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: 5.972e24 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 Crust
40- 400 Upper mantle
400- 650 Transition region
650-2700 Lower mantle
2700-2890 D'' layer
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 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
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:
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:
North American Plate - North America, western North Atlantic and Greenland
South American Plate - South America and western South Atlantic
Antarctic Plate - Antarctica and the "Southern Ocean"
Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
African Plate - Africa, eastern South Atlantic and western Indian Ocean
Indian-Australian Plate - India, Australia, New Zealand and most of Indian
Nazca Plate - eastern Pacific Ocean adjacent to South America
Pacific Plate - most of the Pacific Ocean (and the southern coast of
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 boundaries (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 responsible 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
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 outer
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 and even
reverse relative to the surface; the geomagnetic north pole is currently located
in northern Canada. (The "geomagnetic north pole" is the position on the Earth's
surface directly above the south pole of the Earth's field; see this diagram.)
The Earth's magnetic field and its interaction with the solar wind also produce
the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas
(or plasma) trapped in orbit around the Earth. The outer belt stretches from
19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and
7,600 km in altitude.
Earth has only one natural satellite, the Moon. But
thousands of small artificial satellites have also been placed in orbit around
Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships
with the Earth; they're not really moons, the term "companion" is being used.
It is somewhat similar to the situation with Saturn's moons Janus and
Lilith doesn't exist but it's an interesting story.
Distance Radius Mass
Satellite (000 km) (km) (kg)
--------- -------- ------ -------
Moon 384 1738 7.35e22
Bill Arnett; last updated: 2003 May 28