Geoscience Reference
In-Depth Information
Part I
Planetary perspective
I want to know how God created this
world. I am not interested in this or
that phenomenon, in the spectrum of
this or that element. I want to know
his thoughts, the rest are details.
Albert Einstein
differentiate at a very early stage in their evolu-
tion, probably during accretion. Although prim-
itive objects have survived in space for the age
of the solar system, there is no evidence for the
survival of primitive material once it has been in
a planet. One would hardly expect large portions
of the Earth to have escaped this planetary differ-
entiation, and to be 'primordial' and undegassed.
The present internal structure of the Earth was
mainly established 4.57 billion years ago. This
is not a central dogma of current geochemical
models but the use of high-precision short-lived
isotope data promises to change this.
A large amount of gravitational energy is
released as particles fall onto an accreting Earth,
enough to raise the temperature by tens of thou-
sands of degrees and to evaporate the Earth
back into space as fast as it forms. Melting and
vaporization are likely once the proto-Earth has
achieved a given size. The mechanism of accre-
tion and its time scale determine the fraction of
the heat that is retained, and therefore the tem-
perature and heat content of the growing Earth.
The 'initial' temperature of the Earth was high. A
rapidly growing planet retains more of the gravi-
tational energy of accretion, particularly if there
are large impacts.
The magma-ocean concept was developed to
explain the petrology and geochemistry of the
Moon. It proved fruitful to apply this to the Earth,
Overview
Earthispartofthesolarsystemanditcannot
be completely understood in isolation. The chem-
istry of meteorites and the Sun provide con-
straints on the composition of the planets. The
properties of the planets provide ideas for and
tests of theories of planetary formation and
evolution. The Earth is often assumed to have
been formed by the slow accumulation of plan-
etesimals -- small cold bodies present in early
solar system history. In particular, types of stony
meteorite called
chondrites
have been adopted as
the probable primary material accreted by the
Earth. This material, however, has to be exten-
sively processed before it is suitable.
Study of the Moon, Mars and meterorites
demonstrates that melting and basaltic volcan-
ism is ubiquitous, even on very small bodies.
Planets form hot, or become hot, and begin to
