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Theories on the Origin of Cometary Ice

  Two types of theories on the origin of cometary ice have been proposed, which are distinguished by the formation place of cometary ice. One is the formation in the primordial solar nebula and the Jovian subnebulae, and the other is formation in an interstellar molecular cloud from which the solar nebula formed. The first type considers quenching of solar nebula gas in its cooling (Prinn and Fegley, 1989; Fegley and Prinn, 1989), formation of clathrate hydrate (Lunine, 1989), and radial transport of grains in the solar nebula (Engel et al., 1990). Here we concentrate on the quenching model. This theory asserts that cometary ice is a mixture of ice condensed from the cooling solar nebula gas whose composition was quenched at a high temperature and ice condensed in the Jovian subnebula. The theory of the second type asserts that cometary ice had originally condensed in the interstellar cloud from which the solar nebula was formed, and lost very volatile components by sublimation at the subsequent solar nebula stage (Yamamoto et al., 1983; Yamamoto, 1985a, b). Namely cometary ice is regarded as sublimation residue of interstellar ice. Let us call this model interstellar-ice residue model.

A main difference in the two theories may be summarized as: Are cometary volatiles (1) condensates in the solar nebula, or (2) a sublimation residue of interstellar ices?




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Mon Sep 16 16:23:29 JST 1996