Conditions for condensation and preservation of amorphous ice and crystallinity of astrophysical ices
Abstract
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_/a^4^=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 H_2_O 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- October 1994
- Bibcode:
- 1994A&A...290.1009K
- Keywords:
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- MOLECULAR PROCESSES;
- SOLAR SYSTEM: FORMATION;
- STARS: CIRCUMSTELLAR MATTER;
- INTERSTELLAR MEDIUM: CLOUDS