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.
|
|