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It is widely believed that comets are one of the most pristine objects in the solar system, preserving the matter of the early solar system and even presolar interstellar matter that had been in the parent interstellar cloud of the solar system. Figure 1 illustrates evolution of interstellar dust from its birth to formation of the solar system.
Interstellar dust is formed in atmospheres of low temperature stars such as redgiants, and in the ejecta of supernova and nova explosions. The dust formed there is ejected into interstellar space, and trapped in interstellar clouds after travelling for many years. Planetary systems including our solar system are formed from interstellar molecular clouds. Interstellar dust may be preserved as cometary materials since they have been preserved in low temperature and pressure environment up to the present time, in contrast with other solar-system bodies like planets. In this sense comets are a terminal of interstellar dust evolution, and cometary nuclei may be a reservoir of interstellar dust. In contrary with dust in interstellar space, cometary materials are accessible by spacecraft, and it is hoped that we can get their samples and can analyze them in laboratories in near future.
Comets may act as a probe for the study of the origin and evolution of the solar system. Among the relatively pristine objects in the solar system, comets are particularly remarkable in that they are composed of abundant volatiles, i.e., ices consisting mainly of hydrogen, carbon, nitrogen, oxygen, and sulfur. The study of cometary matter will provide us clues that reveal not only the origin of comets but also the physical conditions of the low-temperature region in the primordial solar nebula and the evolution of matter at the stages from interstellar clouds to planetary systems.
This article concentrates on the origin and thermal history of ices in a cometary nucleus. Ices are sensitive to the temperature and radiation environment where they have been placed, and thus act as an excellent probe for studying the origin and evolution of cometary matter and primordial materials in the solar system. For comprehensive recent reviews for the origin of comets and its relevance to the solar system formation, see Donn (1991), Yamamoto (1991), Mumma et al. (1993), Levasseur-Regourd (1992), and Mumma (in this volume).