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Earth

Earth is the third planet from the Sun. It is the largest and densest of the rocky inner planets, and is the only one with a large moon. It is the only object in the solar system whose surface is covered with liquid water. And it is the only object in the known universe which supports life.

The Blue Marble
Our Earth gets its name from an Old English word for soil or dry land. The equivalent Latin (and Italian) word is terra; compare with Terre and Tierra, the French and Spanish words for Earth. To the ancient Greeks, Earth was "Gaea", the daughter of Chaos who first emerged from the void at the start of creation. Gaea or Gaia was the Great Mother, the primary source of fertility of land and life itself. Gaea bore all the heavenly gods with Ouranos (the Sky). Gaea's body is the Earth in many ancient cultures, as are all her mountains, seas, and other geographic features. Indeed, the derivation of the English word "geographic" from its Greek root, Gaea, is hard not to notice!

The Earth orbits the Sun at an average distance of 150 million kilometers (93 million miles), or exactly 1 Astronomical Unit (A.U.) from the Sun, and takes just over 365 days to complete one orbit. Earth rotates once on its axis every 23 hours, 56 minutes, and 4 seconds. The Earth's rotation causes the daily cycle of day and night. The tilt of Earth's axis - about 23.5 degrees to the plane of its orbit - causes the seasons: one hemisphere is bathed in sunlight and experiences summer, while the other hemisphere points away from the sun and experiences winter. Six months later, the hemispheres are reversed, and each one experiences the opposite season.

The Earth has one natural satellite: the Moon. It is large, as moons go, with approximately one quarter the diameter of the Earth. The Moon orbits the Earth at an average distance of 240,000 miles (384,000 km), with an orbital period of 27.5 days. The center of gravity of the Earth-Moon system lies inside the Earth, displaced about 4700 km from the Earth's center. The gravity of the Moon is largely responsible for the tides in the fluid oceans on the Earth's surface. By a remarkable coincidence of nature, the Sun and Moon appear almost exactly the same size as seen from the Earth's surface. This creates spectacular eclipses when the Moon passes in front of the Sun.

Atmosphere and Climate

The surface of the Earth is dominated by water. The Earth's surface presents the right combination of atmospheric pressure and temperature for water to exist as both a liquid, a solid, and a gas. About 70% of the Earth's surface is covered by oceans of liquid water, making the Earth appear blue from space. The Earth's striking white polar caps are composed of water frozen solid as ice. Water exists in the Earth's atmosphere both as invisible vapor and as clouds composed of trillions of microscopic water droplets.
The Earth's atmosphere and oceans spread heat around the planet, and help keep the surface temperature even. Water vapor in Earth's atmosphere plays a pivotal role in regulating its climate. Like carbon dioxide, water vapor is an extremely effective "greenhouse gas", trapping infrared radiation and warming the surface. Without the presence of its atmospheric water vapor, the Earth's average surface temperature would be about -20°C, and the oceans would be frozen solid. The Earth's climate would be much like that of Mars. Conversely, with too much carbon dioxide and/or water vapor in the atmosphere, the Earth's temperature would rise - eventually leading to a "runaway greenhouse effect" which boiled away the oceans, leading to conditions much like those on Venus. The Earth's current climate exists in the balance between these two extremes.

The presence of liquid water makes possible, and in return is made possible by, the existence of organic life. Life is the most important - and so far, unique - quality of planet Earth. The green plants which cover much of the Earth's land surface photosynthesize atmospheric carbon dioxide into oxygen, regulating the amount of this greenhouse gas that is present in the atmosphere. The chemical composition of the Earth's atmosphere - 78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide, and water vapor - would be impossible without life. Pure oxygen is highly flammable, and would quickly react to form an inert blanket of carbon dioxide much like the atmosphere of Venus, without the photosynthesis of living plants to maintain it.

Geology

The Earth is an extremely geologically active place, with frequent volcanic eruptions and earthquakes.
This activity, combined with erosion by wind and water, gives the Earth's surface a topography much unlike any other in the solar system. To begin with, the Earth's surface contains almost no impact craters - only about 120 have been recognized.
The Earth's geological and erosional activity wipes craters away after only a few million years. Instead, the Earth's land surface is dominated by long mountain ranges, huge erosional plains, and extensive river valleys.

The Earth's major mountain ranges, and the outlines of its continents, are shaped by plate tectonics - a process in which huge sections of the Earth's crust slide around over millions of years above the plastic mantle layer. As the crustal plates drift, at a rate of a few centimeters per year, they carry the continents on top of them. Where they collide, they thrust up enormous mountain ranges; where they plunge beneath one another, they create deep undersea trenches. The earth's highest mountain (Mt. Everest in the Himalayas at 29,028 ft or 8848 meters) and its deepest undersea valley (the Marianas trench, almost 7 miles or 11 km beneath sea level) both exist at the boundaries of colliding plates.
The Earth's tallest mountain (though not its highest) is Hawaii's Mauna Kea volcano. The Hawaiian island chain, of which Mauna Kea is a part, is also the result of plate tectonics. Hawaii's volcanos sit atop a "hot spot" of molten material rising from the deep interior, bursting through the crust, and then cooling to form the island chain as the Pacific plate carries each volcano westward, away from the hot plume.

All of this activity is powered by the tremendous amount of energy at the Earth's core. Much of that energy is residual heat left over from the Earth's formation; some of it is also generated by the decay of naturally occurring radioactive elements like Uranium and Thorium.

The Earth's inner core is thought to have a temperature of about 7000°C. It is composed mainly of iron and nickel, and is solid - despite the high temperature - due to the tremendous pressure of the overlying layers. The solid inner core has a radius of about 1220 km, and is surrounded by a liquid outer core about 3400 km in radius. The outer core, in turn, is surrounded by the mantle, comprised mainly of silicate rocks, and extends to a depth of about 2890 km, making it the thickest layer of the Earth. Although it is mostly solid, the mantle's high temperature (500°C at the crust to 4000°C at the core) causes it to flow like a plastic over millions of years. This slow, viscous convection is what drives the tectonic motion of the crustal plates which lie on top of it.

The dynamic motion of the Earth's metallic core gives rise to another important property of our planet: its magnetic field. The Earth's magnetic axis is tiled 11.5° to the Earth's rotational axis. Field line arrows point from south to north. The north magnetic pole is in fact a south pole, since north poles on compasses point toward it.

The Earth's Magnetic Field. (Peter Reid)
The magnetic field shields the Earth's surface from the majority of the deadly radiation particles emitted by the Sun. Instead of bombarding the surface, these particles are funneled into the Earth's upper atmosphere at high latitudes near the poles, giving rise to harmless but colorful auroras. The existence of Earth's magnetic field is another factor that may have made life possible here.