Atmosphere, Composition and Structure (Encyclopedia of Science)
Earth's atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, and a 1-percent mixture of minor gases dominated by argon. The atmosphere can be divided into vertical layers. Traveling from the surface of the planet upward, the major layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Although the atmosphere was formed over billions of years, there is growing concern that present-day human activity may be altering the atmosphere to the point that it may affect Earth's climate.
The atmosphere's past
When Earth formed 4.5 million years ago, its atmosphere was probably composed of hydrogen, methane, and ammonia gasesuch like the outer planets in our solar system. Some scientists theorize that this original atmosphere may have been lost when the Sun violently emitted material that swept away this gaseous envelope from around Earth.
It is believed that Earth's current atmosphere then began to form when gases were released by early volcanic activity. These gases included water vapor, carbon dioxide, nitrogen, and sulfur or sulfur compounds. The water vapor formed clouds that continually rained on Earth, forming the oceans. Since carbon dioxide dissolves easily in water, the new oceans gradually absorbed most of it. Early plants on the planet then began absorbing sunlight, water, and the remaining carbon...
(The entire section is 1110 words.)
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