When seen from space, the Earth appears as a blue ball, partially covered
with swirls of white clouds. A bluish envelope of gas surrounds the Earth
- our atmosphere. At a closer range, you can see land, oceans,
mountains, and cloud patterns. The surface is not smooth, but covered
with mountains, plains, and valleys. There are two polar caps - white
areas of frozen water at the North and South Poles. Two-thirds of the
Earth's surface is covered with water, while the other one-third is land .
In time, you could detect seasonal changes on Earth's surface. The first
evidence of life you would see would probably be the proliferation of
artificial outdoor lighting on the night side of the planet. The Earth is
the only planet known to have life.
A planet actually has two types of days: a sidereal day, which is its
rotation with respect to the stars and a solar day, which is its rotation
with respect to the Sun. For most planets, like Earth, which rotate very
rapidly compared to their orbit speed , the two types of days are almost
the same. For example, Earth's sidereal day is only four minutes shorter
than its solar day (which is what our clocks use.)
Much of what we know about the interior of the Earth is from studies of
earthquakes and man-made underground explosions. The Earth appears to be
divided into several shells. At the center there is a very dense metallic
core about 2,580 kilometers (1,600 miles) in diameter. This inner core is
surrounded by an outer core of molten iron, nickel and cobalt (over 2,100
kilometers or 1,300 miles thick), and then a crystalline mantle (2,900
kilometers or 1,800 miles thick). It is primarily granite under the
continents and basalt under the oceans. On the surface, hydrogen and
oxygen form the water in the oceans.
Bound by gravity to the Earth, practically all of the atmosphere lies
within one hundred miles of the Earth's surface. It is 78% nitrogen, 21%
oxygen, and has small amounts of carbon dioxide and other gases, and
unfortunately, an increasing quantity of man-made pollutants which can
damage other parts of the atmosphere. For instance, some of these
pollutants reduce the amount of ozone in the upper atmosphere. Ozone is
vital because it protects us from harmful ultraviolet and x-ray radiation
from the Sun.
The other planets in the solar system do not seem to have abundant free
oxygen. We believe that a billion years of plant life on the Earth
explains why we do. The Earth also has a magnetic field which traps
charged particles such as protons and electron s which make up the solar
wind.
Revised May 1996
IS - 04
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