Measurement of Diamond Nucleation Rates from Hydrocarbons at Conditions Comparable to the Interiors of Icy Giant Planets

We present measurements of the nucleation rate into a diamond lattice in dynamically compressed polystyrene obtained in a pump-probe experiment using a high energy laser system and in situ femtosecond X-ray diffraction. Different temperature-pressure conditions that occur in planetary interiors were probed. For a single shock reaching 70GPa and 3000K no diamond formation was observed while with a double shock driving polystyrene to pressures around 150GPa and temperatures around 5000K nucleation rates between 1029 m-3s-1 and 1034 m-3s-1 were recorded. These nucleation rates do not a agree with predictions of recent theoretical models for carbon-hydrogen mixtures by many orders of magnitude. Our data suggests that there is indeed significant diamond formation to be expected inside icy giant planets like Neptune and Uranus.