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.
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 Crust
10- 400 Upper mantle
400- 650 Transition region
650-2700 Lower mantle
2700-2890 D'' layer (sometimes included in the lower mantle)
2890-5150 Outer core
5150-6378 Inner core
The 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 fluid.
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):
atmosphere = 0.0000051
oceans = 0.0014
crust = 0.026
mantle = 4.043
outer core = 1.835
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 of this only from
seismic techniques; samples from the upper mantle arrive at the
surface as lava from volcanoes but 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:
34.6% Iron
29.5% Oxygen
15.2% Silicon
12.7% Magnesium
2.4% Nickel
1.9% Sulfur
0.05% Titanium
The Earth is the densest major body in the solar system.
The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.
Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:
There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundariess (right).
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
a frigid -21 C to a comfortable +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.
The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.
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.
Earth has only one natural satellite, the Moon.
Distance Radius Mass Satellite (000 km) (km) (kg) --------- -------- ------ ------- Moon 384 1738 7.35e22