**Chapter 7** Chiral Ice Crystals in Space

*Akira Kouchi,Takashi Shimonishi,Tomoya Yamazaki, Masashi Tsuge, Naoki Nakatani, Kenji Furuya, Hiromasa Niinomi, Yasuhiro Oba,Tetsuya Hama, Hiroyasu Katsuno, Naoki Watanabe and Yuki Kimura*

## **Abstract**

We observed the formation of CO, CH3OH, and H2O ices using a cryogenic transmission electron microscope, to determine if chiral ice crystals could form under the conditions of interstellar molecular clouds and young stellar objects (protoplanetary disks) and to clarify the crystalline structure of these ices. Our results suggest that the following ice crystals are chiral: crystalline CO (α-CO) formed on amorphous H2O (a-H2O) grains in a 10-K molecular cloud, crystalline CH3OH formed by the heating of amorphous CH3OH on a-H2O grains at 40–60 K in young stellar objects, and several polymorphs of hydrogen-ordered cubic ice crystals formed by the heating of a-H2O at 80–100 K and direct condensation at 120–140 K in protoplanetary disks. We also investigated candidates for other chiral ices using published data. We found that NH3 I and NH3H2O I are chiral at low temperature and pressure conditions. If one-handed circularly polarized light is irradiated during the nucleation of these chiral ice crystals, homochiral crystals can be formed. These results have important implications for the origin of interstellar organic molecule homochirality.

**Keywords:** ice crystals, chirality, CO, H2O, CH3OH, NH3, interstellar molecular cloud, protoplanetary disk, circularly polarized light, asymmetric nucleation

#### **1. Introduction**

The origin of biomolecular homochirality is one of the most important mysteries of the origin of life. However, asymmetric adsorption and/or asymmetric synthesis on inorganic crystal surfaces is a possible candidate for chiral selection [1–3]. Quartz and cinnabar are regarded as chiral crystals, and it has been suggested that the surfaces of achiral crystals (e.g., gypsum, calcite, and alkali feldspar) can act as chiral faces [2, 4]. These minerals, as both chiral and achiral crystals, could be formed in evolved bodies, such as meteoritic parent bodies and terrestrial planets; however, it is implicitly considered that there were/are no chiral crystals in interstellar grains. Using transmission electron microscopy (TEM), we demonstrated that chiral crystalline CO (α-CO) would form on icy grains in interstellar molecular clouds [5]; therefore, α-CO in molecular clouds could be regarded as the first chiral crystal in space. To build on this finding, we searched for other chiral ices in space via further laboratory

experiments and literature searches. We used the term "ice" to describe a solid at low temperatures (e.g., H2O, CO2, CO, NH3, CH3OH, and their hydrates).

In this chapter, after a brief explanation of icy grains in space in Section 2, we describe the crystal structures of each chiral ice in Section 3. In Section 4, a formation mechanism for homochiral ice crystals in space is discussed. In Section 5, we suggest further areas of study for the determination of the origin of the homochirality of organic molecules on icy grains in space.
