38 Encyclopedia of the Solar System
FIGURE 13 The estimated composition of the silicate portion
of the Earth as a function of the calculated temperature at which
half the mass of the element would have condensed. The
concentrations of the various elements are normalized to the
average composition of the solid matter in the disk as
represented by CI carbonaceous chondrites. Open circles:
lithophile elements; shaded squares: chalcophile elements;
shaded triangles: moderately siderophile elements; solid
diamonds: highly siderophile elements. It can be seen that
refractory lithophile elements are enriched relative to CI
concentrations. This is because of core formation and volatile
losses compared with CI chondrites. The moderately volatile
lithophile elements like K are depleted because of loss of
volatiles. Siderophile elements are depleted by core formation.
However, the pattern of depletion is not as strong as expected
given the ease with which these elements should enter the core.
The explanation is that there was addition of a late veneer of
chondritic material to the silicate Earth after core formation.
(From A. N. Halliday, 2003, The origin and earliest history of the
Earth, in “Meteorites, Comets and Planets” (A. M. Davis, ed.),
Vol. 1, “Treatise of Geochemistry” (H. D. Holland and K. K.
Turekian, eds.), pp 509–557, Elsevier-Pergamon, Oxford.)
collisions. Energetic collisions between large bodies would
have generated high temperatures and could have caused
further loss of moderately volatile elements. For this rea-
son, the terrestrial planets have compositions that differ
from one another and also from chondritic meteorites. The
Moon is highly depleted in moderately volatile elements
(Fig. 9) and is thought to be the product of such an ener-
getic planetary collision.
4. Nucleosynthesis and Short-lived Isotopes
With the exception of hydrogen and helium the elements
were mainly made by stellar nucleosynthesis. If one exam-
ines Fig. 5, seven rather striking features stand out.
- The estimated abundances of the elements in the Sun
and the solar nebula span a huge range of 13 orders of
magnitude. For this reason, they are most easily com-
pared by plotting on a log scale such that the number
of atoms of Si is 10^6.- Hydrogen and helium are by far the most abundant
elements in the Sun as they are elsewhere in the uni-
verse. These two elements were made in the Big Bang. - The abundances of the heavier elements generally de-
crease with increasing atomic number. This is because
most of the elements are themselves formed from
lighter elements by stellar nucleosyntheis. - Iron is about 1000 times more abundant than its neigh-
bors in the periodic table because of a peak in the
binding energy providing enhanced stability during
nucleosynthesis. - Lithium, beryllium, and boron are all relatively under-
abundant compared to other light elements because
they are unstable in stellar interiors. - A saw-toothed variability is superimposed on the over-
all trend reflecting the relatively high stability of even-
numbered isotopes compared to odd-numbered ones. - All the elements in the periodic table are present in the
solar system except those with no long-lived or stable
isotopes, namely technetium (Tc), promethium (Pm),
and the trans-uranic elements.
Those elements lighter than Fe can be made by fusion
because the process of combining two nuclei to make a
heavier nuclide releases energy. This produces the energy
in stars and is activated when the pressure exceeds a critical
threshold (i.e., when a star reaches a certain mass). Larger
stars exert more pressure on their cores such that fusion re-
actions proceed more quickly. When a star has converted all
of the hydrogen in its center to helium, it will either die out
if it is small or proceed to the next fusion cycle such as the
conversion of helium to carbon if it is sufficiently massive to
drive this reaction. Lithium, beryllium, and boron are un-
stable at the temperatures and pressures of stellar interiors,
hence the drop in abundance in Fig. 5. They are made by
spallation reactions from heavier elements by irradiation in
the outer portions of stars.
Nearly all nuclides heavier than Fe must be made by
neutron irradiation because their synthesis via fusion would
consume energy. Neutron addition continues until an un-
stable isotope is made; it will decay to an isotope of another
element, which then receives more neutrons until another
unstable nuclide is made and so forth. These are s-process
isotopes (produced by a slow burst of neutrons). However,
some of these isotopes cannot be made simply by adding a
neutron to a stable nuclide because there is no stable isotope
with a suitable mass. Such nuclides are instead created with
a very high flux of neutrons such that unstable nuclides pro-
duced by neutron irradiation receive additional neutrons
before they have time to decay, jumping the gap to very
heavy nuclides. These are r-process isotopes (produced by