Present-Day Composition
C. Argon - 0.9% - Used in light bulbs.
D. Carbon Dioxide - 0.03% - Plants use it to make oxygen. Acts as a blanket and prevents the escape of heat into outer space. Scientists are afraid that the buring of fossil fuels such as coal and oil are adding more carbon dioxide to the atmosphere.
E. Water Vapor - 0.0 to 4.0% - Essential for life processes. Also prevents heat loss from the earth.
F. Trace gases - gases found only in very small amounts. They include neon, helium, krypton, and xenon.