Land, sea and air are familiar terms that are defined by scientists as the three major parts of our planet. The lithosphere is the solid outer layer of earth and extends down about 50 miles. Its upper part, which we walk upon, is called the crust. The hydrosphere includes the fresh water of the land as well as salt water in the oceans. It covers 80% of the earth's crust. The third sphere, where flight takes place, is the atmosphere, a mixture of gases surrounding the earth and held by earth's gravity. Our world, then, is made up of solids, liquids, and gases.

It is now thought that our solar system was created almost five billion years ago from a cloud of swirling gas and dust. Matter collided and merged under its own gravitational force to form into planets with, at their center, the largest body of all, the sun, which contains almost all the matter in its solar system. Our planet was at first a collection of cold matter without any atmosphere. Over a period of a billion years gravity pressed this matter together and caused the interior to become magma, rock melted into liquid by extreme heat. Volcanoes grew where the magma, held under pressure like air in a car's tire, erupted through cracks in the earth's crust. Our atmosphere began when gases were forced out of these volcanoes.

Scientists believe that earth's early atmosphere was a combination of water vapor, carbon dioxide, carbon monoxide, nitrogen, methane and ammonia. There was no oxygen, which all animal life needs to survive. Yet our atmosphere today is 21% oxygen (the rest is nitrogen, argon, and carbon dioxide). What caused such a change in the mixture over 4 billion years? One major cause was condensation, in which much of the volcanic water vapor became liquid and formed our oceans. More than two billion years after the planet's birth, the first plants floating in these oceans developed a process called photosynthesis. When the water and carbon dioxide absorbed by a plant is struck by sunlight, chlorophyll molecules - which give the plant its green color - generate tiny electric currents that split the water inside the plant into separate particles of oxygen and hydrogen. The oxygen is given off into the atmosphere to form a vital part of the air we breathe. The hydrogen is combined with carbon dioxide to form a sugar. Such sugars are the basic building blocks of plants. Photosynthesis, then, not only provides the oxygen that keeps most animals alive, but is also the basis of all the world's food chains. On a geologic time line, about 500 million years ago the oxygen content became high enough for marine life to develop respiration. 200 million years later reptiles, the first air-breathing land animals, had evolved. The development of our atmosphere over such enormous periods of time is truly . . . breathtaking.

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Last modified: Thu Oct 9 13:51:20 PDT 1997

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