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 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.
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.
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
Ocean
Nazca Plate - eastern Pacific Ocean adjacent to South America
Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)
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).
ATMOSPHERE
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.
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 the Earth. Asteroid 3753 (1986 TO) has a
complicated orbital relationship with the Earth; it's not really a moon, the term
"companion" is being used. It is somewhat similar to the situation with Saturn's
moons Janus and Epimetheus.
Lilith doesn't exist but it's an interesting story.
