**3.1 CO**

α-CO is a thermodynamically stable phase of solid CO at low temperatures [31]. We observed the morphology of CO deposited on a-H2O by TEM, as shown in **Figure 3** [5]. The CO formed three-dimensional polyhedral crystals, and the diffraction pattern confirmed that the CO crystals were α-CO. It has long been debated whether the crystal structure of α-CO is an orientationally ordered phase (space group: *P*213) or a disordered one (space group: *Pa*3). Wang et al. [21] confirmed by neutron diffraction that α-CO belongs to *P*213, as proposed by Vegard [32]. Therefore, we concluded that α-CO in molecular clouds would be chiral.
