The third planet in distance outward from the Sun. It is the only planetary body in the solar system that has conditions suitable for life, at least as known to modern science.
A brief treatment of the Earth follows. For the relationship of the Earth to other components of the solar system, see Solar System: Earth. For detailed coverage of the shape, structure, and composition of the Earth, as well as of its gravitational and magnetic fields, see Earth. For specific information about terrestrial surface features and the factors governing their formation, see Continental Landforms; Oceans; Plate Tectonics. For a description of the layers of air and water that envelop the Earth, see Atmosphere; Hydrosphere. For a discussion on the so-called zone of life at the Earth's surface, see Biosphere.
Basic planetary data. The mean distance of the Earth from the Sun is about 149,573,000 km (92,960,000 miles). The planet orbits the Sun at a speed of 29.8 km (18.5 miles) per second, making one complete revolution in 365.25 days. As it revolves around the Sun, the Earth spins on its axis and rotates completely once every 23 hours 56 minutes 4 seconds.
The fifth largest planet of the solar system, the Earth has an equatorial circumference of 40,076 km (24,902 miles), an equatorial radius of 6,378 km (3,963 miles), a polar radius of 6,357 km (3,950 miles), and a mean radius of 6,371 km (3,960 miles). The planet's total surface area is roughly 509,600,000 square km (197,000,000 square miles), of which about 29 percent, or 148,000,000 square km (57,000,000 square miles), is land. The balance of the surface is covered by the oceans and smaller seas. The Earth has a mass of 5.976 1027 grams (or roughly 6 1021 metric tons) and a mean density of 5.517 grams per cubic cm (0.2 pound per cubic inch).
The Earth has a single natural satellite, the Moon. The latter orbits
the planet at a mean distance of slightly more than 384,400 km (238,870
miles).
The centrifugal force of the Earth's rotation makes the planet
bulge at the Equator. Because of this, the Earth has the shape of an oblate
spheroid, being flatter near the poles than near the Equator. Correspondingly,
one degree of latitude is longer in high latitudes than it is in low ones.
The gravitational field, or gravity, of the Earth is manifested as
the force acting upon a free, unsupported body causing it to move in the
general direction of the centre of the planet. The Earth's gravity is not
fixed, but rather varies from place to place on the surface, with the main
variation occurring with latitude. It averages approximately 983.22 cm
(32.26 feet) per second per second at the poles, which is somewhat higher
than at the Equator, where it is only about 973.03 cm (31.92 feet) per
second per second.
The Earth's atmosphere consists of a mixture of gases, chiefly
nitrogen (78 percent) and oxygen (21 percent). Argon makes up much of the
remainder of the gaseous envelope, with trace amounts of water vapour,
carbon dioxide, and various other gases also present. At lower altitudes,
minute dust particles and water droplets occur in suspension.
The Earth is surrounded by a magnetosphere, a region of strong magnetic
forces that extends upward from about 140 km (90 miles) in the upper atmosphere.
In the magnetosphere, the magnetic field of the Earth traps rapidly moving
charged particles (e.g., electrons and high-energy protons), the majority
of which appear to be emitted by the Sun during periods of intense activity.
If it were not for this shielding effect, such particles would bombard
the terrestrial surface and destroy life. High concentrations of the trapped
particles make up two doughnut-shaped zones called the Van Allen radiation
belts. These belts play a key role in several geophysical phenomena, as,
for example, auroras.
The Earth is the only planet known to have liquid water. Together with ice, the liquid water constitutes the hydrosphere. Seawater makes up more than 98 percent of the total mass of the hydrosphere and covers about 71 percent of the Earth's surface. Significantly, seawater constituted the environment of the earliest terrestrial life forms. The rest of the hydrosphere consists of fresh water, occurring principally in lakes, streams (including rivers), and glaciers.
The Earth's surface is subdivided into continental masses, of which there are seven: Europe, Asia, Africa, Australia, North America, South America, and Antarctica. These continents are surrounded by the so-called World Ocean, which is commonly broken down into three major bodies--namely, the Atlantic, Pacific, and Indian oceans. The continents and oceanic depressions may be further subdivided into major relief features, such as mountain systems, volcano chains, rift valleys, and plateaus on land, and mid-oceanic ridges, trenches, submarine canyons, and clusters of guyots (flat-topped, deeply submerged undersea mountains) on the ocean floor. Most such third- and fourth-order topographic features were formed as a result of vertical tectonic movements (i.e., uplift and subsidence of the Earth's crust) and the extrusion of molten-rock material called magma. Continental landforms of smaller scale, including pediments, caves, and cliffs, have been produced by denudational processes, which involve the weathering and erosion of rocks and the accumulation of the resultant sedimentary debris.
Broadly speaking, the Earth consists of two regions: a core composed largely of molten, iron-rich metallic alloy; and a solid shell of silicate minerals comprising both the mantle and crust. In continental regions, the crust is made up chiefly of granitic rock, whereas the composition of the ocean floor corresponds mainly to that of basalt and gabbro. On average, the crust extends 35 km (22 miles) downward from the surface to the underlying mantle from which it is separated by the Mohorovicic discontinuity (often simply called the Moho). The mantle, consisting of rock material in which olivines, pyroxenes, and silicate perovskite predominate, ends at a depth of approximately 2,900 km (1,800 miles). The core is thought to have a two-part structure: an outer fluid region and a solid, extremely dense inner region that measures only about 2,400 km (1,500 miles) across. The inner core is most likely composed exclusively of iron, whereas the outer core appears to include such alloying elements as sulfur or oxygen, as suggested by its lower density.
Recent studies indicate that more than 90 percent of the magnetic field
at the Earth's surface originates in an electric-current system in the
outer core. The magnetic field is thought to be caused by the flow of electric
charges in the region's fluid molten metal. Variations in the field through
geological time, as revealed by paleomagnetic research, suggest that the
electrically conductive fluid in the outer core circulates. The fluid motions,
responsible for surface variations, presumably also generate and maintain
the currents. This process is analogous to the one that occurs in the dynamos
(rotating generators) used by power stations, in which current is produced
by the motion of metallic wires past each other. The streams of molten
iron alloy in the Earth's outer core are the equivalent of the moving wires.
©Copyright Andrew Do