What Makes the Earth Unique?

The image of our blue and white planet taken from space reveals much of what makes the Earth unique. Although the terrestrial planets are alike in many ways, they differ greatly in the composition of their atmospheres, the characteristics of their surfaces, and the presence or absence of water and life. The Earth has an overall blue-and-white hue because it is surrounded by an atmosphere of gases, predominantly nitrogen, oxygen, argon, and water vapor, with small amounts of some other gases, such as carbon dioxide. No other planet in the solar system has such an atmosphere. The Earth's atmosphere contains clouds of condensed water vapor. The clouds form because water evaporates from the hydrosphere, another unique feature of the Earth.

The hydrosphere ("watery sphere") consists of the oceans, lakes, and streams; underground water; and snow and ice. Planets farther from the Sun are too cold for liquid water to exist on their surfaces. Planets closer to the Sun are so hot that any surface water evaporated long ago. Other planetary bodies have ice on the surface and water vapor in the atmosphere, but only the Earth has the right surface temperature to maintain a hydrosphere consisting of liquid water, ice, and water vapor.

Another reason the Earth is special is the biosphere ("life sphere"), which includes the totality of the Earth's living matter. When the Earth is viewed from space, the biosphere is reveled by blankets of green plants on some of the land masses. The biosphere embraces innumerable living things, large and small, which belong to millions of different species. It also includes dead plants and animals that have not yet been completely decomposed.

The nature of the Earth's solid surface is another special characteristic. The Earth is covered by an irregular blanket of loose debris formed as a result of weathering-the chemical alteration and mechanical breakdown of rock caused by exposure to the atmosphere, hydrosphere, and biosphere. This layer is called regolith (from the Greek rhegos, blanket, and lithos, stone). Soils, muds in river valleys, sands in the desert, rock fragments, and all other unconsolidated debris are part of the regolith. Other planets and planetary bodies with rocky surfaces have regolith too, but in those cases the regolith has formed primarily from endless pounding by meteorite impacts. The Earth's regolith is unique because it is formed by complex interactions among physical, chemical, and biological processes, usually involving water. The Earth's regolith is also unique because it teems with life; most plants and animals live on or in the regolith or in the hydrosphere. The regolith is like a loose blanket that lies on top of the rocks that make up the outermost part of the Earth. This tough, rocky outermost part of the Earth is called the lithosphere. The lithosphere is defined by strength and rigidity, not by composition. It is about 100 km (67 miles) 3 thick, and consists of the crust and the outermost part of the mantle.

The Earth differs from all other known planets because of the unique relationship between its lithosphere and the hotter, weaker, putty-like rocks that lie immediately below, in the asthenosphere. The lithosphere-asthenosphere boundary (that is, the strong rock-weak rock boundary) lies in the upper mantle and does not coincide with the compositional boundary between the crust and the mantle.

The Earth's lithosphere is very thin relative to the Earth as a whole. The solid rock that makes up the lithosphere is strong, but not strong enough to withstand the constant movement of hotter, weaker material in the underlying mantle. Consequently, the lithosphere has broken into a set of enormous rocky fragments we call plates. These plates move around on top of the asthenosphere, driven by the movement of hot material in the mantle. They collide, split apart, and slide past one another. This generates a lot of geologic activity, such as earthquakes and mountain-building, much of which occurs along the boundaries where plates interact with one another. Collectively, these processes are referred to as plate tectonics.

Plate tectonic activity has been an important process throughout much of the Earth's history. It is responsible for the creation of mountains, the locations of volcanoes, the intensities of earthquakes, and the shapes of continents and deep ocean basins. It has influenced the formation of the atmosphere, the development of climatic zones, and the evolution of life. Because of plate tectonics, the Earth has developed two fundamentally different kinds of crust: The oceanic crust that underlies the deep ocean basins is relatively thin (average thickness 8 km, or 5.4 mi) and dense; it is composed mostly of basaltic rocks. The continental crust is much thicker (average thickness 45 km, or 30 mi) and is made of generally less dense rocks. As we will see in later chapters, the concept of plate tectonics enables us to understand many years' worth of observations of natural processes in a unified context.

In summary, what makes the Earth unique? We know of no other planet where plate tectonics has played, and continues to play, such an important role in forming the environment. We know of no other planet where the temperature permits water to exist near the surface in solid, liquid, and gaseous forms. We know of no other planet that would have been hospitable to the origin and evolution of life as we know it. There are billions upon billions of stars in the universe, so it is almost inevitable that there are billions of planets too; surely a few of those planets must be Earthlike and therefore might be capable of supporting life. However, if a relatively advanced civilization does exist on a planet somewhere out in space, so far we have not heard or seen any sign of it. So the existence of plate tectonics, the presence of an atmosphere rich in oxygen and the abundance of water and life, are all unique features of our planet Earth.