Planet Earth Our Solar System Earth is part of a large physical system containing countless objects, all of which revolve around the sun. Eight spheres, called planets, are the largest objects in the solar system. Terrestrial planets have solid, rocky crusts; these four inner planets are Mercury, Venus, Earth, and Mars. Gas giant planets are more gaseous and less dense then the other planets; these four outer planets are Jupiter, Saturn, Uranus, and Neptune.

Getting to Know Earth The atmosphere, hydrosphere, and lithosphere form the biosphere, the part of Earth that supports life for all people, animals, and plants. Atmosphere is a thin layer of gases that surround Earth and contains the air we breathe. Hydrosphere includes the liquid and frozen surface water, groundwater, and water vapor in and around oceans, lakes, and rivers on Earth. Lithosphere is the land or surface areas on Earth, including the continents, islands, and ocean basins. Planet Earth

Forces of Change Earth’s Structure For millions of years, the surface of the Earth has been moving. The Earth is composed of three layers—the core at Earth’s center, the mantle layer of dense rock on the outer core, and the crust forming Earth’s surface. Many scientists believe Earth was once a single land mass called Pangaea, but then continental drift slowly spread the continents apart. Plate tectonics describes the activities of continental drift and magma flow; referring to the physical processes that create and shape continents, islands, oceans, and mountain ranges.

Internal Forces of Change Mountains are formed when Earth’s giant continental and oceanic plates collide. Moving plates sometimes cause Earth’s surface to buckle forming folds; in other cases the moving plates form cracks called faults. Violent movements of Earth’s crust along fault lines are called earthquakes, which dramatically change the surface of the land and the floor of the ocean. Volcanoes are mountains formed by lava or magma that breaks Earth’s surface. Forces of Change

External Forces of Change Wind and water break down the Earth’s surface through weathering and erosion. Physical weathering happens when large masses of rock are broken down into smaller pieces. Glaciers are large bodies of ice that move across the Earth’s surface, changing the landscape as they flow. Soil building takes thousands of years of weathering, erosion, and biological activity and is influenced by five factors; climate, topography, geology, biology, and time. Forces of Change

Earth’s Water The Water Cycle Almost all of the hydrosphere is saltwater found in oceans, seas, and some lakes. The amount of water on Earth never changes, but it is constantly moving through the processes of evaporation, condensation, and precipitation. Evaporation is when the sun’s energy causes water to change into vapor or gas. Condensation is an excess of water vapor that changes into liquid water after warm air cools. Precipitation is the release of moisture through rain, snow, or sleet.

Bodies of Salt Water Seventy percent of the Earth’s surface is water, primarily salt water. Salt water cannot be used for drinking, farming, or manufacturing. Due to a growing population and the need for more freshwater, some countries are focusing on the desalination process, which removes the salt from seawater. Desalination is expensive and is also controversial; critics believe the process has negative environmental and economic impacts. Earth’s Water

Bodies of Freshwater More than two-thirds of the world’s scarce freshwater is frozen in glaciers. Less than one-third of a percent of freshwater is found in lakes, rivers, and streams and their tributaries. Aquifers and groundwater are important sources of freshwater found underground within the Earth. Earth’s Water