the age of the Earth being more than 4 billion years. It is interesting to note that the atmosphere of Mars has a similar atmospheric abundance of 40Ar, with a smaller amount of 36Ar than that of the Earth. This is due to Mars’s lower gravity, causing a loss of the lighter isotope. The presence of argon in the atmosphere of both planets is clearly due to the radioactive decay of 40K over a long period of time.

There are two proposed sources for the large amount of nitrogen gas (N2) in our atmosphere. One is that plate tectonics caused the movement of rocks containing ammonium to become heated, resulting in the decomposition of the ammonium in the subsurface, producing nitro-gen gas which was then released to the atmosphere over time during volcanic activity. This is based on a comparison of the nitrogen levels in the atmosphere of our neighboring planets, Venus and Mars, which have much lower levels of nitrogen than in the Earth’s atmosphere. The other proposed source of nitrogen is the addition of ammonia directly to the Earth’s atmosphere, either from an interaction with Jupiter’s ammonia ice clouds or from comets striking the Earth during the planet’s formation. Over time, this ammonia could react in the atmosphere to form nitrogen gas. This theory is consistent with a comparison between nitrogen isotopic abundances of meteorites and the Earth’s atmospheric nitrogen. Interestingly enough, ammonia currently exists in the atmosphere at very trace levels, and is the only basic gas in our modern atmosphere. It is currently uncertain which of these proposed mechanisms was the most dominant source of nitrogen gas in the Earth’s atmosphere.

The primary gases in the Earth’s early atmosphere – methane, carbon dioxide, and water –kept the planet from freezing, since they are greenhouse gases and at that time the Sun was producing only about 70% of the energy that it does today. There was little or no molecular oxygen in the atmosphere at this time. All of the oxygen in the atmosphere was bound up in water or carbon dioxide. During this period, called the Archean Eon, the first life on the planet was thought to be anaerobic bacteria, single-celled organisms that were able to use sulfur and other elements as energy sources on which they could survive. At the end of this period, about 2.7 billion years ago, cyanobacteria evolved in the early oceans. These bacteria used sun-light, carbon dioxide, and water to produce organic compounds, while releasing oxygen into the atmosphere in the process called photosynthesis. These microorganisms were the major source of oxygen in the planet’s early atmosphere. Once oxygen levels began to increase, life on the planet evolved rapidly with increasing plant life, which allowed for increased conversion of carbon dioxide and water to oxygen. This led to the oxygen levels that we have in the modern atmosphere, which is approximately 21%. The production of this oxygen led to further evolution of the atmosphere due to photochemical reactions in the presence of ultraviolet (UV) radiation from our Sun.

While photosynthesis is responsible for the increased oxygen levels as plant populations increased both on land and in the oceans, plant respiration is also an important source of carbon dioxide. In the early atmosphere, with high levels of carbon dioxide, significant amounts of water-soluble carbon dioxide would have interacted with the oceans. The equilibrium of carbon dioxide with water leads to carbonic acid, which is in equilibrium with bicarbonate and carbonate. This carbon dioxide dissolution in water initiated the carbonate cycle, which formed insoluble inorganic compounds such as calcium and magnesium carbonates. The carbonate cycle also acted to reduce the carbon dioxide levels in the atmosphere, while the less reactive gases such as nitrogen and argon continued to build up to the current levels. While the increasing oxygen content led to animal life on the planet, the increasing carbonate levels in the oceans led to shellfish, which produced calcium carbonate shells. Limestone deposits formed over hundreds of millions of years from shellfish and other carbonate-fixing ocean life account for most of the sequestered carbon on the planet.

The production of oxygen in our atmosphere also has an important outcome with regard to protecting organisms from damaging UV radiation from the Sun. As most of the early life was in the oceans, it was protected from harmful UV radiation due to absorption by water. At some depth the UV radiation is reduced sufficiently that plant and animal life can be maintained. But the development of life in terrestrial areas required significant oxygen levels to be formed in order to screen the harmful UV radiation. The increased levels of oxygen also produced ozone in the upper atmosphere that would further protect life in terrestrial environments from UV radiation. Thus, animal life – including man – evolved along with our atmosphere to its current state.