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A basis of the chemical composition of astrophysical dust and ices including cometary ice is the solar-system abundance of elements, which gives relative amount of elements in the solar-system bodies. The solar-system abundance is deduced from spectroscopic observations of solar and stellar atmospheres, chemical analysis of C1 carbonaceous chondrites, which are considered to be one of the most primitive (i.e. unaltered) materials of the solar system, measurements of the elemental composition of solar wind, and so on. In particular the elemental abundances determined from observations of solar atmosphere and analyses of carbonaceous chondrites show excellent agreement for a wide variety of heavy elements (see, for example, Ringwood, 1979). The solar-system abundance is regarded as a representative elemental abundance in various astrophysical environment, and is sometimes called ``cosmic abundance".
Figure 3 illustrates the solar-system abundance of elements compiled by Anders and Grevesse (1989).
The elemental abundance has the following characteristics:
Here
indicates the difference by more than one order of magnitude, whereas >
indicates the difference within an order of magnitude.
Table 1 summarizes a rough idea on the chemical properties of abundant
elements with their abundances in order of magnitude. Of course, the chemical
properties such as phases and compounds formed from the elements depend
on the temperature and pressure of the relevant environment. The chemical
properties shown here are those for temperatures of 10 to 1000K and a pressure
of
atm, a typical presumed pressure of the solar nebula at heliocentric distance
of 1AU. In practice the pressure does not affect the chemical properties
so long as it is low.
Each of the elements form specific chemical compounds and phases. The
most abundant elements H and He are very volatile, and exist as a gas such
as H
and He under the temperatures and pressure stated above. The major elements
that form solids are those heavier than carbon. C, N, and O are more abundant
than Mg, Si, Fe, and Ni by about an order of magnitude. Note that C, N,
O, and S combined with H are the elements that form ices and organics,
whereas Mg, Si, Fe, and Ni are the elements that form rocks and metals.
This implies that ice and organics have a potential to be more abundant
than rocks and metals in space under appropriate conditions.
The volatility depends on the nature of bonds which combine molecules
or atoms in solids. Molecules that form ices such as H
O ice and NH
ice are bonded by hydrogen bond in which a hydrogen ion (i.e. proton)
is exchanged between adjacent oxygen atoms for H
O ice, for example, and act as a glue for combining them, while ices like
CH
ice are bonded by van der Waals bond, which originates from fluctuating
dipole-dipole interaction. On the other hand, atoms in rocks are bonded
by valence (chemical) bond, and atoms in metal by metallic bond; in both
bonds electrons act as a glue. Van der Waals and hydrogen bonds are weak
with the binding energy
on the order of 0.01 to
eV, whereas valence and metallic bonds are strong with
of typically a few to
eV. In terms of temperature, the binding energies for van der Waals and
hydrogen bonds correspond to the temperature of
to
K, and those for valence and metallic bonds to
K, where k is the Boltzmann constant. However, actual sublimation
temperatures for van der Waals and hydrogen bonded solids are
K or lower, and those for valence and metallic bonds are
K; both are lower than the corresponding
by about one order of magnitude. This is due to the fact that the binding
energy is the internal energy of solids, whereas sublimation proceeds
so as to minimize the free energy of the relevant system. Sublimation
increases entropy of the system, which decreases the free energy, and thus
occurs temperatures well below the temperature
.