↵† Present address: Centre de Recherches Pétrographiques et Géochimiques, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre-lès-Nancy, France. Interstellar ice is believed to be a cradle of complex organic compounds, commonly found within icy comets and interstellar clouds, in association with ultraviolet (UV) irradiation and subsequent warming. We found that UV-irradiated amorphous ices composed of H2O, CH3OH, and NH3 and of pure H2O behave like liquids over the temperature ranges of 65 to 150 kelvin and 50 to 140 kelvin, respectively. This low-viscosity liquid-like ice may enhance the formation of organic compounds including prebiotic molecules and the accretion of icy dust to form icy planetesimals under certain interstellar conditions. Water-dominated ice is the most abundant solid component of dense molecular clouds and the outer cold part of protoplanetary disks (1) and must have played a critical role in the formation of gas and icy giants in the early solar system as a dominant component of their building blocks. Remnants of icy planetesimals (small bodies formed before planets in the early solar system) are now known as comets. Complex organic materials, including prebiot...