Transmission Electron Microscopy Study of the Morphology of Ices Composed of H<sub>2</sub>O, CO<sub>2</sub>, and CO on Refractory Grains
Akira Kouchi, Masashi Tsuge, Tetsuya Hama, Yasuhiro Oba, Satoshi Okuzumi, Sin-iti Sirono, Munetake Momose, Naoki Nakatani, Kenji Furuya, Takashi Shimonishi, Tomoya Yamazaki, Hiroshi Hidaka, Yuki Kimura, Ken-ichiro Murata, Kazuyuki Fujita, Shunichi Nakatsubo, Shogo Tachibana, Naoki Watanabe
Abstract
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 H 2 O deposition onto refractory substrates and the crystallization of amorphous ices (H 2 O, CO 2 , and CO) using an ultra-high-vacuum transmission electron microscope. In the H 2 O-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 H 2 O became three-dimensional islands of polyhedral crystals; amorphous CO 2 , on the other hand, became a thin film of nano-crystalline CO 2 covering the amorphous H 2 O. 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 H 2 O covered the refractory grain uniformly, CO 2 nano-crystals were embedded in the amorphous H 2 O, and a polyhedral CO crystal was attached to the amorphous H 2 O. 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.