Global Dimensions

Talk of mysteries!— Think of our life in nature—


—Henry David Thoreau (1848), Ktaadn top to bottom

The Earth system, as an entity in the cosmos, exists within an envelope of gases that are kept from escaping into space by gravitational attraction to the planet below (Fig. 1.1a). Hydrogen, which is the smallest and least dense atom, with a single proton in its nucleus, composes the outermost layer of the atmosphere to a height of more than 10,000 kilometers above the Earth's surface. Underneath, there are distinct atomic layers of helium (two protons) and oxygen (eight protons) overlying a molecular layer that is composed principally of two bound nitrogen atoms (seven protons each). Each denser than the layer above, these different spheres of individual gases collectively comprise the heterosphere from 90 kilometers to the boundary of outer space—beyond the layer of hydrogen atoms rotating with the Earth.

Below 90 kilometers is the thin atmospheric region called the homosphere, where gases are well mixed with a highly uniform composition throughout. By volume, pure dry air in the homosphere is composed mostly of nitrogen (78.084%) and oxygen (20.946%). The homosphere also contains small amounts of argon (0.934%) and carbon dioxide (0.033%) along with several trace gases (including neon, krypton, and xenon). Together, these gases in the homosphere—across one-

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