Part 5, Chapter 17 Summary and Analysis

The atmosphere keeps the Earth warm. Without it, the Earth would be a ball of ice with an average temperature of -60?F. In addition, the atmosphere absorbs or deflects a variety hazards, including cosmic rays, charged particles, ultraviolet rays, etc. At 120 miles deep, the atmosphere seems thick. However, if the Earth were the size of a desk globe, the atmosphere would be no thicker than a few coats of varnish.

The first layer of atmosphere, next to the ground, is called the troposphere. This layer contains the majority of oxygen in the air and all water. It accounts for 80 percent of the mass of the total atmosphere. In a fast elevator, one could reach the top of the troposphere in 20 minutes. Anyone would be uncomfortable there, though, because the average temperature is -70?F and the oxygen is...