1994A&A...290.1009K

Conditions for formation and preservation of amorphous ice formed through condensation of water vapor on a substrate is investigated both theoretically and experimentally. The kinetic consideration of deposition of vapor leads to the condition for formation of amorphous ice on a cold substrate: the flux of water vapor onto the substrate should be larger than a critical flux D_s/a⁴=F_c, where D_s is the surface diffusion coefficient of water molecules on the substrate and a the lattice constant of the substrate. The validity of the derived condition has been confirmed by measuring the critical flux of water vapor onto a substrate of polycrystalline cubic ice as a function of temperature. From the measured F_c, a surface diffusion coefficient of H₂O molecules on polycrystalline cubic ice has been determined. With regard to the preservation condition, we derive a theoretical expression of the time scale t_c for crystallization of amorphous ice, which has the same form at low temperatures as the empirical formula used so far. A self-diffusion coefficient of amorphous ice is estimated from a comparison between the theoretical and empirical formulas of t_c. The results are applied to condensation of ice in molecular clouds, circumstellar envelopes of late-type stars, and the primordial solar nebula. Discussion is given on the crystallinity of ices in these sites.