Geoscience Reference
In-Depth Information
Table 1.1
Approximate sequence of conden-
sation of phases and elements from a gas of
solar composition at 10
−
3
Table
1.2
Properties
of
the
terrestrial
planets
atm total pressure
GM
R
D
∗
ρ
Phase
Formula
Temperature
10
18
cm
3
/s
2
km
g/cm
3
I/MR
2
km
Hibonite
CaAl
12
O
19
1770 K
Earth
398.60
6371
5.514
0.3308
14
Corundum
Al
2
O
3
1758 K
Moon
4.903
1737
3.344
0.393
75
Platinum
Pt, W, Mo, Ta
Mars
42.83
3390
3.934
0.365
>
28
metals
Zr, REE, U, Th
Sc, Ir
Venus
324.86
6051
5.24
?
?
Mercury
22.0
2440
5.435
?
?
Perovskite
CaTiO
3
1647 K
∗
Estimated crustal thickness.
Melilite
Ca
2
Al
2
SiO
7
−
Ca
2
Mg
2
Si
2
O
7
1625 K
Co
flow is due to cooling of the Earth, which means
that only an upper bound can be placed on the
uranium and thorium content. Nevertheless, this
is a useful constraint particularly when com-
bined with the lower bound on potassium pro-
vided by argon-40 and estimates of K/U and Th/U
provided by magmas and the crust. There is little
justification for assuming that the volatile ele-
ments joined the planets in constant propor-
tions. In this context the volatiles include the
alkali metals, sulfur and so forth in addition to
the gaseous species.
Spinel
MgAl
2
O
4
1513 K
Al
2
SiO
5
Metallic iron
Fe, Ni
1473 K
Diopside
CaMgSi
2
O
6
1450 K
Forsterite
Mg
2
SiO
4
1444 K
Anorthite
CaAl
2
Si
2
O
8
1362 K
Ca
2
SiO
4
CaSiO
3
Enstatite
MgSiO
3
1349 K
Cr
2
O
3
P, Au, Li
MnSiO
3
MnS, Ag
As, Cu, Ge
Theories of planetary formation
Feldspar
(Na,K)AlSi
3
O
8
Ag, Sb, F, Ge
Sn, Zn, Se, Te, Cd
The nature and evolution of the solar nebula
and the formation of the planets are complex
subjects. The fact that terrestrial planets did in
fact form is a sufficient motivation to keep a
few widely dispersed scientists working on these
problems. There are several possible mechanisms
of planetary growth. Either the planets were
assembled from smaller bodies (planetesimals), a
piece at a time, or diffuse collections of these
bodies, clouds, became gravitationally unstable
and collapsed to form planetary-sized objects.
The planets, or protoplanetary nuclei, could have
formed in a gas-free environment or in the pres-
ence of a large amount of gas that was subse-
quently dissipated. Some hypotheses speculate
that large amounts of primordial helium dis-
solved in an early molten Earth. Others assume
that the bulk of the Earth assembled gas-free and
volatiles were brought in later. The intermediate
Reaction
(Mg,Fe)
2
SiO
4
1000 K
products
(Mg,Fe)SiO
3
Troilite,
FeS, (Fe, Ni)S
700 K
pentlandite
Pb, Bi, In, Tl
Magnetite
Fe
3
O
4
405 K
Hydrous
Mg
3
Si
2
O
7
2H
2
O, etc.
minerals
Calcite
CaCO
3
<
400 K
Ices
H
2
O, NH
3
,CH
4
<
200 K
Anders (1968), Grossman (1972), Fuchs and
others (1973), Grossman and Larimer (1974).
the inferred abundance of their heat-producing
members, uranium and thorium, and the global
heat flux. But the present surface heat flow does
not accurately represent the current rate of heat
production. A large fraction of the present heat
