The Origin of the Solar System 35
FIGURE 8 The current best estimates for the timescales over
which very early inner solar system objects and the terrestrial
planets formed. The approximated mean life of accretion is the
time taken to achieve 63% growth at exponentially decreasing
rates of growth. The dashed lines indicate the mean lives for
accretion deduced for the Earth based on W isotopes. (Based on
a figure that first appeared in A. N. Halliday and T. Kleine, 2006,
Meteorites and the timing, mechanisms and conditions of
terrestrial planet accretion and early differentiation, in
“Meteorites and the Early Solar System II” (D. Lauretta, L.
Leshin, and H. MacSween., eds.), pp. 775–801, Univ. Arizona
Press, Tucson.)
innermost regions of the solar nebula close to the
Sun. Dating based on radioactive isotopes suggest
that CAIs are the oldest surviving materials to have
formed in the solar system. CAIs in the Efremovka
chondrite are 4.5672±0.0006 Ga old based on the235 / (^238) U– 207 / (^206) Pb system, and this date is often used
to define the canonical start to the solar system. The
oldest chondrules appear to have formed at about the
same time, but most chondrules are 1–3 million years
(Ma) younger than this (Fig. 8). The space between
the chondrules and CAIs in chondrites is filled with
fine-grained dust called matrix.Most chondrites are
variably depleted in moderately volatile elements like
potassium (K) and rubidium (Rb) (Fig. 9). This de-
pletion is more a feature of the chondrules and CAIs
rather than the matrix. Chondrites are subdivided into
groups of like objects thought to come originally from
the same parent body. Currently, about 15 groups
are firmly established, 8 of which are collectively re-
ferred to as carbonaceous chondrites. These tend to
be richer in highly volatile elements such as carbon
and nitrogen compared to other chondrites, although
as with all meteorites these elements are less abun-
dant than they are in the Sun. Ordinary chondrites
are more depleted in volatile elements than carbona-
ceous chondrites, and are largely made of silicates
and metal grains. Enstatite chondrites are similar but
FIGURE 9 Comparison between the K/U and Rb/Sr ratios of
the Earth and other differentiated objects compared with
chondrites. The alkali elements K and Rb are both relatively
volatile compared with U and Sr, which are refractory. Therefore,
these trace element ratios provide an indication of the degree of
volatile element depletion in inner solar system differentiated
planets relative to chondrites, which are relatively primitive. It
can be seen that the differentiated objects are more depleted in
moderately volatile elements than are chondrites. (Based on a
figure that first appeared in A. N. Halliday and D. Porcelli, 2001,
In search of lost planets—The paleocosmochemistry of the inner
solar system,Earth Planet. Sci. Lett. 192 , 545–559.)
highly reduced. Chondrules are absent from the most
primitive, volatile-rich group of carbonaceous chon-
drites (the CI group), either because their parent
body formed entirely from matrix-like material or
because chondrule structures have been erased by
subsequent reactions with water in the parent body.
Chondrites also contain presolar grains, which are
submicron grains that are highly anomalous isotopi-
cally and have compositions that match those pre-
dicted to form by condensation in the outer envelopes
of various stars. These represent a remarkable source
of information on stellar nucleosyntheis and can be
used to test theoretical models.
- Achondritesare silicate-richmaficand ultramafic
igneous rocks not too dissimilar from those form-
ing on Earth but with slightly different chemistry
and isotopic compositions. They clearly represent the
near-surface rocks of planets and asteroids that have
melted and differentiated. A few achondrites come
from asteroids that appear to have undergone only
partial differentiation. In principle, it is possible to
group achondrites and distinguish which planet or
asteroid they came from. The oxygen isotopic com-
position of a meteorite is particularly useful in this
respect. Isotopically, oxygen is extremely heteroge-
neous in the solar system, and planets that formed