Asteroid 65 Cybele: Detection Of Small Silicate Grains, Water-Ice And Organics
Asteroid 65 Cybele: Detection Of Small Silicate Grains, Water-Ice And Organics
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小行星 65 Cybele：检测小硅酸盐颗粒、水冰和有机物
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
J. Emery
中科院分区：
文献类型：
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作者：
Z. Landsman;J. Licandro;H. Campins;M. Kelley;K. Hargrove;N. Pinilla;D. Cruikshank;A. Rivkin;J. Emery
上下文(65)西布莉是位于主带外缘的原始小行星群中最具代表性的成员，西布莉小行星。最近的动力学模型表明，它们中的很大一部分起源于原始的外海王星盘，因此，对这些小行星的物理特性的研究可能是对这些模型的有用测试。目标。我们的目标是获得关于这颗小行星表面成分的信息。特别是，我们希望获得有关风化层的组成和性质以及可能存在的冰和有机物质的信息。方法.本文介绍了用NASA IRTF望远镜和斯皮策空间望远镜分别获得的（65）Cybele的2-4 μm和5-14 μm光谱。并与特洛伊小行星和（24）忒弥斯小行星的光谱进行了比较。我们使用散射模型分析2-4 μm光谱，并将热模型应用于5-14 μm数据。结果（65）Cybele的2-4 μm光谱呈现出一个中心在3.1 μm的吸收带，在3.2-3.6 μm区域有更多较弱的吸收带，与（24）Themis中观察到的吸收带非常相似。未检测到水合硅酸盐。由5-14 μ m区的谱，用NEATM热模型导出有效直径D = 290 ± 5 km，聚束参数η = 0.967 ± 0.014，几何可见光散射pV = 0.05 ± 0.01。在5-14 μm范围内，发射谱在9 - 12 μm处有一个发射平台，光谱对比度为1.5%。这种发射类似于特洛伊小行星和活跃彗星，可能是由于小硅酸盐颗粒嵌入相对透明的基质中，或者是由于非常低密度（童话城堡）的表面结构。西布莉的光谱中硅酸盐发射的幅度比特洛伊小行星的低，这可能是由于更大的尘埃颗粒和/或稍微密集的结构。结论.（65）Cybele表面覆盖着细小的无水硅酸盐颗粒地幔，有少量的水冰和复杂的有机固体。这类似于彗星表面，其中非平衡相共存。水冰和无水硅酸盐的存在表明水合作用没有发生或不完全，表明温度总是足够低。
Context. (65) Cybele is the most representative member of a population of primitive asteroids in the outer edge of the main belt, the Cybele asteroids. Recent dynamical models suggest that a significant fraction of them originated in the primordial transneptunian disk, so the study of the physical properties of these asteroids is potentially a useful test of these models. Aims. Our aim is to obtain information on the surface composition of this asteroid. In particular we want to obtain information on the composition and properties of the regolith and the possible presence of ices and organic materials. Methods. We present 2–4 μm and 5–14 μm spectroscopy of (65) Cybele obtained with the NASA IRTF telescope and Spitzer Space Telescope respectively. We compare the results with spectra of Trojan asteroids and asteroid (24) Themis. We analyze the 2–4 μm spectrum using scattering models and we apply thermal models to the 5–14 μm data. Results. The 2–4 μm spectrum of (65) Cybele presents an absorption band centered at ∼3.1 μm and more weaker bands in the 3.2–3.6 μm region, very similar to those observed in (24) Themis. No hydrated silicates are detected. From the spectrum in the 5–14 μ mr egion an effective diameter D = 290 ± 5 km, a beaming paramete η = 0.967 ± 0.014, and a geometric visible albedo pV = 0.05 ± 0.01 are derived using the NEATM thermal model. The emisivity spectrum in the 5–14 μm range exhibits an emission plateau at about 9 to 12 μm with an spectral contrast of ∼5%. This emission is similar to that of Trojan asteroids and active comets and may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. The lower amplitude of the silicate emission in Cybele’s spectrum with respect to that of Trojan asteroids could be attributed to larger dust particles and/or a slightly denser structure. Conclusions. The surface of (65) Cybele is covered by a fine anhydrous silicate grains mantle, with a small amount of water ice and complex organic solids. This is similar to comet surface where non-equilibrium phases coexist. The presence of water-ice and anhydrous silicates is indicative that hydration did not happened or is incomplete, suggesting that the temperatures were always sufficiently low.