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Title Previous: Alteration
Processes of Cometary
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
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
(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
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
O ice is expected from the interstellar-ice residue model. For simplicity,
volatile molecules other than H
O ice (e.g. CO) in the ice mantle are not taken into account, neither the
effect of the organic mantle.