Transmission Electron Microscopy Study of the Morphology of Ices Composed of H2O, CO2, and CO on Refractory Grains
Transmission Electron Microscopy Study of the Morphology of Ices Composed of H2O, CO2, and CO on Refractory Grains
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DOI：
10.3847/1538-4357/ac0ae6
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发表时间：
2021-09-01
影响因子：
4.9
通讯作者：
Watanabe, Naoki
中科院分区：
文献类型：
--
作者：
Kouchi, Akira;Tsuge, Masashi;Watanabe, Naoki
人们隐含地假设，分子云和原行星盘中颗粒上的冰是由均一层形成的，无论它们的组成或结晶度如何。为了验证这一假设，我们用超高真空电子显微镜观察了H2O在耐火衬底上的沉积和无定形冰(H2O、CO2和CO)的结晶。在H2O沉积实验中，我们发现在130K以上形成了晶态冰岛(I-c)。晶化实验表明，均匀的非晶态CO和H2O薄膜形成了三维多面体晶体孤岛，而非晶态CO2则成为覆盖在非晶态H2O上的纳米晶态CO2薄膜。我们的观察表明，晶体的形态不仅强烈地依赖于冰的组成，而且还取决于衬底。利用沉积冰的结晶度和无定形冰的结晶时间标度的实验数据，阐明了空间冰结晶度的判据，并概述了分子云中冰颗粒的宏观形态：无定形H2O均匀地覆盖在耐火颗粒上，CO2纳米晶体嵌入无定形H2O中，多面体CO晶体附着在无定形H2O上。此外，还显示了原行星盘中颗粒形态的变化。这些结果对分子的化学演化、非热解吸、冰粒碰撞和烧结都有重要的意义。
It has been implicitly assumed that ices on grains in molecular clouds and protoplanetary disks are formed by homogeneous layers regardless of their composition or crystallinity. To verify this assumption, we observed the H2O deposition onto refractory substrates and the crystallization of amorphous ices (H2O, CO2, and CO) using an ultra-high-vacuum transmission electron microscope. In the H2O-deposition experiments, we found that three-dimensional islands of crystalline ice (I-c) were formed at temperatures above 130 K. The crystallization experiments showed that uniform thin films of amorphous CO and H2O became three-dimensional islands of polyhedral crystals; amorphous CO2, on the other hand, became a thin film of nano-crystalline CO2 covering the amorphous H2O. Our observations show that crystal morphologies strongly depend not only on the ice composition but also on the substrate. Using experimental data concerning the crystallinity of deposited ices and the crystallization timescale of amorphous ices, we illustrated the criteria for ice crystallinity in space and outlined the macroscopic morphology of icy grains in molecular clouds as follows: amorphous H2O covered the refractory grain uniformly, CO2 nano-crystals were embedded in the amorphous H2O, and a polyhedral CO crystal was attached to the amorphous H2O. Furthermore, a change in the grain morphology in a protoplanetary disk is shown. These results have important implications for the chemical evolution of molecules, nonthermal desorption, collision of icy grains, and sintering.