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- ItemMeasurement of diamond nucleation rates from hydrocarbons at conditions comparable to the interiors of icy giant planets(Woodbury, NY : Inst., 2020) Schuster, A.K.; Hartley, N.J.; Vorberger, J.; Döppner, T.; Van Driel, T.; Falcone, R.W.; Fletcher, L.B.; Frydrych, S.; Galtier, E.; Gamboa, E.J.; Gericke, D.O.; Glenzer, S.H.; Granados, E.; MacDonald, M.J.; MacKinnon, A.J.; McBride, E.E.; Nam, I.; Neumayer, P.; Pak, A.; Prencipe, I.; Voigt, K.; Saunders, A.M.; Sun, P.; Kraus, D.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 70 GPa and 3000 K no diamond formation was observed, while with a double shock driving polystyrene to pressures around 150 GPa and temperatures around 5000 K nucleation rates between 1029 and 1034m-3 s-1 were recorded. These nucleation rates do not agree with predictions of the state-of-the-art theoretical models for carbon-hydrogen mixtures by many orders of magnitude. Our data suggest that there is significant diamond formation to be expected inside icy giant planets like Neptune and Uranus. © 2020 authors. Published by the American Physical Society.
- ItemDirect Observation of Shock-Induced Disordering of Enstatite Below the Melting Temperature(Hoboken, NJ [u.a.] : Wiley, 2020) Hernandez, J.-A.; Morard, G.; Guarguaglini, M.; Alonso-Mori, R.; Benuzzi-Mounaix, A.; Bolis, R.; Fiquet, G.; Galtier, E.; Gleason, A.E.; Glenzer, S.; Guyot, F.; Ko, B.; Lee, H.J.; Mao, W.L.; Nagler, B.; Ozaki, N.; Schuster, A.K.; Shim, S.H.; Vinci, T.; Ravasio, A.We report in situ structural measurements of shock-compressed single crystal orthoenstatite up to 337 ± 55 GPa on the Hugoniot, obtained by coupling ultrafast X-ray diffraction to laser-driven shock compression. Shock compression induces a disordering of the crystalline structure evidenced by the appearance of a diffuse X-ray diffraction signal at nanosecond timescales at 80 ± 13 GPa on the Hugoniot, well below the equilibrium melting pressure (>170 GPa). The formation of bridgmanite and post-perovskite have been indirectly reported in microsecond-scale plate-impact experiments. Therefore, we interpret the high-pressure disordered state we observed at nanosecond scale as an intermediate structure from which bridgmanite and post-perovskite crystallize at longer timescales. This evidence of a disordered structure of MgSiO3 on the Hugoniot indicates that the degree of polymerization of silicates is a key parameter to constrain the actual thermodynamics of shocks in natural environments. © 2020. The Authors.