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.