Earth Planet

The Earth, man's home, is a planet. The Earth has special characteristics, and
these are important to man. It is the only planet known to have the right
temperature and the right atmosphere to support the kind of environments and
natural resources in which plants and man and other animals can survive. This
fact is so important to man that he has developed a special science called
ecology, which deals with the dependence of all living things will continue to
survive on the planet. Many millions of kinds of plants and animals have
developed on Earth. They range in size from microscopic plant and animals to
giant trees and mammoth whales. Distinct types of plants or animals may be
common in many parts of the world or may be limited to a small area. Some kinds
thrive under conditions that are deadly for others. So some persons suggest that
forms of life quite different from those known on Earth might possibly survive
on planets with conditions that are far different from conditions on Earth. Many
persons believe that the Earth is the only planet in the solar system that can
support any kind of life. Scientists have theorized that some primitive forms of
life may exist on the surface of Mars, but evidence gathered in 1976 by unmanned
probes sent to the Martian surface seems to indicate that this is unlikely.

Scientist at one time also believed that Venus might support life. Clouds always
hide the surface of Venus, so it was thought possible that the temperature and
atmosphere on the planet's surface might be suitable for living things. But it
is now known that the surface of Venus is too hot--an average of 800 F (425

C)--for liquid water to exist there. The life forms man is familiar with could
not possibly live on Venus. The Earth has excellent conditions for life. The
temperature is cool enough so that liquid water can remain on Earth's surface.

In fact, oceans cover more than two thirds of the surface. But the temperature
is also warm enough so that a small fraction of this water is permanently
frozen--near the North and South Poles and on some mountain tops. The Earth's
atmosphere is dense enough for animals to breathe easily and for plants to take
up the carbon dioxide they need for growth. But the atmosphere is not so dense
that it blocks out sunlight. Although clouds often appear in the sky, on the
average enough sunlight reaches the surface of the Earth so that plants
flourish. Growing plants convert the energy of sunlight into the chemical energy
of their own bodies. This interaction between plants and the sun is the basic
source of energy for virtually all forms of life on Earth. Extensive exploration
of the sea floor since 1977, however, has uncovered the existence of biological
communities that are not based on solar energy. Active areas of sea floor
spreading, such as the centers in the eastern Pacific that lie far below the
limit of light penetration, have chimney like structures known as smokers that
spew mineral-laden water at temperatures of approximately 660 F (350 C).

Observations and studies of these active and inactive hydrothermal vents have
radically altered many views of biological, geological, and geochemical
processes that exist in the deep sea. One of the most significant discoveries is
that the vents and associated chemical constituents provide the energy source
for chemosynthetic bacteria. These bacteria form, in turn, the bottom of the
food chain, sustaining the lush biological communities at the hydrothermal vent
sites. Chemosynthetic bacteria are those that use energy obtained from the
chemical oxidation of inorganic compounds, such as hydrogen sulfide, for the
fixation of carbon dioxide into organic matter. Although the atmosphere allows
sunlight to reach the Earth's surface, it blocks out certain portions of solar
radiation, especially X rays and ultraviolet light. Such radiation is very
harmful, and, if the atmosphere did not filter it out, probably none of the life
forms on Earth could ever have developed. So, the necessary conditions for these
life forms--water, the planet in the solar system known to have all these
"right" conditions. THE EARTH'S PLACE IN SPACE Despite its own special
conditions, the Earth is in some ways similar to the other inner planets--the
group of planets nearer to the sun. Of these planets, Mercury is the closest to
the sun; Venus is second; the Earth is third; and Mars is forth. All of these
planets, including the Earth, are basically balls of rock. Mercury is the
smallest in size. It diameter is about two thirds the greatest width