Geoscience Reference
In-Depth Information
Chapter 1
Origin and early history
Earth is the namesake of the terrestrial planets,
also known as the inner or rocky planets. The
chemistry of meteorites and the Sun provide
constraints on the composition of the bulk of
these planets and they provide tests of theories of
planetary formation and evolution. In trying to
understand the origin and structure of the Earth,
one can take the geocentric approach or the ab
initio approach. In the former, one describes the
Earth and attempts to work backward in time.
For the latter, one attempts to track the evolu-
tion of the solar nebula through collapse, cool-
ing, condensation and accretion, hoping that one
ends up with something resembling the Earth
and other planets. Planets started hot and had
a pre-history that cannot be ignored. The large-
scale chemical stratification of the Earth reflects
accretionary processes.
condense, and metallic iron condenses near
1470 K (Table 1.1 and Figure 1.2). Below about
1000 K, sodium and potassium condense as
feldspars, and a portion of the iron is stable
as fayalite and ferrosilite with the proportion
increasing with a further decrease in tempera-
ture. FeS condenses below about 750 K. Hydrated
silicates condense below about 300 K.
Differences in planetary composition may
depend on the location of the planet, the location
and width of its feeding zone and the effects of
other planets in sweeping up material or perturb-
ing the orbits of planetesimals. In general, one
would expect planets closer to the Sun and the
median plane of the nebula to be more refractory
rich than the outer planets. On the other hand, if
the final stages of accretion involve coalescence
of large objects of different eccentricities, then
there may be little correspondence between bulk
chemistry and the present position of the terres-
trial planets (Table 1.2).
There is evidence that the most refractory
elements condensed from the solar nebula as a
group, unfractionated from one another, at tem-
peratures above the condensation temperature
of the Mg-silicates. Hence, the lithophile refractory
elements (Al, Ca, Ti, Be, Sc, V, Sr, Y, Zr, Nb, Ba,
rare-earth elements, Hf, Ta, Th and U and, to
some extent, W and Mo) can be treated together.
From the observed abundance in samples from
the Moon, Earth and achondrites, there is strong
support for the idea that these elements are
present in the same ratios as in Cl chondrites.
The abundance of the refractory elements in a
given
Condensation of the nebula
The equilibrium assemblage of solid compounds
that exists in a system of solar composition
depends on temperature and pressure and, there-
fore, location and time. The condensation behav-
ior
of
the
elements
is
given
in
Figures
1.1
and 1.2.
Atanominalnebularpressureof10 1 atm,
the material would be a vapor at temperatures
greater than about 1900 K. The first solids to
condense at lower temperature or higher pres-
sure are the refractory metals (such as W, Re,
Ir and Os). Below about 1750 K refractory oxides
of aluminum, calcium, magnesium and titanium
planet
can
be
weakly
constrained
from
 
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