  It has been implicitly assumed that ices on grains in molecular clouds and
proto planetary 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 (Ic) 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 proto-planetary disk is shown. These results have important
implications for the chemical evolution of molecules, non-thermal desorption,
collision of icy grains, and sintering.
