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A Brief History of the Solar System

  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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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).
  6. 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.
  7. 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.


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Next: Solar-System Abundance of Elements Up: No Title Previous: Introduction

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Mon Sep 16 16:23:29 JST 1996