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Crystallization of amorphous water ice in the solar systemElectron diffraction studies of vapor-deposited water ice have characterized the dynamical structural changes during crystallization that affect volatile retention in cometary materials. Crystallization is found to occur by nucleation of small domains, while leaving a significant part of the amorphous material in a slightly more relaxed amorphous state that coexists metastably with cubic crystalline ice. The onset of the amorphous relaxation is prior to crystallization and coincides with the glass transition. Above the glass transition temperature, the crystallization kinetics are consistent with the amorphous solid becoming a "strong" viscous liquid. The amorphous component can effectively retain volatiles during crystallization if the volatile concentration is approximately 10% or less. For higher initial impurity concentrations, a significant amount of impurities is released during crystallization, probably because the impurities are trapped on the surfaces of micropores. A model for crystallization over long timescales is described that can be applied to a wide range of impure water ices under typical astrophysical conditions if the fragility factor D, which describes the viscosity behavior, can be estimated.
Document ID
20040089722
Acquisition Source
Ames Research Center
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Jenniskens, P.
(NASA Ames Research Center Moffett Field CA United States)
Blake, D. F.
Date Acquired
August 21, 2013
Publication Date
December 20, 1996
Publication Information
Publication: The Astrophysical journal
Volume: 473
Issue: 2 Pt 1
ISSN: 0004-637X
Subject Category
Lunar And Planetary Science And Exploration
Distribution Limits
Public
Copyright
Other
Keywords
NASA Center ARC
NASA Discipline Exobiology

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