Atmosphere

The atmosphere is the body of air that surrounds our planet. But what is air? Air is a mixture of gases that constitutes the Earth’s atmosphere. What is the Earth’s atmosphere? The atmosphere is that thin shell, veil, or envelope of gases that surrounds Earth like the skin of an apple—thin, very thin—but very, very vital. The approximate composition of dry air is, by volume at sea level, nitrogen, 78 percent; oxygen, 21 percent (necessary for life as we know it); argon, 0.93 percent; and carbon dioxide, 0.03 percent, together with very small amounts of numerous other constituents (see table ). The water vapor content is highly variable and depends on atmospheric conditions. Air is said to be pure when none of the minor constituents is present in sufficient concentration to be injurious to the health of human beings or animals, to damage vegetation, or to cause loss of amenity (e.g., through the presence of dirt, dust, or odors or by diminution of sunshine).

Where does air come from? Genesis 1:2 states that God separated the water environment into the atmosphere and surface waters on the second day of creation. Many scientists state that 4.6 billion years ago a cloud of dust and gases forged the Earth and also created a dense molten core enveloped in cosmic gases. This was the protoatmosphere or proto-air, composed mainly of carbon dioxide, hydrogen, ammonia, and carbon monoxide, but did not last long before it was stripped away by a tremendous outburst of charged particles from the Sun. As the outer crust of Earth began to solidify, a new atmosphere began to form from the gases outpouring from gigantic hot springs and volcanoes. This created an atmosphere of air composed of carbon dioxide, nitrogen oxides, hydrogen, sulfur dioxide, and water vapor. As the Earth cooled, water vapor condensed into highly acidic rainfall, which collected to form oceans and lakes.

For much of Earth’s early existence (the first half ), only trace amounts of free oxygen were present. But then green plants evolved in the oceans, and they began to add oxygen to the atmosphere as a waste gas, and later oxygen increased to about 1 percent of the atmosphere and with time to its present 21 percent. How do we know for sure about the evolution of air on Earth? Are we guessing, using ‘‘voodoo’’ science? There is no guessing or voodoo involved with the historical geological record. Consider, for example, geological formations that are dated to two billion years ago. In these early sediments, there is a clear and extensive band of red sediment (‘‘red bed’’ sediments)—sands colored with oxidized (ferric) iron. Previously, ferrous formations had been laid down showing no oxidation. But there is more evidence. We can look at the time frame of 4.5 billion years ago, when carbon dioxide in the atmosphere was beginning to be lost in sediments. The vast amount of carbon deposited in limestone, oil, and coal indicate that carbon dioxide concentrations must once have been many times greater than today, which stands at only 0.03 percent. 

The first carbonated deposits appeared about 1.7 billion years ago, the first sulfate deposits about 1 billion years ago. The decreasing carbon dioxide was balanced by an increase in the nitrogen content of the air. The forms of respiration practiced advanced from fermentation 4 billion years ago to anaerobic photosynthesis 3 billion years ago to aerobic photosynthesis 1.5 billion years ago. The aerobic respiration that is so familiar today only began to appear about five hundred million years ago. The atmosphere itself continues to evolve, but human activities—with their highly polluting effects—have now overtaken nature in determining the changes. And, when you get right down to it, that is one of the overriding themes of this text—human beings and their affect on planet Earth. The atmosphere is an important geologic agent and is responsible for the processes of weathering that are continually at work on the Earth’s surface.

Gas Chemical     Symbol       Volume (%)

nitrogen                N2               78.08

oxygen                 O2                20.94

carbon dioxide     CO2               0.03

argon                   Ar                  0.093

neon                    Ne                  0.0018

helium                 He                 0.0005

krypton                Kr                 trace

xenon                  Xe                  trace

ozone                   O3               0.00006

hydrogen             H2               0.00005

Earth’s Systems

Scientists who study Earth have identified four main Earth systems: the geosphere, atmosphere, hydrosphere, and biosphere. Each system is unique, yet each interacts with the others.

Geosphere The area from the surface of Earth down to its center is called the geosphere. The geosphere is divided into three main parts: the crust, mantle, and core. 

The rigid outer shell of Earth is called the crust. There are two kinds of crust—continental crust and oceanic crust. Just below the crust is Earth’s mantle. The mantle differs from the crust both in composition and behavior. The mantle ranges in temperature from 100°C to 4000°C — much warmer than the temperatures found in Earth’s crust. Below the mantle is Earth’s core. 

Atmosphere The blanket of gases that surrounds our planet is called the atmosphere. Earth’s atmosphere contains about 78 percent nitrogen and 21 percent oxygen. The remaining 1 percent of gases in the atmosphere include water vapor, argon, carbon dioxide, and other trace gases. Earth’s atmosphere provides oxygen for living things, protects Earth’s inhabitants from harmful radiation from the Sun, and helps to keep the planet at a temperature suitable for life. 

Hydrosphere All the water on Earth, including the water in the atmosphere, makes up the hydrosphere. About 97 percent of Earth’s water exists as salt water, while the remaining 3 percent is freshwater contained in glaciers, lakes and rivers, and beneath Earth’s surface as groundwater. Only a fraction of Earth’s total amount of freshwater is in lakes and rivers.

Biosphere The biosphere includes all organisms on Earth as well as the environments in which they live. Most organisms live within a few meters of Earth’s surface, but some exist deep beneath the ocean’s surface, and others live high atop Earth’s mountains. All of Earth’s life-forms require interaction with at least one of the other systems for their survival.