Dissociation of methane into hydrocarbons at extreme (planetary) pressure and temperature.
ABSTRACT Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature. Contrary to the current interpretation of shock-wave experiments, the simulations suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascals. The simulation conditions (100 to 300 gigapascals; 4000 to 5000 kelvin) were chosen to follow the isentrope in the middle ice layers of Neptune and Uranus. Implications on the physics of these planets are discussed.
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ABSTRACT: The phase diagram of the carbon-hydrogen system is of great importance to planetary sciences, as hydrocarbons comprise a significant part of icy giant planets and are involved in reduced carbon-oxygen-hydrogen fluid in the deep Earth. Here we use resistively- and laser-heated diamond anvil cells to measure methane melting and chemical reactivity up to 80 GPa and 2,000 K. We show that methane melts congruently below 40 GPa. Hydrogen and elementary carbon appear at temperatures of >1,200 K, whereas heavier alkanes and unsaturated hydrocarbons (>24 GPa) form in melts of >1,500 K. The phase composition of carbon-hydrogen fluid evolves towards heavy hydrocarbons at pressures and temperatures representative of Earth's lower mantle. We argue that reduced mantle fluids precipitate diamond upon re-equilibration to lighter species in the upwelling mantle. Likewise, our findings suggest that geophysical models of Uranus and Neptune require reassessment because chemical reactivity of planetary ices is underestimated.Nature Communications 09/2013; 4:2446. · 10.02 Impact Factor
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ABSTRACT: Water and hydrogen at high pressure make up a substantial fraction of the interiors of giant planets. Using ab initio random structure searching methods we investigate the ground-state crystal structures of water, hydrogen, and hydrogen-oxygen compounds.We find that, at pressures beyond 14 Mbar, a novel phase withH4O stoichiometry is stable relative to separate water ice and hydrogen phases.We also predict two new ground-state structures, P21/m and I4/mmm, for post-C2/m water ice.Physical Review B 01/2013; 87:024112. · 3.66 Impact Factor
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ABSTRACT: A new design for targets employed in laser induced shock-compression experiments is presented. Numerical simulations to optimize target parameters and to clarify shock dynamics are realized. The experiments proved the new scheme is reliable and appropriate for reflectivity measurements of thermodynamical states lying out of the standard graphite or diamond Hugoniot: the final state reached in compression can be varied tuning the carbon layer characteristics (initial density and thickness) and the laser intensity, with the possibility to determine the reflectivity of carbon and the position on the phase diagram. An increase of reflectivity in carbon has been observed at 260 GPa and 14 000 K while no increase in reflectivity is found at 200 GPa and 20 000 K.The European Physical Journal D 07/2013; 67(7). · 1.51 Impact Factor