Geology Reference
In-Depth Information
Depletion factor
Enrichment factor
10
-7
10
-5
10
-3
10
-1
1
10
Refractory
Ca
Al Ti U
Pt metals
Si Mg Li
Siderophile
Fe Ni Co P
Volatile
Chalcophile
Na K
Rb Mn F
S Cu Zn Ag
C
Cl Br l
Pb Hg Cd
HHe
N
Relative
abundance
in Moon
Relative
abundance
in Earth
Figure 11.5
Element fractionation in the Earth (white bars) and the Moon (grey bars). The length of the bars indicates the
approximate degree of depletion (left) or enrichment (right) of each 'volatility group' of elements relative to Solar-System
average abundances - see
logarithm
ic scale at top. The arrow on the right indicates the predicted order of condensation as
the solar nebula cooled. Because Si is the reference element for expressing both terrestrial/lunar and Solar-System abun-
dances (Figure 11.2), the Si group of elements registers neither enrichment nor depletion in this diagram.
make-up. The small inner planets Mercury, Venus,
Earth and Mars - known as the
terrestrial planets
- have
high densities characteristic of mixtures of metal and
silicates in varying proportions. Their atmospheres
make up only a tiny proportion of the planetary mass.
The remaining planets
9
- the
major planets
- have
masses several
orders of magnitude
greater than
Earth's. Their low mean densities (0.69 kg dm
-3
for
Saturn to 1.64 kg dm
-3
for Neptune) indicate compos-
itions closer to the Solar-System average (Figure 10.2),
in which atmophile elements predominate: the largest
planet, Jupiter, consists almost entirely of hydrogen
and helium, although with a small rock and ice core of
10-20 times the Earth's mass.
More is known about the constitution of the Earth
and Moon than the other planets, and geochemists
have been able to assemble quite detailed models of
the overall chemical composition of these two bodies.
They are illustrated in Figure 11.5. The Earth and Moon
are strongly depleted not just in atmophile (gaseous)
elements, but in the other volatile elements too. This
depletion is more marked in the Moon than the Earth.
Evolution of the Solar System
10
The current consensus is that the Sun and the Solar
System developed together by gravitational contrac-
tion of a large cloud of interstellar gas and dust more
than 4.5 billion years ago. The overall composition of
this
pre-solar nebula
must have been close to the
10
The on-line illustrated 'time-line' at
http://www.lpi.usra.edu/
education/timeline/mural.shtml#right
may be a helpful
introduction to this section.
9
Not including Pluto, which is no longer classed as a planet.
Search WWH ::
Custom Search