DESPITE current interest1-8 in the effects of ultraviolet radiation and high-energy particles on interstellar ice and on solid mixtures of H2O and other molecules, most attention has so far been paid to the initiation of chemical reactions, although it has been recognized that electron-beam bombardment will convert ice crystals to the amorphous form7,8. Accordingly, we have studied the effect of ultraviolet radiation on ice, using electron diffraction to follow the structural changes that take place. We find that, below 70 K, ultraviolet irradiation transforms cubic ice (ice Ic) into amorphous ice (a-H2O). This is a novel means of forming a-H2O, supplementing previously known routes8-14. We infer that other high-energy rays or particles will amorphize ice crystals below 70 K. We conclude that crystalline ice in stellar atmospheres and on asteroids and planetary satellites will be rendered amorphous by ultraviolet radiation in times short compared with the rimescale of stellar evolution.