In situ cryogenic transmission electron microscopy observation on the formation of hydrogen-ordered hexagonal ices and its astrophysical implications

Abstract

ABSTRACT Water ices play a critical role in various astrophysical phenomena and store information about the thermal history of icy grains. To understand this history, clarifying the formation conditions in astrophysical environments is critical. In addition to taking the form of amorphous ices, ice can take four crystalline forms: hexagonal ice I (ice Ih), cubic ice I (ice Ic) and hydrogen-ordered hexagonal ice (ice XIh) and cubic ice (ice XIc). The aim of this article is to study the conditions for phase transitions among these different crystalline ices. We found through cryogenic transmission electron microscopy observations that some polymorphs of ice XIh were formed by simple annealing of ice Ih at 120–150 K without any dopant or irradiation. We also investigated the formation conditions of ice Ih by vapour deposition and heating of ice Ic in protoplanetary discs. The transition kinetics from ice Ih to ice XIh suggested that ice XIh exists between the transition temperature from ice Ic to ice Ih and 150 K. From these results, we constructed a water-vapour flux–temperature–time diagram that clarifies the amorphous, Ic, Ih, XIc and XIh ice phases. We found that, under the conditions encountered in most protostars, hydrogen-ordered ices XIh and XIc can exist just outside the snow line and below 125 K, respectively. The infrared libration modes should enable these phases to be distinguished observationally. Because some of these phases are chiral, their presence could fundamentally affect the evolution of other homochiral organic molecules that develop on their surface.