Scientists believe the Earth formed about 4.6 billion years ago. The Earth formed from gas, interstellar dust particles, and ice, which lumped together into a continually larger mass until about 200 million years had passed. At the end of this period, Earth's mass growth was essentially complete. Initially, the surface temperature was quite high, estimated at about 8000°C (14,400°F). By about 4.4 billion years ago, the surface had cooled enough for magma to solidify into a solid outer crust. This cooling also resulted in the initial release of large quantities of gases from the lithosphere, mainly hydrogen and helium, into the early atmosphere. These gases were hot and, as a result, had high molecular velocities, enabling them to overcome gravity and travel into space.
Table 5.1 describes the three major stages of atmospheric development. Most of the early atmosphere formed during the first million years after the crust solidified (4.4 billion years ago). The temperature of the Earth's surface during this period was about 85 to 110°C (185 to 230°F). The main gases found in this early atmosphere consisted of carbon dioxide (CO2), nitrogen in the form of N2, ammonia (NH3), and water vapor (H2O). This mixture had a density of 10 to 20 times greater than today's atmosphere. This higher density was primarily due to large amounts of evaporated water.
The second stage (secondary atmosphere) in the evolution of the Earth's atmosphere occurred between 4.0 and 3.3 billion years ago. By about 3.8 billion years ago, atmospheric temperatures were cool enough for water vapor to condense. Rain events were numerous, heavy, and geographically widespread. At first, the rain evaporated before hitting the ground because of high temperatures in the lower atmosphere. Eventually, the lower atmosphere and the ground surface cooled enough to allow surface water to accumulate, forming rivers, lakes, and oceans. The volume of water in the world's oceans leveled off about 3.5 billion years ago. The falling rain also washed out large amounts of carbon dioxide from the air. This carbon dioxide is chemically combined with calcium and magnesium at the ground surface to form sedimentary rocks. Oxygen began accumulating in the atmosphere through the chemical breakdown of water by sunlight (known as photodissociation), releasing molecular oxygen (O2), and by way of photosynthesis. The emergence of living organisms around 3.8 billion years ago was essential in creating atmospheric oxygen (O2) and ozone (O3).
The last period of atmospheric evolution is known as the living atmosphere. About 2.5 billion years ago, life began significantly influencing the atmosphere's chemical composition. Through photosynthesis, organisms produce large amounts of oxygen, which begins to accumulate at higher concentrations. Most of the buildup of oxygen in our planet's atmosphere occurred between 850 and 500 million years ago, as a direct result of photosynthesis by ocean-based plants such as algae. At the end of this period, there was enough oxygen in the atmosphere to develop a stratospheric ozone layer thick enough to protect terrestrial life from the harmful effects of ultraviolet radiation. As a result, evolutionary development and the expansion of terrestrial life began at this time. The atmosphere's composition remained relatively fixed for most of the last 500 million years. Over the last 300 years, the concentrations of several atmospheric gases have changed significantly due to human activities. In the next section, we will examine this form of environmental change in greater depth.
