Next: A
key to testing Up: Interstellar-ice
residue model Previous: Interstellar-ice
residue model
The interstellar-ice residue model makes a few assumptions which are too simplified in view of the recent results of experiments on thermal properties of ice and of theoretical and observational development of interstellar chemistry.
One of the assumptions made in this model is simplification of the sublimation
process for ice mixtures. Namely, it is assumed that each of the molecular
species composing the ice sublimes independently according to their vapor
pressures (see eq. (1)).
However, experimental studies have revealed complex behavior of ice sublimation.
Even for pure
amorphous ice, it has been demonstrated (Kouchi, 1987) that the vapor pressure
depends not only on the temperature, but also on the temperature of the
substrate on which the vapor condenses and on the rate of condensation.
For ices composed of two or more molecular species, the sublimation rates
of each molecular species show complicated behavior as a function of temperature
(Bar-Nun et al., 1985, 1987; Schmitt and Klinger, 1987; Grim and
Greenberg, 1987; Laufer et al., 1987; Moore et al., 1988;
Sandford and Allamandola, 1988, Sandford et al., 1988; Schmitt et
al. 1989; Kouchi, 1990). For H
O-CO ice (
) condensed at
K, for example, distinct sublimation of H
O and CO vapors is observed in seven temperature ranges, which are summarized
in Table 3 together with identification of the processes (Kouchi, 1990).
These results have the following important implications for the origin
and evolution of cometary ice. First, these experiments show that CO, for
instance, is retained in
ice up to higher temperatures than the sublimation temperature of pure
CO. This implies that the presence of CO in cometary ice does not necessarily
indicate the formation temperature as low as
to 25 K. Second implication is that the molecular abundance observed in
the coma might not simply equal the composition of the ice in the nucleus.
Namely, there will occur fractionation in the sublimation process,
when the sublimation temperature is lower than that of H
O ice. Although the experimental data have increased, theoretical work
on has hardly been done at all. Theoretical modelling of the sublimation
process of ice mixtures is very much needed as well as further experimentation
to understand the elementary processes of sublimation of multi-component
ice.
The second assumption is on the composition of interstellar ice. This model adopts the interstellar molecule (i.e., gaseous abundances). However, the composition of the ice mantle (i.e., solid composition) is not the same as the gaseous composition in general (e.g., d'Hendecourt et al., 1985). The study of interstellar chemistry including both solid and gaseous phases is very important not only to determine the initial composition of cometary ice but also for clarifying evolutionary connection between interstellar and cometary ices (see papers of interstellar chemistry in this volume).