The Origin of the Solar System 37
FIGURE 12 The pallasite Esquel is a mixture of silicate
(olivine) and iron metal that may have formed at a planetary
core-mantle boundary. (Photograph courtesy of Drs. M. Grady
and S. Russell and the Natural History Museum, London.)
Note that there are no clear examples of mantle material
within meteorite collections. The isotopic compositions of
some elements in irons reveal that they have been exposed
to cosmic rays for long periods—up to hundreds of millions
of years. This means their parent bodies broke up a long
time ago. Because they are extremely hard, they survived
the collisions that destroyed their parent body as well as any
subsequent impacts. In contrast, fragments of mantle ma-
terial (as with samples excavated by volcanoes on the Earth)
are extremely friable and would not survive collisions.
Survivability is also an issue for meteorites entering
Earth’s atmosphere and being recovered in recognizable
form. Chondrites and achondrites are mainly composed
of silicates that undergo physical and chemical alteration
on the surface of Earth more rapidly than the material in
iron meteorites. Furthermore, iron meteorites are highly
distinctive, so they are easier to recognize than stony mete-
orites. For this reason, most meteorites found on the ground
are irons, whereas most meteorites that are seen to fall from
the sky (referred to as falls) are actually chondrites. Most
falls are ordinary chondrites, which probably reflects the
fact that they survive passage through the atmosphere bet-
ter than the weaker carbonaceous chondrites. The parent
bodies of ordinary chondrites may also have orbits in the
Asteroid Belt that favor their delivery to Earth. IDPs are
less prone to destruction during passage through the at-
mosphere than meteorites so they probably provide a less
biased sample of the true population of interplanetary mate-
rial. Most IDPs are compositionally similar to carbonaceous
rather than ordinary chondrites and this suggests that the
Asteroid Belt is dominated by carbonaceous-chondrite-like
material.
Mass spectrometric measurements on meteorites and
lunar samples provide evidence that the isotopes of most
elements are present in similar proportions in the Earth,
Moon, Mars, and the asteroids. The isotopes of elements
heavier than hydrogen and helium were made bynucle-
osynthesisin stars that generate extremely varied isotopic
compositions. Since the solar nebula probably formed from
material from a variety of sources, the observed isotopic
homogeneity was originally interpreted as indicative that
the inner solar nebula was very hot and planetary material
condensed from a∼2000 K gas of solar composition. How-
ever, a variety of observations including the preservation of
presolar grains in chondrites suggest that the starting point
of planet formation was cold dust and gas. This homogeneity
is therefore nowadays interpreted as indicating that the in-
ner nebula was initially turbulent, allowing dust to become
thoroughly mixed. CAIs sometimes contain nucleosynthetic
isotopic anomalies. This suggests that CAIs sampled varied
proportions of the isotopes of the elements before they be-
came homogenized in the turbulent disk. With improved
mass spectrometric measurements evidence has been ac-
cumulating for small differences in isotopic composition in
some elements between certain meteorites and those of
the Earth and Moon. This area of study that searches for
nucleosythetic isotopic heterogeneity in the solar system is
ongoing and is now providing a method for tracking the
provenance of different portions of the disk.
However, oxygen and the noble gases are very different
in this respect. Extreme isotopic variations have been found
for these elements. The different oxygen and noble gas iso-
tope ratios provide evidence of mixing between composi-
tions of dust and those of volatile (gaseous) components.
Some of this mixing may have arisen later when the neb-
ula cooled, possibly because large amounts of isotopically
distinct material are thought to have arrived from the outer
nebula in the form of water ice. There are also possibili-
ties for generating some of the heterogeneity in oxygen by
irradiation within the solar nebula itself.
The terrestrial planets and asteroids are not just depleted
in nebular gas relative to the Sun. They are also depleted in
moderately volatile elements (elements such as lead, potas-
sium, and rubidium that condense at temperatures in the
range 700–1350 K) (Figs. 9 and 13). In chondritic mete-
orites, the degree of depletion becomes larger as an el-
ement’s condensation temperature decreases. It was long
assumed that this is the result of the loss of gas from a hot
nebula before it cooled. For example, by the time tempera-
tures became cool enough for lead to condense, much of the
lead had already accreted onto the Sun as a gas. However,
it is clear that moderately volatile elements are depleted in
chondrites at least in part because they contain CAIs and
chondrules that lost volatiles by evaporation during heat-
ing events. The least depleted chondrites (CI carbonaceous
chondrites) contain no CAIs or chondrules. Another mech-
anism for losing moderately volatile elements is planetary