The Atmosphere
Earth is surrounded by layers of gases. These are collectively called air, or the atmosphere. The air surrounding Earth is composed of gases that are essential to animal and plant life. Earth's air is about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases. Dust is also present. The atmosphere is subdivided into five layers. They are, from lowest to highest, the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
The atmosphere also includes water vapor that is essential to life. It dissipates carbon dioxide and other waste products of life and of human industries. The atmosphere transmits the radiation from the Sun (solar radiation) that is essential to photosynthesis. At the same time, it shields Earth from lethal ultraviolet radiation. It protects the planet from powerful cosmic rays and from the meteors that constantly shower down on the planet from space.
Furthermore, the atmosphere acts as a blanket. It moderates the planet's climate and maintains a generally higher temperature on the planet than would occur without it.
The Hydrosphere
Earth appears to be the only planet that contains large amounts of liquid water. In fact, pictures from space show Earth as a blue planet. About three-fourths of the surface is covered by the oceans. These bodies of water, together with large inland lakes, constitute the hydrosphere.The hydrosphere contributes great amounts of water vapor to the air. It plays a large part in the atmospheric changes known as weather.
Of Earth's surface water, almost 96 percent by weight is made up of hydrogen and oxygen. Sodium, chlorine, and many other elements are also found in oceanic waters. Highly sensitive instruments would probably reveal traces of many more elements.
The plants and animals found in the sea are an immensely valuable resource. They provide food, fertilizers, and industrial materials. The ocean is also a vast storehouse of minerals. These include common salt (sodium chloride), magnesium, manganese, gold, iron, copper, uranium, and silver. Salt and magnesium are obtained from the sea in quantity. Others will no doubt be made available as more-efficient methods for extracting them are developed.
The Lithosphere
The solid part of Earth is its lithosphere. It is made up of three types of rock -igneous, sedimentary, and metamorphic- and soil. Soil consists of rock debris combined with organic materials. Igneous rock is derived from the molten, rock-producing matter called magma.
Sedimentary rock consists principally of rock fragments. The fragments have accumulated through untold millennia and have been pressed together. Igneous rock or sedimentary rock is altered through changes in temperature and pressure and other forces within Earth. The process gives rise to metamorphic rock.
Geologists study the rocks and minerals of Earth's surface. Geophysicists use the methods of physics to study Earth's interior. Geophysicists do not know for certain what is found at the center of the planet. However, they have gathered information about the core from various indirect sources such as seismic waves. Seismic waves are shock waves that radiate through Earth when its layers shift, as during earthquakes.
Two kinds of seismic waves travel through the interior: primary, or P, waves, which travel very quickly through any material; and secondary, or S, waves, which move about half as fast as P waves and only travel through solids. The slowest-moving seismic waves are the long, or L, waves, which ripple just beneath Earth's surface. By studying the movements of seismic waves, scientists have developed a fairly clear picture of Earth's interior.
Earth has a solid outer mantle about 1,740 miles (2,800 kilometers) thick. The crust makes up only a small part of this mantle. Earth's crust extends only 25 miles (40 kilometers) or so below the surface. Beneath the mantle is the core. The core is some 4,400 miles (7,100 kilometers) in diameter. The core is made up of a solid inner core and a molten outer core.
