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Solar-System Abundance of Elements and Their Chemical Properties

  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:

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 tex2html_wrap_inline1142 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 tex2html_wrap_inline1146 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 tex2html_wrap_inline1146 O ice and NH tex2html_wrap_inline1150 ice are bonded by hydrogen bond in which a hydrogen ion (i.e. proton) is exchanged between adjacent oxygen atoms for H tex2html_wrap_inline1146 O ice, for example, and act as a glue for combining them, while ices like CH tex2html_wrap_inline1154 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 tex2html_wrap_inline1156 on the order of 0.01 to tex2html_wrap_inline1158 eV, whereas valence and metallic bonds are strong with tex2html_wrap_inline1156 of typically a few to tex2html_wrap_inline1162 eV. In terms of temperature, the binding energies for van der Waals and hydrogen bonds correspond to the temperature of tex2html_wrap_inline1164 to tex2html_wrap_inline1166 K, and those for valence and metallic bonds to tex2html_wrap_inline1168 K, where k is the Boltzmann constant. However, actual sublimation temperatures for van der Waals and hydrogen bonded solids are tex2html_wrap_inline1172 K or lower, and those for valence and metallic bonds are tex2html_wrap_inline1174 K; both are lower than the corresponding tex2html_wrap_inline1176 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 tex2html_wrap_inline1176 .


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Mon Sep 16 16:23:29 JST 1996