STRUCTURAL TRANSITIONS IN AMORPHOUS WATER ICE AND ASTROPHYSICAL IMPLICATIONS

Selected area electron diffraction is used to monitor structural chang es of vapor-deposited water ice in vacuum during warm-up from 15 to 18 8 K. A progression of three amorphous forms of water ice is found with well-defined transitions. The formation of a high-density amorphous f orm (I(a)h) at 15 K is confirmed, and the transition to the more famil iar low-density form (I(a)l) occurs gradually over the range 38 to 68 K. At 131 K, the ice transforms into a third amorphous form (I(a)r), w hich precedes the crystallization of cubic ice (I-c) and coexists meta stably with I-c from 148 K until at least 188 K. These structural tran sformations of amorphous water ice can be used to explain hitherto ano malous properties of astrophysical ices. The structural transition fro m I(a)h to I(a)l is responsible for the diffusion and recombination of radicals in ultraviolet-photolyzed interstellar ices at low temperatu res. The occurrence and persistence of I(a)r explains anomalous gas re tention and gas release from water-rich ices at temperatures above 150 K.