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Characteristics of the Chemical Composition of the Ice of a Cometary Nucleus

A cometary nucleus is composed of ices composed mainly of H, C, N, O and S, and refractory dust including silicates and organic refractories. We note that ices are sensitive to the environmental conditions as mentioned in §1, and use them as a probe for studying the formation history of cometary matter. The composition of ice of a cometary nucleus is inferred from abundances of gaseous molecules in the coma. Most of the molecular species observed in the coma, however, are secondary products such as radicals and ions produced from parent molecules composing the ice of the nucleus via photolysis by solar UV and other chemical reactions in the coma.

Table 2 lists candidate parent molecules and their abundances adopted from Mumma et al. (1993) (see also Weaver 1989, Yamamoto 1991).

Although the abundance values have large uncertainties, it is clear that both oxidized (CO, tex2html_wrap_inline1184 ) and reduced ( tex2html_wrap_inline1186 ) species constitute cometary ice, indicating that the cometary ice is a mixture of oxidized and reduced compounds. What does this fact imply for the origin of cometary ice? The ice is formed through condensation of a gas. If the condensation occurred from a gas of the solar-system elemental composition in chemical equilibrium, the composition of the ice may be calculated from thermodynamics. The result is shown in Fig. 4, which is taken from Lewis (1974).

Figure 4 indicates that the composition of C or N-bearing ices is CH tex2html_wrap_inline1154 , NH tex2html_wrap_inline1150 , and their clathrates. Namely the equilibrium condensation theory of a gas of solar-system abundance predicts reduced compounds. In contrast, the observed abundance of the cometary molecules indicates that cometary ice was formed under non-equilibrium conditions.


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Mon Sep 16 16:23:29 JST 1996