Geoscience Reference
In-Depth Information
stages of planetary assembly involved impacts of
large objects. The final stages involved sweeping
up the debris and collecting an outer veneer of
exotic materials from the Sun and the outer solar
system.
The planets originated in a slowly rotating
disk-shaped 'solar nebula' of gas and dust with
solar composition. The temperature and pres-
sure in the hydrogen-rich disk decreased radially
from its center and outward from its plane. The
disk cooled by radiation, mostly in the direction
normal to the plane, and part of the incandes-
cent gas condensed to solid 'dust' particles. As
the particles grew, they settled to the median
plane by collisions with particles in other orbits,
by viscous gas drag and gravitational attraction
by the disk. The total gas pressure in the vicin-
ity of Earth's orbit may have been of the order
of 10
−
1
to 10
−
4
of the present atmospheric pres-
sure. The particles in the plane formed rings and
gaps. The sedimentation time is rapid, but the
processes and time scales involved in the collec-
tion of small objects into planetary-sized objects
are not clear. Comets, some meteorites and some
small satellites may be left over from the early
stages of accretion.
The accretion-during-condensation, or inho-
mogeneous-accretion, hypothesis leads to radi-
ally zoned planets with refractory and iron-rich
cores, and a compositional zoning away from
the Sun; the outer planets are more volatile-rich
because they form in a colder part of the neb-
ula. Superimposed on this effect is a size effect:
the larger planets, having a larger gravitational
cross section, collect more of the later condens-
ing (volatile) material but they also involve more
gravitational heating.
In the widely used
Safronov cosmogoni-
cal theory
(1972) it is assumed that the Sun ini-
tially possessed a uniform gas--dust nebula. The
nebula evolves into a torus and then into a disk.
Particles with different eccentricities and incli-
nations collide and settle to the median plane
within a few orbits. As the disk gets denser, it
breaks up into many dense accumulations where
the self-gravitation exceeds the disrupting tidal
force of the Sun. As dust is removed from the
bulk of the nebula, the transparency of the neb-
ula increases, and a large temperature gradient
is established.
If the relative velocity between planetesimals
is high, fragmentation rather than accumula-
tion will dominate and planets will not grow.
If relative velocities are low, the planetesimals
will be in nearly concentric orbits and the col-
lisions required for growth will not take place.
For plausible assumptions regarding dissipation
of energy in collisions and size distribution
of the bodies, mutual gravitation causes the
mean relative velocities to be only somewhat less
than the escape velocities of the larger bodies.
Thus, throughout the entire course of planetary
growth, the system regenerates itself such that
the larger bodies would always grow. The for-
mation of the giant planets, however, may have
disrupted planetary accretion in the inner solar
system and the asteroid belt.
The initial stage in the formation of a planet
is the condensation in the cooling nebula. The
first solids appear in the range 1750--1600 K and
are oxides, silicates and titanates of calcium and
aluminum and refractory metals such as the plat-
inum group. These minerals (such as corundum,
perovskite, melilite) and elements are found in
white inclusions (chondrules) of certain mete-
orites, most notably in Type III carbonaceous
chondrites. These are probably the oldest surviv-
ing objects in the solar system. Metallic iron con-
denses at relatively high temperature followed
shortly by the bulk of the silicate material as
forsterite and enstatite. FeS and hydrous minerals
appear at very low temperature, less than 700 K.
Volatile-rich carbonaceous chondrites have for-
mation temperatures in the range 300--400 K, and
at least part of the Earth must have accreted from
material that condensed at these low tempera-
tures. The presence of He, CO
2
and H
2
Ointhe
Earth has led some to propose that the Earth is
made up almost entirely of cold carbonaceous
chondritic material -- the
cold-accretion hypothe-
sis
. Even in some current geochemical models,
the lower mantle is assumed to be gas-rich,
and is speculated to contain as much helium
as the carbonaceous chondrites. This is unlikely.
The volatile-rich material may have come in
as a late veneer -- the inhomogenous accretion
