Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994sci...265..753j&link_type=abstract
Science, Volume 265, Issue 5173, pp. 753-756
Astronomy and Astrophysics
Astrophysics
170
Scientific paper
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.
Blake David F.
Jenniskens Peter
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