Geoscience Reference
In-Depth Information
the impactor was highly refractory, greatly depleted in volatile substances, and intensely
reduced.
The initial state of the terrestrial planets depended on their size and composition. The
energy sources that may have contributed to heating the interior of these planets are well
understood:
•
Initial gravitational energy, which has markedly heated and almost certainly melted the
external parts of the largest planets.
•
Gravitational energy of separation and crystallization of the iron and nickel core. This
energy is enough to raise the Earth's temperature instantaneously by 2000 K.
Radioactive heating by decay of short half-life nuclides such as
26
Al whose descendant
26
Mg has been identified in several planetary objects.
•
We will see upon discussing the extinct radioactivity of
182
Hf that the Earth's core
formed soon after the formation of the Earth itself. We will also see that the extinct radioac-
tivity of
146
Sm requires that the upper mantle of the Earth, the Moon, and Mars went
through a stage of wholesale melting, as what is termed a magma ocean. If so, it may be
that the deep mantle still contains rocks formed at that time, an idea that still remains a
challenge to geodynamicists.
12.3 Condensation of planetary material
The prediction of reasonable condensation sequences relevant to planetary accretion is one
of the most successful contributions of thermodynamics to our understanding of the Solar
System. The gravitational energy given off by the collapse of the solar nebula to form the
Sun, and the radiative transfer of thermal energy thus produced, heated the proto-planetary
material and vaporized it at temperatures in excess of 2000 K. As it cooled, the hot gas,
initially dominated by molecular H
2
,He,N
2
,O
2
,H
2
O, CO, and SiO, recombined and
recondensed to form the solids, liquids, and gases observed at ambient temperature. Astro-
physical models provide temperature and pressure distributions throughout the nebula and
their evolution over time. By applying elementary thermodynamic principles we should
therefore be able to deduce the order of condensation of the various minerals involved in the
formation of planetary bodies. Although it would be incorrect to assume that condensation
is a well-understood equilibrium process, the insight into the diversity of planetary com-
positions and the mineralogy of planetary interiors provided by thermodynamic modeling
is quite invaluable.
The method is very similar to that described in
Section 7.3
for the calculation of spe-
ciation in solutions. A first task is to draw up an inventory of the number of moles
N
of
components (O, Al, Si, etc.) and potential species (CH
4
,Al
2
O
3
,H
2
O) regardless of their
state, allowing the mass balance to be written, e.g. for silicon:
N
Si
=
N
Si
(g)
+
N
SiO
(g)
+
N
Mg
2
SiO
4
(s)
+
N
MgSiO
3
(s)
+···
(12.4)
