8. The Machinery Behind The      Periodic Table   Previous PageNext Page
     Postscript: The Making of a Universe

It was then subjected to further fractionation at several stages, as indicated by the last four columns of the atomic composition table on page 32. The interstellar debris from which second-generation stars like our sun condensed already was enriched in the heavier elements. As the Earth was built up by accretion of dust, rocks, and debris, it too acquired an unusually rich supply of iron and other heavy elements.

The energy from collisions of accreting particles, and from radioactive decay, was sufficient to melt the interior of the early Earth and create high surface temperatures. At these temperatures, the gravitational field of such a small planet was insufficient to hold onto its original gaseous atmosphere. This is why the Earth is so poor in the noble gases helium, neon, argon, and xenon, even though these elements are not particularly rare in the universe as a whole. Unable to form solid or high-melting compounds, they leaked into interplanetary space during this high-temperature era. Oxygen was retained in large quantities because it is a reactive element and was combined in many solid oxides, carbonates, phosphates, and silicates. Nitrogen, being less reactive and having fewer solid compounds, was largely lost.

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