EARTH

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