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Transmission Electron Microscopy Study of the Morphology of Ices Composed of H2O, CO2, and CO on Refractory Grains

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Title: Transmission Electron Microscopy Study of the Morphology of Ices Composed of H2O, CO2, and CO on Refractory Grains
Authors: Kouchi, Akira Browse this author →KAKEN DB
Tsuge, Masashi Browse this author
Hama, Tetsuya Browse this author
Oba, Yasuhiro Browse this author
Okuzumi, Satoshi Browse this author
Sirono, Sin-iti Browse this author
Momose, Munetake Browse this author
Nakatani, Naoki Browse this author
Furuya, Kenji Browse this author
Shimonishi, Takashi Browse this author
Yamazaki, Tomoya Browse this author
Hidaka, Hiroshi Browse this author →KAKEN DB
Kimura, Yuki Browse this author →KAKEN DB
Murata, Ken-ichiro Browse this author →KAKEN DB
Fujita, Kazuyuki Browse this author
Nakatsubo, Shunichi Browse this author
Tachibana, Shogo Browse this author
Watanabe, Naoki Browse this author →KAKEN DB
Issue Date: Sep-2021
Publisher: IOP Publishing
Journal Title: Astrophysical journal
Volume: 918
Issue: 2
Start Page: 45
Publisher DOI: 10.3847/1538-4357/ac0ae6
Abstract: 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.
Type: article
Appears in Collections:低温科学研究所 (Institute of Low Temperature Science) > 雑誌発表論文等 (Peer-reviewed Journal Articles, etc)

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