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Characteristics of a Cometary Nucleus

Among many possible alteration processes, radiogenic heating in the interior of a cometary nucleus is one of the most plausible processes, and may be studied on a firm basis compared with other processes. In the following sections, we shall discuss thermal history of ice in the interior of a cometary nucleus under radiogenic heating during residence in the Oort cloud for tex2html_wrap_inline1386 yr, the age of the solar system. For the thermal evolution of short-period comets, see excellent review by Rickman (1991). The results of the evolution in the Oort cloud provide initial conditions for the study of the evolution of short-period comets.

The thermal history depends on the characteristics of a cometary nucleus such as its structure, thermal conductivity of the ice, abundance of radiogenic nuclides, and so on. There have been proposed several models of a cometary nucleus (see a review by Donn, 1991); many of them remain qualitative because there have been no direct observational data except that for comet Halley (Keller et al., 1986; Sagdeev et al., 1986). One of the important observational results to infer the structure of a cometary nucleus is its bulk density, which is as low as 0.28 to tex2html_wrap_inline1388 (Rickman, 1989), much smaller than the densities of compact ice and silicate; both are main composition of the nucleus. The low density indicates that a cometary nucleus is very porous structure. We assume that cometary nuclei are initially composed of aggregates of grains with a silicate core coated by an amorphous H tex2html_wrap_inline1146 O ice mantle. Mass fraction x of the silicate core is about 0.5 according to the Greenberg model of cometary grains (Greenberg, 1982). Amorphous H tex2html_wrap_inline1146 O ice is expected from the interstellar-ice residue model. For simplicity, volatile molecules other than H tex2html_wrap_inline1146 O ice (e.g. CO) in the ice mantle are not taken into account, neither the effect of the organic mantle.


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Mon Sep 16 16:23:29 JST 1996