Earth Planet Essays - Planetary Science, Terrestrial Planets

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