Keywords:Amorphous ices, Crystallization, Polyhedron
Collision and following sticking or disruption of grains and grain aggregates are one of the most important elementary process to discuss the formation mechanism of planetesimals. It has been hitherto assumed that grains are sphere regardless of materials (silicates, organics, ices) and crystallinity (amorphous, crystalline). To check this assumption, we observed crystallization of amorphous ices (a-H2O, a-CO2, a-CO) using transmission electron microscope. We found that uniform thin films of amorphous ices (a-H2O, a-CO) changed to 3-dimensional islands due to crystallization. Furthermore, number of crystalline islands decreased and the size of islands increased due to Ostwald reipening. The structure of icy grains composed of crystalline ice and core materials became heterogeneous structure; one or two crystalline ices (polyhedron with planer faces) attached to spherical core. In the case of CO2, on the other hand, uniform thin films of a-CO2did not change after crystallization. As shown here, it is found that morphology of crystalline ices depends greatly on the composition of ices. Therefore, care should be taken for the application of models on collision-sticking or sintering. Even in low temperature molecular clouds, morphology of CO ice is not uniform amorphous mantle but heterogeneous crystalline polyhedron, because a-CO crystallized within short time, suggesting non-thermal desorption of various molecules occurs simultaneously.