Geology Reference
In-Depth Information
11
THE ELEMENTS IN THE UNIVERSE
The significance of element abundance
A second question to be addressed in this chapter is
how the Earth has come to have its present composition
and structure. In seeking an answer to this problem,
we again consider element abundances, noting this
time how they differ between the Sun, the Earth and
the other planets. Such differences provide clues to the
kind of chemical processing that has produced the
Solar System and the Earth in their present form.
Having considered the behaviour of some impor-
tant elements in the lithosphere, hydrosphere and
atmosphere (Chapters 4, 8 and 9) and their isotopes
(Chapter 10), it is natural to ask the question: how
were the chemical elements formed in the first place?
Have they always existed at their present levels, or has
there been a progressive building up of the cosmic
inventory through the life of the universe? Can we
identify the process(es) by which they were formed?
Current opinion favours the gradual synthesis of
heavier elements from lighter ones, by a complex
series of nuclear fusion reactions occurring in stars.
This process, called
stellar nucleosynthesis
, leaves its
fingerprint on all forms of cosmic matter. We can
learn how this works by studying the relative abun-
dances of the elements in the universe as a whole, or
in some representative part of it.
Measuring cosmic and Solar System
abundances
Our knowledge of the overall composition of visible
matter
1
in the Universe rests mainly on two kinds of
analysis:
Invisible (including so-called 'dark') matter will be discussed
a little later.
1
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