Amorphous Mixtures of Ice and C60 Fullerene

Carbon and ice make up a substantial proportion of our Universe. Recent space exploration has shown that these two chemical species often coexist including on comets, asteroids and in the interstellar medium. Here we prepare mixtures of C60 fullerene and H2O by vapor co-deposition at 90 K with molar C60:H2O ratios ranging from 1:1254 to 1:5. The C60 percolation threshold is found between the 1:132 and 1:48 samples, corresponding to a transition from matrix-isolated C60 molecules to percolating C60 domains that confine the H2O. Below this threshold, the crystallization and thermal desorption properties of H2O are not significantly affected by the C60, whereas the crystallization temperature of H2O is shifted towards higher temperatures for the C60-rich samples. These C60-rich samples also display exotherms corresponding to the crystallization of C60 as the two components undergo phase separation. More than 60 volume percent C60 is required to significantly affect the desorption properties of H2O. A thick blanket of C60 on top of pure amorphous ice is found to display large cracks due to water desorption. These findings may help understand the recently observed unusual surface features and the H2O weather cycle on the 67P/Churyumov–Gerasimenko comet.