Physical properties of carbon cast light on Neptune and Uranus

May 11, 2006
Researchers at Lawrence Livermore National Laboratory (LLNL) have determined the solid/liquid and solid/ solidphase boundaries of carbon for pressures up to 20 million Earth atmospheres and more than 10,000°K.

Researchers at Lawrence Livermore National Laboratory (LLNL) have determined the solid/liquid and solid/ solidphase boundaries of carbon for pressures up to 20 million Earth atmospheres and more than 10,000°K. The simulations reveal physical properties of carbon, which are important for modeling Neptune, Uranus, and white dwarf stars, as well as extrasolar carbon-rich planets.

In its elemental form, the carbon found in coal, graphite, diamond, buckyballs, and nanotubes has drastically different properties. But at the microscopic level, it only differs by the geometrical arrangements of carbon atoms.

Despite important experimental work over the last few decades studying compressed diamond, scientists know little about phase boundaries and melting properties of elemental carbon or its electronic properties under extreme conditions. There have been few experiments because of difficulties in reaching megabar (1 million atmospheres) pressures and temperatures of thousands of degrees Kelvin in the laboratory.

"Our results show a consistent description of elemental carbon in a broad range of temperature and pressures and a description of its electronic properties within the same framework," says Alfredo Correa of UC Berkeley, who works in LLNL's Quantum Simulations Group.

Researchers also discovered that the diamond/BC8/liquid triple point (the temperature and pressure at which these three phases coexist in thermodynamic equilibrium) happens at a lower pressure than previously thought. (BC8 denotes a solid phase of carbon into which diamond transforms above 12 megabar and absolute zero, an in-between form of carbon where it's not as hard as diamond and not as soft as a liquid.) The conditions at which the triple point is found resemble estimates of the temperatures and pressures in Neptune and Uranus' cores.

"Our results call for a partial revision of planetary models, especially the description of their cores," Correa says.

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