From interstellar material to comet particles and molecules
From interstellar material to comet particles and molecules
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从星际物质到彗星粒子和分子
DOI：
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发表时间：
2004
期刊：
影响因子：
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通讯作者：
D. Wooden
中科院分区：
文献类型：
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作者：
P. Ehrenfreund;S. Charnley;D. Wooden
恒星、行星和小天体的诞生地是主要由氢和氦气体以及少量固体粒子组成的分子云。来自这样一个星际云的尘埃和气体坍缩形成一个中心凝聚体，形成了太阳，以及周围的圆盘-太阳星云。在这个原行星盘中，无数的亚微米粒子--本质上是冰冷的和难熔的--聚集成更大的星子，随后形成行星。彗星是由未被行星吸收的残留内盘物质形成的。彗星聚集在木星轨道之外，因此可能提供了来自母星际云的原始物质的记录。我们对彗星组成的了解主要是基于当明亮的彗星接近太阳时挥发性物质的蒸发和硅质和碳质尘埃的热发射。对明亮彗星放气曲线的研究提供了一个与在稠密星际云中探测到的冰和气相分子丰度的一般联系。理论模型表明，彗星核中的大块物质在密度、孔隙度和成分上是分层的，并含有共存的冰相，可能是笼形物和被困气体。因此，很明显，彗星核中物质的释气本质上不会模拟星际成分的冰的纯升华。目前缺乏与太阳型恒星形成区有关的天文学数据，而且迄今为止所研究的彗星样本在统计上很小，这是在星际物质和太阳系物质之间建立连贯联系之前需要克服的主要障碍。本章试图汇编目前的知识星际和彗星物质之间的联系，基于星际尘埃和气体的观测，彗星挥发物的观测，模拟实验，和地外物质的分析。
The birthplace of stars, planets, and small bodies are molecular clouds consisting mainly of H and He gas as well as tiny amounts of solid particles. Dust and gas from such an interstellar cloud collapsed to form a central condensation that became the Sun, as well as a surrounding disk — the solar nebula. In this protoplanetary disk innumerable submicrometer particles — icy and refractory in nature — agglomerated to larger planetesimals and subsequently into planets. Comets were formed from remnant inner disk material that was not incorporated into planets. Comets assembled beyond the orbit of Jupiter and may therefore provide a record of some pristine material from the parent interstellar cloud. Our knowledge of the composition of comets is predominantly based on evaporation of volatile species and thermal emission from siliceous and carbonaceous dust when bright comets approach the Sun. The investigation of outgassing curves from bright comets has provided a general link with abundances of ices and gas phase molecules detected in dense interstellar clouds. Theoretical models indicate that bulk material in cometary nuclei is stratified in density, porosity, and composition and contains coexisting ice phases, possibly clathrates and trapped gases. It is therefore apparent that the outgassing of species from cometary nuclei will not essentially mimic that expected from pure sublimation of ices of interstellar composition. The current lack of astronomical data related to solar-type star-forming regions, and the statistically small sample of comets studied to date, are the major obstacles to be overcome before a coherent link between interstellar and solar system material can be established. This chapter attempts to compile the current knowledge on the connection between interstellar and cometary material, based on observations of interstellar dust and gas, observations of cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter.