Sustainability of Life *credits from DiscoveryEducation

Earth EARTH is one of the planets in the solar system, the third in distance from the sun and the fifth largest of the planets in diameter. The average distance of the earth from the sun is 149,597,890 km (92,955,820 mi). It is the only planet known to support life, although some of the other planets have atmospheres and contain water. The earth is not a perfect sphere but is slightly pear-shaped. Calculations based on perturbations in the orbits of artificial satellites reveal that the earth is an imperfect sphere, because the equator bulges, or is distended, 21 km (13 mi); the North Pole bulges 10 m (33 ft); and the South Pole is depressed about 31 m (about 100 ft). The circumference of the earth at the equator is 40,075 km (24,901 mi).

BRIEF SURVEY OF THE EARTH Distance from sun Perihelion (closest) 147,100,000 km (91,400,000 mi) Aphelion (farthest) 152,100,000 km (94,500,000 mi) Period of revolution 365.24 days Rotation period 23.934 hours Eccentricity of orbit 0.0167 Inclination of orbit 0 Mass 5,973,700,000,000,000,000,000 metric tons Radius at equator 6378.14 km (3963.19 mi) Mean density 5.515 g/cm3 Surface gravity 9.766 m/sec2 (32.041 ft/sec2) Atmospheric pressure at surface 1 bar Surface temperature (min./max.) -88° to 58° C (-126 to 136° F) Known natural satellites 1

Motion In common with the entire solar system, the earth is moving through space at the rate of approximately 20.1 km/sec or 72,360 km/hr (approximately 12.5 mi/sec or 45,000 mph) toward the constellation of Hercules. The Milky Way galaxy as a whole, however, is moving toward the constellation Leo at about 600 km/sec (about 375 mi/sec). The earth and its satellite, the moon, also move together in an elliptical orbit about the sun. The eccentricity of the orbit is slight, so that the orbit is virtually a circle. The approximate length of the earth's orbit is 938,900,000 km (583,400,000 mi), and the earth travels along it at a velocity of about 106,000 km/hr (about 66,000 mph). The earth rotates on its axis once every 23 hr 56 min 4.1 sec (based on the solar year)

Composition The earth consists of five parts: the first, the atmosphere, is gaseous; the second, the hydrosphere, is liquid; the third, fourth, and fifth, the lithosphere, mantle, and core, are largely solid. The atmosphere is the gaseous envelope that surrounds the solid body of the planet. Although it has a thickness of more than 1100 km (more than 700 mi), about half its mass is concentrated in the lower 5.6 km (3.5 mi). The lithosphere, consisting mainly of the cold, rigid, rocky crust of the earth, extends to depths of 100 km (60 mi). The hydrosphere is the layer of water that, in the form of the oceans, covers approximately 70.8% of the surface of the earth. The mantle and core are the heavy interior of the earth, making up most of the earth's mass.

Composition Continued The hydrosphere consists chiefly of the oceans, but technically it encompasses all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The average depth of the oceans is 3794 m (12,447 ft), more than five times the average height of the continents. The mass of all the world's oceans is approximately 1,350,000,000,000,000,000 (1.35 × 1018) metric tons, or about 1/4400 of the total mass of the earth.

Composition Continued The rocks of the lithosphere have an average density of 2.7 (water = 1) and are almost entirely made up of 11 elements, which together account for about 99.5% of its mass. The most abundant is oxygen (about 46.60% of the total), followed by silicon (about 27.72%), aluminum (8.13%), iron (5.0%), calcium (3.63%), sodium (2.83%), potassium (2.59%), magnesium (2.09%) and titanium, hydrogen, and phosphorus (totaling less than 1%). In addition, 11 other elements are present in trace amounts of from 0.1 to 0.02%. These elements, in order of abundance, are carbon, manganese, sulfur, barium, chlorine, chromium, fluorine, zirconium, nickel, strontium, and vanadium. The elements are present in the lithosphere almost entirely in the form of compounds rather than in their free state. These compounds exist almost entirely in the crystalline state, so each is, by definition, a mineral.

Composition Continued The lithosphere comprises two shells--the crust and upper mantle--that are divided into a dozen or so rigid tectonic plates (see Plate Tectonics). The crust itself is divided in two. The sialic or upper crust, of which the continents consist, is made up of igneous and sedimentary rocks whose average chemical composition is similar to that of granite and whose density is about 2.7. The simatic or lower crust, which forms the floors of the ocean basins, is made of darker, heavier igneous rocks such as gabbro and basalt, with an average density of about 3.The lithosphere also includes the upper mantle. Rocks at these depths have a density of about 3.3. The upper mantle is separated from the crust above by a seismic discontinuity, called the Moho, and from the lower mantle below by a zone of weakness known as the asthenosphere. Shearing of the plastic, partially molten rocks of the asthenosphere, 100 km (60 mi) thick, enables the continents to drift across the earth's surface and oceans to open and close.

Composition Continued The dense, heavy interior of the earth is divided into a thick shell, the mantle, surrounding an innermost sphere, the core. The mantle extends from the base of the crust to a depth of about 2900 km (about 1800 mi). Except for the zone known as the asthenosphere, it is solid, and its density, increasing with depth, ranges from 3.3 to 6. The upper mantle is composed of iron and magnesium silicates, as typified by the mineral olivine, the chief constituent of peridotite. The lower part may consist of a mixture of oxides of magnesium, silicon, and iron.Seismological research has shown that the core has an outer shell about 2225 km (about 1380 mi) thick with an average density of 10. This shell is probably rigid, and studies show that its outer surface has depressions and peaks, the latter forming where warm material rises. In contrast, the inner core, with a radius of about 1275 km (about 795 mi) is probably solid. Both core layers are thought to consist largely of iron, with a small percentage of nickel and other elements. Temperatures in the inner core may be as high as 6650° C (12,000° F), and the average density is estimated to be 13. In 1996, after nearly 30 years of research, it was found that, like the earth itself, the inner core spins from west to east, but at its own rate, outpacing the earth by about one degree per year.

Origin of Earth .Radiometric dating has enabled scientists to arrive at an estimate of 4.6 billion years for the age of the earth (see Dating Methods). Although the oldest earth rocks dated this way are not quite 4 billion years old, meteorites, which correlate geologically with the earth's core, give dates of about 4.5 billion years, and crystallization of the core and meteorites is considered to have occurred at the same time, some 150 million years after the earth and solar system first formed (see Solar System: Theories of Origin). After originally condensing, by gravitational attraction of cosmic dust and gas, the earth would have been almost homogeneous and relatively cool. But continued contraction of these materials caused them to heat, as did the radioactivity of some of the heavier elements. In the next stage of its formation, as the earth became hotter, it began melting under the influence of gravity. This caused the differentiation into crust, mantle, and core, with the lighter silicates moving up and outward to form the mantle and crust, and the heavier elements, mainly iron and nickel, sinking downward toward the center of the earth to form the core. Meanwhile, by volcanic eruption, light volatile gases and vapors continually escaped from the mantle and crust. Some of these, mainly carbon dioxide and nitrogen, were held by the earth's gravity and formed the primitive atmosphere, while water vapor condensed to form the world's first oceans.