The Origin of the Solar System 47
mass for most of its history. The spectrum of asteroid Vesta,
located 2.4 AU from the Sun, shows that it has a basaltic
crust. The HED meteorites, which probably come from
Vesta, show this crust formed only a few million years after
the solar system, according to several isotopic systems. The
survival of Vesta’s crust suggests that the crust formed the
impact rate in the belt has never been much higher than it
is today. For these reasons, it is thought that most of the As-
teroid Belt’s original mass was removed at a very early stage
by a dynamical process rather than by collisional erosion.
The Asteroid Belt currently contains a number oforbital
resonancesassociated with the giant planets. Resonances
occur when either the orbital period or precession period
of an asteroid has a simple ratio with the corresponding pe-
riod for one of the planets. Many resonances induce large
changes in orbital eccentricity, causing asteroids to fall into
the Sun, or to come close to Jupiter, leading to close en-
counters and ejection from the solar system. For this rea-
son, there are very few asteroids that orbit the Sun twice
every time Jupiter orbits the Sun once, for example. When
the nebular gas was still present, small asteroids moving on
eccentric orbits would have drifted inward rapidly due to
gas drag. After the giant planets had formed, a combination
of resonances and gas drag may have transferred most ob-
jects smaller than a few hundred kilometers from the Aster-
oid Belt into the terrestrial-planet region. Larger planetary
embryos would not have drifted very far. However, once
oligarchic growth ceased, embryos began to gravitationally
scatter one another across the belt. Numerical simulations
show that most or all of these bodies would eventually en-
ter a resonance and be removed, leaving an Asteroid Belt
greatly depleted in mass and containing no objects bigger
than Ceres. The timescale for the depletion of the belt de-
pends sensitively on the orbital eccentricities of the giant
planets at the time, which are poorly known. The belt may
have been cleared in only a few million years, but it may
have required as much as several hundred million years if
the giant planets had nearly circular orbits.
The albedos and spectral features of asteroids vary widely
from one body to another, but clear trends are apparent as
one moves across the Asteroid Belt. S-type asteroids, which
generally lie in the inner Asteroid Belt, appear to be more
thermally processed than the C-type asteroids that domi-
nate the middle belt. These may include the parent bod-
ies of ordinary and carbonaceous chondrites respectively.
C-types in turn seem more processed than the P-type as-
teroids that mostly lie in the outer belt. These differences
may reflect differences in the composition of solid materials
in different parts of the nebula, or differences in the time at
which asteroids formed. Ordinary and enstatite chondrites,
which probably come from the inner Asteroid Belt, tend
to be dry, while carbonaceous chondrites from the middle
and outer belt contain up to 10% water by mass in the form
of hydrated minerals. This suggests that temperatures were
cold enough in the outer Asteroid Belt for water ice to form
and become incorporated into asteroids where it reacted
with dry rock. Temperatures were apparently too high for
water ice to condense in the inner Asteroid Belt. It is pos-
sible that some of the objects currently in the Asteroid Belt
formed elsewhere. For example, it has been proposed that
many of the parent bodies of the iron meteorites, and pos-
sibly Vesta, formed in the terrestrial-planet region and
were later gravitationally scattered outward to their current
orbits.
Iron meteorites from the cores of melted asteroids are
common, whereas meteorites from the mantles of these
asteroids are rarely seen. This suggests that a substantial
amount of collisional erosion took place at an early stage,
with only the strong, iron-rich cores of many bodies sur-
viving. A number of other meteorites also show signs that
their parent asteroids experienced violent collisions early
in their history. Chondrites presumably formed somewhat
later than the differentiated asteroids, when the main ra-
dioactive heat sources had mostly decayed. Chondrites are
mostly composed of chondrules, which typically formed 1–3
Ma after CAIs. Chondrite parent bodies cannot be older
than the youngest chondrules they contain, so they must
have formed several million years after the start of the so-
lar system. For this reason, it appears that the early stages
of planet formation were prolonged in the Asteroid Belt.
Chondrites have experienced some degree of thermal pro-
cessing, but their late formation meant that their parent
bodies never grew hot enough to melt, which has allowed
chondrules, CAIs, and matrix grains to survive.8. Growth of Gas and Ice Giant Planets
Jupiter and Saturn are mostly composed of hydrogen and
helium. These elements do not condense at temperatures
and pressures found in protoplanetary disks, so they must
have been gravitationally captured from the gaseous com-
ponent of the solar nebula. Observations of young stars
indicate that protoplanetary disks survive for only a few
million years, and this sets an upper limit for the amount of
time required to form giant planets. Uranus and Neptune
also contain significant amounts of hydrogen and helium
(somewhere in the range 3–25%), and so they probably
also formed quickly, before the solar nebula dispersed.
Jupiter and Saturn also contain elements heavier than
helium and they are enriched in these elements compared
to the Sun. The gravitational field of Saturn strongly sug-
gests it has a core of dense material at its center, containing
roughly one fifth of the planet’s total mass. Jupiter may also
have a dense core containing a few Earth masses of ma-
terial. The interior structure of Jupiter remains quite un-
certain because we lack adequate equations of state for the
behavior of hydrogen at the very high pressures found in
the planet’s interior. The upper atmospheres of both planets
are enriched in elements such as carbon, nitrogen, sulfur,