Earth

Astronomers now believe that the Earth was formed from the collision of a Mars-sized protoplanet with the primordial Earth. Much of the protoplanet's core merged with the Earth's own, while the lighter materials of the collision reformed as the Moon (see schematic view of the collison). The gravitational interaction of the Earth with its Moon slows the Earth's rotation by about two milliseconds per century, so that about 900 million years ago, Earth's "year" was comprised of 481 "days" that lasted only 18 hours long.

NASA -- larger image


Viewed from space, Earth's classic image is that of a bluish ball with shifting white clouds, the result of an atmosphere actively sustained by its widespread life. Its atmosphere helps to shield its surface from meteors, most of which burn up before they can strike the surface. The atmosphere is composed of mostly nitrogen (78 percent) and oxygen (21 percent), with traces of argon, carbon dioxide, and water. Its presence of free oxygen is quite unusual because oxygen is a very reactive gas. Without the constant and massive respiration of plants worldwide, oxygen would quickly combine with other elements, and there would be little free oxygen, as has happened on Venus and Mars.

The primordial Earth probably had much more carbon dioxide, but virtually all of it has been incorporated into carbonate rocks, dissolved into the oceans, or incorporated into living plants. However, the tiny amount of carbon dioxide still in the atmosphere at any time warms the Earth's average surface temperature by about 35 °C (95 °F) above what it would otherwise be (from a frigid -21 °C/-17 °F to a comfortable 15 °C/59 °F) via the greenhouse effect. Without this improved retention of solar heat, the oceans would freeze. On the other hand, the activities of Earth's dominant lifeform (Humans) is increasing the level of carbon dioxide in Earth's atmosphere which appears to be promoting a rising trend in global temperatures.

Earth is the only planet on which water can exist in liquid form on the surface, and most (71 percent) of the planet's surface is indeed covered with water. The heat-absorbing capacity of Earth's oceans is crucial to keeping the planet's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process that is unique in the solar system today, although it may have occurred on Mars in the past).

Most of Earth's surface is very young. Within an astronomically short period (500 million years or so), erosion and tectonic processes destroy and recreate most of its surface, and so evidence of earlier geologic history including even large impact craters is quickly erased. Thus, while the Earth is at least 4.5 billion years old, the oldest known rocks are about four billion years old, and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old, and no record of the critical period when life was first getting started has yet been detected and recognized.

NASA

Although not quite as large as the Chicxulub impact structure associated with the demise of the dinosaurs, Manicouagan, a 214-million-year-old crater in eastern Canada (Quebec) that has been eroded nearly flat by glaciers, is easily seen from space due to its huge 62-mile (100-km) diameter. It was made by the largest known fragment of an object that also created: the 25-mile (40-km) diameter, Saint Martin crater in western Canada (Manitoba); the 15-mile (25-km) Rochechouart crater in the Massif Central of France; the 9.3 mile (15-km) Obolon' impact structure in the Ukraine; the 5.6-mile (9-km) Red Wing crater in the western U.S.; and possibly also the 7.4 mile (12-km) Wells Creek, Tennessee impact structure and the 2-mile (3-km) Newporte, North Dakota crater -- both in the U.S.

Unlike the other three terrestrial planets, Earth's crust is divided into eight large and 20 small, solid "plates" which float around independently on top of the semi-fluid mantle below. Under the modern the theory of plate tectonics, some plates move away from each other and new crust is created by upwelling magma from below in a process called "spreading." Other plates are being destroyed through the process of "subduction," where plates collide and the edge of one dives beneath the other to be melted in the hot mantle below.

Once in a while, a truly massive plume of hot rock from the Earth's mantle can erupt through the crust for centuries or even millenia, producing acid rains, destruction of the ozone layer from emissions of chlorine-bearing compounds, and a chill down in climate from the resulting increase in atmospheric dust. The last such eruption occurred some 65 million years ago, created India's Deccan plains, and contributed to the extinction of the dinosaurs. However, an earlier and even larger event in Siberia may have caused the Permian-Triassic extinction over 240 million years ago, when 95 percent of all species were wiped out.

By mass, the Earth's is composed of mostly iron (35 percent), oxygen (30 percent), silicon (15 percent), and magnesium (13 percent). It is made of distinct layers: a thin crust, upper mantle, lower mantle, outer core, and inner core, as well as transition zones. The crust is thinner under the oceans but thicker under the continents. While the inner core and crust are solid, the outer core and mantle layers are semi-fluid. Most of the mass of the Earth is in the mantle, which is mostly made of the minerals olivene and pyroxene (iron and magnesium silicates). The elements calcium and aluminum are abundant in the upper mantle and while silicon, magnesium, and oxygen are major components of the lower mantle. The core is probably composed mostly of iron (perhaps with some nickel), where the temperature may be as high as 7227 °C (13,040 °F) at the center -- which is hotter than the surface of the Sun. (See schematic diagram depicting the Earth's interior structure by William K. Hartmann.)

Earth has a modest magnetic field produced by electric currents generated in the metal core by its rapid spin, which is distorted into a tear-drop shape by the solar wind. Along with the atmosphere, the magnetic field shields life on Earth's surface from most harmful solar and cosmic radiation. This field traps radiation in a pair of doughnut-shaped rings of ionized gas in orbit around the Earth called the Van Allen radiation belts. The outer belt stretches from 19,000 to 41,000 km (11,800 to 25,500 miles) in altitude, while the inner belt lies between 7,600 to 13,000 km (4,700 to 8,100 miles) in altitude.

According to the geological record, Earth's magnetic field dwindles down to nothing for about a hundred years every few hundred thousand years. Then, it gradually reappears but with the north and south poles flipped. The last reversal of the poles occurred about 780,000 years ago, and the strength of the magnetic field has diminished by about five percent over the past century. Hence, the Earth may be overdue for this cyclical event. Given the anticipated loss of the magnetic field's shielding against the Sun's energetic subatiomic radiation and more ultraviolet radiation from the consequent erosion of the ozone layer, however, a magnetic reversal may cause serious ecological disruption to all surface lifeforms on Earth as well as threaten human well-being.