Amorphous Mixtures Of Ice And C-60 Fullerene

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