To provide background knowledge for the origin
of cometary matter, I shall briefly summarize processes of formation of
the solar system relevant to formation of cometary materials. Although
the present theories of solar-system formation have provided a picture
of zeroth order approximation, many problems have remained unsolved such
as the time scale of growth of outer planets and the turbulent state of
the primordial solar nebula. Readers can find comprehensive reviews of
the present status of the theories in the books Protostars and Planets
II (1985) and III (1993). A scenario given below should be regarded as
a rough sketch of the solar-system history. Rather the study of cometary
and other primordial materials of the solar system is hoped to constrain
the various theories on the origin of the solar system, and this is one
of the reasons to study cometary materials as stated in the previous section.
Figure 2 illustrates the scenario of formation of the solar system.
Gravitational collapse and fragmentation of an interstellar cloud:
The first stage is gravitational collapse and fragmentation of an interstellar
molecular cloud. Gaseous molecules in the cloud condense, adsorb, and react
on grain surfaces to form ice mantles on the surfaces. Irradiation of UV
induced by ionization of hydrogen molecules by cosmic rays penetrating
into the cloud and by irradiation of cosmic ray particle itself alter the
ices at this stage (see also Fig. 1). The grains in the cloud may
be characterized by those proposed by Greenberg (1982). Since the molecular
cloud is generally inhomogeneous, the dense parts collapse faster and fragment
into many subclouds. In general each subcloud rotates around a certain
axis because of turbulent motion of the gas in the parent cloud, so that
the subcloud contracts along the axis resulting in the formation of a rotating
flattened disk composed of gas and dust. One of the subclouds is the parent
cloud of our primordial solar nebula.
Formation of the primordial solar nebula: At the formation
of the primordial solar nebula, the nebular gas was heated by shock wave
induced by accretion of the gas. As a result, the inner region of the solar
nebula would have become so hot that the grains in the parent subcloud
would have sublimed, since high gravitational energy was released in the
inner region. Subsequently grains recondensed as the solar nebula cooled
by thermal emission form the nebular surfaces. It should be noted that
the newly condensed grains have different structure and composition in
particular for volatiles compared with those in the parent subcloud because
of different chemistry. Namely, the chemistry in the solar nebular is mainly
thermal chemistry, whereas the chemistry in molecular clouds is nonthermal
one and is far from thermal equilibrium (see §5).
In the outer region of the solar nebula, on the other hand, the temperature
would not have become high enough for the grains in the parent subcloud
to sublime, and the grains would have survived and suffered only mild thermal
processing. The degree of the thermal processing depends on the distance
from the center of the nebula.
Growth and sedimentation of dust toward the midplane of the solar
nebula: After the end of accretion of the gas onto the solar nebula,
the grains begins to settle toward the midplane of the solar nebula with
revolving around the protosun. Because of the difference in the sedimentation
velocity due to grain's size (mass) difference, the grains collide and
stick with each other. Grains grow up to mm size at this stage.
Formation of the dust layer: As a result of the sedimentation
of grains, a dust layer is formed in the midplane of the solar nebula.
It should be pointed out, however, that the sedimentation might be disturbed
in the boundary layer near the midplane because of turbulence of the nebular
gas resulting from the difference in the Keplerian velocities of the gas
in the nebula and grains in the midplane (Weidenshilling, 1980, 1984; Weidenshilling
and Cuzzi, 1993). If this actually occurs, grains grow by mutual collisions
with relative velocity on the order of turbulent velocity. In this case,
grains may suffer alteration at collision because the turbulent velocity
is much larger than the difference in the sedimentation velocity. Alteration
at grain-grain collisions for various velocities is discussed by Donn (1991).
When grains grow to the size larger than a few times ten meter, gas drag
due to turbulence becomes ineffective, and these bodies finally settle
to the midplane as in the non-turbulent case.
Gravitational fragmentation of the dust layer - Formation of planetesimals:
When the density in the midplane reaches a critical value, the dust layer
becomes gravitationally unstable, and fragments into many pieces, which
are called planetesimals. It is highly probable that cometary nuclei are
icy planetesimals formed in the outer region of the solar nebula without
accretion to the planets (Greenberg et al., 1984; Yamamoto and Kozasa,
1988). Kuiper-belt objects recently discovered (Jewitt and Luu, 1992; see
also Cruikshank et al., in this volume) may be remnant planetesimals
survived up to the present time (Yamamoto et al., 1994).
Growth of planetesimals to planets: Planetesimals accrete to
planets by collisional growth due to mutual gravitational scattering. Dynamical
processes of the growth have been studied extensively (see Protostars
and Planets III, 1993). In the growth from planetesimals to planets,
primitive materials in the solar nebula suffer alteration due to heating
at accretion and high pressure in the interior of the planets.
Dissipation of the nebular gas: At a stage of planetary growth
the nebular gas is dissipated by solar radiation energy. It has been shown
that the amount of UV energy radiated from the protosun during the T Tauri
phase is sufficient to dissipate the nebular gas (e.g. Hayashi et
al. 1985), though the detailed dynamics of the nebular dissipation has
not yet been clarified. Furthermore it is not clear whether the time of
the T Tauri phase of the sun coincides with the final stage of the planetary
growth, since the evolutionary ``clocks" of the sun and solar nebula
have not been adjusted in the present theories of formation of the solar
system.