he 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.
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).
he 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.
he 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.
he 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
arth 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