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The Birth and Life of Our Atmosphere
Nature Bulletin No. 554-A   February 15, 1975
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

In Bulletin No. 542-A we described the atmosphere that surrounds the earth, held there by the pull of gravity, including the canopy of air immediately above us. That canopy is responsible for our weather and climates, shields us from the killing rays of the sun, and furnishes substances necessary for all life: oxygen, nitrogen, carbon dioxide, and water vapor.

Astronomers believe that none of the other planets -- excepting Mars, perhaps -- has an atmosphere anything like ours. The principal elements, free nitrogen (78 percent) and free oxygen (21 percent), apparently are rare in the universe. On the other hand, the earth's atmosphere contains only traces of free hydrogen and helium -- the two lightest gases -- whereas they are by far the most common elements elsewhere in the universe.

Originally, those two gases must have been present here in vast quantities. What happened, and what was the origin of our atmosphere? The answer, according to modern theory, is that the earth has had other atmospheres which escaped; and that the latest one was slowly accumulated during eons of time.

Most scientists believe that this planet condensed from a whorl of star dust and gases into a naming ball where the heaviest elements gravitated inward and lighter ones rose to the surface. Fiery spouts of gases, especially hydrogen and helium, jetted away into space. As the earth lost that heat, its surface gradually cooled and became a fluid crust with hardening islands of granite and basalt which eventually formed continents. Torrents of naming lava erupted through innumerable weak spots in the crust, and also terrific jets of gases more complex and heavier than hydrogen and helium: water vapor (as steam), carbon dioxide, and probably other compounds such as ammonia and methane.

Those gases formed enormous hot clouds that grew and grew until they were many miles high, shutting out sunlight and shrouding the earth in perpetual darkness for perhaps millions of years. From their upper reaches into cold atmosphere, wave after wave of rains fell but boiled and became rising steam before they reached the earth's hot crust.

Thus, each rainfall sapped the earth of some of its heat energy and carried that upward into the clouds where it radiated into outer space. Other losses of heat and energy resulted from the awful lightning that must have been continual, and from the tremendous jets of fiery gases. The earth cooled until at last there came a time when the rains fell and did not boil away. Meanwhile the crust had heaved and wrinkled. When the clouds thinned and those deluges ended, the sun shone upon glittering seas collected in the depths.

Life apparently began, somehow, in shallow waters warmed by sunshine. Green plants developed until some emerged and grew along the beaches. Later, others invaded higher ground. Primitive animals developed in the seas and their descendants crept out to live on land. Green plants, in daytime, take in carbon dioxide and give off oxygen. At night the process is reversed. Animals consume oxygen and exhale carbon dioxide. As dead plants and animals decay, carbon dioxide is released into the air and nitrogen into the soil. However, because plants give off far more oxygen than they consume and take in far more carbon dioxide than they give off, the supply of oxygen in the air increased and that of carbon dioxide gradually dwindled.

The dust, smoke, fumes and nuclear explosions produced by modern civilization threaten to disrupt that vital cycle of use and renewal.

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