SECONDARY ORIGIN OF THE ATMOSPHERE

The seemingly simple conclusion that the Earth was not born in a space filled with air, but that the air was formed after the birth of the Earth, is the starting point in considering the origin of our atmosphere. This was pointed out clearly for the first time in 1947 by Harrison Brown of Caltech.[1] When considering the origin of the Earth’s atmosphere, Brown turned his attention to the abundance of rare gases in the atmosphere. There are five types of rare gases. In order from the lightest to the heaviest atomic weight, they are helium, neon, argon, krypton, and xenon. Since none has any chemical affinity, they hardly ever combine with other atoms and are extremely inactive. Because of this, a rare gas is also called a noble gas (i.e. too “noble” to bind with other elements), but the term “rare gas” actually hints at the origin of the atmosphere. As this term implies, the atmosphere contains only an extremely small amount of these (argon-40 is the only exception, which will be discussed later). In terms of volume, xenon constitutes no more than about 0.000 01 percent, or 0.1 ppm (parts per million), of the atmosphere, and even neon, which is the most abundant rare gas (excluding argon-40), constitutes only about 20 ppm.

Looking at the whole Solar System, however, rare gases are by no means rare, compared with other elements with similar atomic weights. Recall the solar abundance of chemical elements discussed in Chapter 2; neon (atomic number 10) is as abundant as nitrogen (atomic number 7) and magnesium (atomic number 12).