Composition of Comets: Observations and Models
Composition of Comets: Observations and Models
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彗星的组成：观测和模型
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发表时间：
2002
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通讯作者：
W. Huebner
中科院分区：
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作者：
W. Huebner
根据对Hale-Bopp彗星(C/1995O1)和Hyakutake彗星(C/1996B2)的现场观测和对Hale-Bopp彗星(C/1995O1)的遥感观测，结合太阳系的元素丰度，分析了彗星的化学成分和丰度。相对于太阳系，彗星中的氮含量较低，因为氮往往存在于化学上相对惰性的氮气中。虽然许多细节仍不确定，但一些粗略的特征正在显现。水：硅酸盐：碳质分子(CO、CO2和碳氢化合物)的质量丰度约为1：1：1。此外，冰：尘埃(硅酸盐和碳氢化合物缩合物)的质量丰度约为1：1。我们将已识别的彗星分子列表与星际介质中检测到的分子进行比较，尽管与它们的相对丰度进行比较，特别是在冰相，会更有意义。然而，目前还没有冰相丰度的数据。人们可以预计，彗星中含碳分子丰度的变化与它们在太阳星云中的发源地有关。然而，我们也注意到彗星是异质的。因此，观察到的差异可能与起源地、核的异质性有关，或者是通过进化获得的。彗星的分子和元素组成很可能与原子核中的不同。对于挥发性冰及其气体，以及尘冰比和尘气比而言，情况尤其如此。分析必须仔细考虑气体的三个来源：来自原子核表面的水，来自原子核内部的比水更易挥发的气体，以及来自分布在彗星中的尘埃升华的气体。原子核表面的地形可能会导致尘气质量比的重要演化差异。原子核表面相对不活跃的区域可能与凸起的地形有关。从凸起区域(丘陵和山脉)升华的气体比从凹陷区域升华的气体发散得更强烈，这可以更有效地携带尘埃。因此，来自凸起区域的尘埃夹带很少，尘埃可能会回落到原子核的表面，形成尘埃地幔，这进一步抑制了放出气体。
We analyze the chemical composition and abundances of comets based on in situ measurements of Comet 1P/Halley and remote sensing observations of several recent bright comets including Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2), in light of the elemental abundances of the solar system. Nitrogen is underabundant in comets relative to the solar system because nitrogen tends to be in N2, which is chemically relatively inert. While many details remain uncertain, some gross features are emerging. The abundance of water : silicates: carbonaceous molecules (CO, CO2, and hydrocarbons) by mass is approximately 1 : 1 : 1. Furthermore, the mass abundance of ice : dust (silicates and hydrocarbon polycondensates) is about1 : 1. We compare a list of identified comet molecules with molecules detected in the interstellar medium, although a comparison with their relative abundances, particularly in the ice phase, would be more meaningful. However, ice-phase abundances are not yet available. One can expect a variation of the abundances of carbon-bearing molecules in comets to be associated with their place of origin in the solar nebula. However, we also note that comets are heterogeneous. Thus, observed differences may be related to the place of origin, heterogeneity of the nucleus, or acquired through evolution. The molecular and elemental compositions of the coma are most likely not the same as those in the nucleus. This is particularly true for volatile ices and their gases and for the dust-to-ice and dust-to-gas ratios. Analyses must carefully consider the three sources of gas: Water from the surface of the nucleus, gases more volatile than water from the interior of the nucleus, and gases from the sublimation of the dust distributed in the coma. Topography on the surface of the nucleus may cause important evolutionary differences in the dust-to-gas mass ratio. Relatively inactive areas on the surface of the nucleus are probably associated with convex topography. Gas sublimated from convex areas (hills and mountains) diverges more strongly relative to gas sublimated from concave areas, which can entrain dust more efficiently. Thus, the entrainment of dust from convex areas is poor and dust may fall back to the surface of the nucleus creating a dust mantle, which further inhibits outgassing.