Geology Reference
In-Depth Information
the Sun's radiance, and will therefore stay dark; such
substellar bodies, intermediate in mass between
Jupiter-like planets and true stars, include the ultra-
dense bodies known as 'brown dwarfs'.
The dark matter making up the other 90-95% of
the mass of the universe, however, is much more
exotic in character and its nature is arguably the
biggest unanswered question in cosmology. Many
bizarre novel subnuclear particles have been
proposed as the constituents of non-baryonic dark
matter, but this cosmological conundrum is still a
long way from being solved. Ferreira (2006) provides
a readable non-specialist summary of our current
understanding.
Although the nature of dark matter is a profoundly
important question for the cosmologist, it falls out-
side the scope of this topic. Dark matter, whatever its
nature, seemingly plays no significant part in the for-
mation or composition of planets such as ours. It is of
course important to acknowledge that, when we gen-
eralize grandly about the composition of matter in
the universe, we are referring solely to the 'visible'
baryonic component that may account for as little as
1% of the mass of the whole. Yet it is this 1% that
determines the character and composition of the
planet we inhabit.
10
8
O
Fe
10
6
Mg
Na
Al
S
Ni
Cr
Mn
Ca
10
4
Co
P
Ti
K
Cu
Zn
10
2
Ge
Rb
Sr
Sc
B
Y
Ba
1
Pr
La
Pb
Ce
Be
Li
Th
Tm
10
-2
10
-2
10
2
Relative abundance in Cl chondrites (Si=10
6
)
10
4
10
6
10
8
1
Figure 11.1
The correlation between element abundances in
the Sun and in CI carbonaceous chondrites. Abundance is
expressed as the number of atoms of each element per 10
6
atoms of silicon (both axes have
logarithm
ic scales).
the key features of this 'abundance curve' are common
to practically all stars and luminous nebulae:
The composite abundance curve
(a) Hydrogen and helium are several orders of mag-
nitude more abundant than any other element. In
atomic terms, helium has one-tenth of the abun-
dance of hydrogen and together they comprise
98% of the Solar System's mass.
(b) Progressing to higher atomic numbers leads to an
overall decrease in abundance, making the heaviest
nuclei among the least abundant.
(c) The elements lithium, beryllium and boron are
sharply depleted compared with the other light
elements. (In the case of Li this depletion is
much more marked for the Sun than for CI
chondrites - Figure 11.1.)
(d) Elements having even atomic numbers (
Z
) are on
average about ten times more abundant than
neighbouring elements having odd atomic num-
bers. This effect, which is apparent in terrestrial
rocks as well, produces a 'sawtooth' profile if
adjacent atomic numbers are joined up (see inset
showing
REE
abundances in Figure 11.2).
The two sources of information outlined above - solar
spectra and analyses of primitive meteorites - allow
us to build a composite picture of the relative abun-
dances of the chemical elements in the Solar System
as a whole. Gaseous elements - hydrogen, the inert
gases, and so on - can of course be determined only
from solar measurements; for other elements, like
boron, spectral measurements are difficult or impos-
sible and reliance on meteorite data is the only feas-
ible course. Fortunately the abundance of most other
elements can be determined by both methods. Since
the two approaches involve different assumptions
and employ different instrumental techniques, it is
reassuring to find a good correlation between them
(Figure 11.1).
The composite abundance data so obtained have
been plotted against atomic number (
Z
) in Figure 11.2.
Although compiled specifically for the Solar System,
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