Structural Transitions in Amorphous Water Ice and Astrophysical Implications
Abstract
Selected area electron diffraction is used to monitor structural changes of vapor-deposited water ice in vacuum during warm-up from 15 to 188 K. A progression of three amorphous forms of water ice is found with well-defined transitions. The formation of a high-density amorphous form (I_ah) at 15 K is confirmed, and the transition to the more familiar low-density form (I_aI) occurs gradually over the range 38 to 68 K. At 131 K, the ice transforms into a third amorphous form (I_ar), which precedes the crystallization of cubic ice (I_c) and coexists metastably with I_c from 148 K until at least 188 K. These structural transformations of amorphous water ice can be used to explain hitherto anomalous properties of astrophysical ices. The structural transition from I_ah to I_aI is responsible for the diffusion and recombination of radicals in ultraviolet-photolyzed interstellar ices at low temperatures. The occurrence and persistence of I_ar explains anomalous gas retention and gas release from water-rich ices at temperatures above 150 K.
- Publication:
-
Science
- Pub Date:
- August 1994
- DOI:
- 10.1126/science.11539186
- Bibcode:
- 1994Sci...265..753J