CRYSTALLIZATION OF AMORPHOUS WATER ICE IN THE SOLAR-SYSTEM

Electron diffraction studies of vapor-deposited water ice have charact erized the dynamical structural changes during crystallization that af fect volatile retention in cometary materials. Crystallization is foun d to occur by nucleation of small domains, while leaving a significant part of the amorphous material in a slightly more relaxed amorphous s tate that coexists metastably with cubic crystalline ice. The onset of the amorphous relaxation is prior to crystallization and coincides wi th the glass transition. Above the glass transition temperature, the c rystallization kinetics are consistent with the amorphous solid becomi ng a ''strong'' viscous liquid. The amorphous component can effectivel y retain volatiles during crystallization if the volatile concentratio n is similar to 10% or less. For higher initial impurity concentration s, a significant amount of impurities is released during crystallizati on, probably because the impurities are trapped on the surfaces of mic ropores. A model for crystallization over long timescales is described that can be applied to a wide range of impure water ices under typica l astrophysical conditions if the fragility factor D, which describes the viscosity behavior, can be estimated.