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on the Origin
This model is based on chemical equilibrium calculations of gaseous molecular composition in a gas of the solar-system elemental composition with taking account of `quenching' (Lewis and Prinn, 1980), which is explained below.
In this model, it is assumed that the solar nebula was initially hot
(
K). The gaseous composition at this temperature is mainly H
O, CO, and N
, plus H
and He, of which the latter two are irrelevant to the condensation. Note
that the nebular gas is of oxidized composition at high temperatures. In
the course of the nebular cooling the gaseous composition is fixed at a
certain temperature called a quenching temperature
, and is maintained at temperatures lower than
. This is because the time scale for achieving chemical equilibrium increases
very rapidly as the temperature gets lower, and becomes much longer than
the nebular dynamical time scale (Lewis and Prinn, 1980). ``Freezing"
of the gaseous composition below the quenching temperature
is the point of this model. Condensation of ices occurs in the quenched
gas. The condensation process itself is treated in an equilibrium manner
in which kinetics of condensation such as nucleation and grain growth,
and resultant supercooling (Yamamoto and Hasegawa, 1977; Draine and Salpeter,
1977; Kozasa and Hasegawa, 1987) are not taken into account.
Figure 5 illustrates the quenching for gaseous carbon compounds, showing
CO/CH
abundance ratio as a function of temperature T and total pressure
P, which is nearly equal to the partial pressure of H
, the main component of the nebular gas.
It is seen that, if chemical equilibrium was achieved at all temperatures,
CO is a dominant gaseous carbon species at high T and low P,
and CH
is a dominant species at low T and high P. Actually, however,
the gaseous composition is ``frozen" at
in the cooling, and as a result CO dominates CH
even at low temperatures under the solar nebula conditions that Fegley
and Prinn (1989) supposed. On the other hand, CH
dominates CO at low temperatures under the conditions of their Jovian subnebula,
since the pressure of their Jovian subnebula is much higher than that of
the solar nebula. A similar situation holds for nitrogen compounds. Namely,
in the solar nebula, and
in the Jovian subnebula. Note that the oxidized CO, N
-rich gaseous composition is realized in the solar nebula, and the reduced
CH
, NH
-rich composition is realized in the Jovian subnebula.
In this model, cometary ice is regarded as a mixture of ices condensed in the solar nebula and the Jovian subnebula. The mixing mechanisms that Prinn and Fegley (1989) suggest are (i) sweep-up of the gas of the reduced composition in the Jovian subnebulae by proto-cometary objects of the oxidized composition formed in the solar nebula, or vice versa, and (ii) partial mixing of the subnebula gas with the solar nebula gas. It is not clear whether or not these mechanisms are efficient enough to be able to produce the estimated total mass of comets.
We point out an important consequence deduced from the quenching model,
which is distinguished from that deduced from the interstellar-ice residue
model described later. That is a condensation temperature of H
O ice. In this model H
O ice condenses at
K under the solar nebula conditions of the pressure of
to
bar (Lewis and Prinn, 1980), and at higher temperatures under the Jovian
subnebula conditions because of higher pressure.