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The Earth - A Planet
Earth is the third planet from the Sun and the fifth largest.
Orbit :
149,600,000 km (1.00 AU) from Sun


Diameter :
12,756.3 km
Mass :
5.9736e24 kg

Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other  names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus-the fertile soil (Greek: Gaia, terra mater - Mother Earth). It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet. Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful. The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

0 - 40
D'' layer
2700 - 2890
Upper mantle
40 - 400
Outer core
2890 - 5150
Lower mantle
650 - 2700
Inner core
5150 - 6378

he crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle. Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole. (values below x10^24 kilograms):

= 0.0000051
= 4.043
= 0.0014
Outer core
= 1.835
= 0.026
Inner core
= 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some  lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly  olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most oft  his only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes, the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:

The Earth is the densest major body in the solar system. The Earth's surface is very young. In the relatively short (by astronomical standards) period of  500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 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. There is no record of the critical period when life was first getting started.
71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also reponsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide  and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated  into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living  plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again.

The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from - 21 C to + 14 C); without it the oceans would freeze and life as we know it would be impossible.

The presence of free oxygen is quite remarkable from a chemical point of view.Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

Earth has a modest magnetic field produced by electric currents in the core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the  auroras (see the Interplanetary Medium). Irregularities in these factors cause the magnetic poles to move relative to the surface; the north magnetic pole is currently located in northern Canada.

© 1994, 1995, 1996, 1997 by William A.Arnett - NASA Solar system
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