Ultrahigh pressures, the kind of pressures found in huge gas giants like Neptune and Uranus are some of the places where ultrahigh temperatures and ultrahigh pressures exist. Eggert and his colleagues placed a small, natural, clear diamond, about a tenth of a carat by weight and half a millimeter thick, and blasted it with lasers at ultrahigh pressures.
The scientists liquefied the diamond at pressures 40 million times greater than what a person feels when standing at sea level on Earth. From there they slowly reduced the temperature and pressure.
When the pressure dropped to about 11 million times the atmospheric pressure at sea level on Earth and the temperature dropped to about 50,000 degrees solid chunks of diamond began to appear. The pressure kept dropping, but the temperature of the diamond remained the same, with more and more chunks of diamond forming.
Then the diamond did something unexpected. The chunks of diamond didn't sink. They floated. Microscopic diamond ice burgs floating in a tiny sea of liquid diamond. The diamond was behaving like water.
With most materials, the solid state is more dense than the liquid state. Water is an exception to that rule; when water freezes, the resulting ice is actually less dense than the surrounding water, which is why the ice floats and fish can survive a Minnesota winter.
An ocean of diamond could help explain the orientation of the planet's magnetic field as well, said Eggert. Roughly speaking, the Earth's magnetic poles match up with the geographic poles. The magnetic and geographic poles on Uranus and Neptune do not match up; in fact, they can be up to 60 degrees off of the north-south axis.
Up to 10 percent of Uranus and Neptune is estimated to be made from carbon. A huge ocean of liquid diamond in the right place could deflect or tilt the magnetic field out of alignment with the rotation of the planet.
Nature Physics - Melting temperature of diamond at ultrahigh pressure
Since Ross proposed that there might be ‘diamonds in the sky’ in 1981, the idea of significant quantities of pure carbon existing in giant planets such as Uranus and Neptune has gained both experimental and theoretical support. It is now accepted that the high-pressure, high-temperature behaviour of carbon is essential to predicting the evolution and structure of such planets. Still, one of the most defining of thermal properties for diamond, the melting temperature, has never been directly measured. This is perhaps understandable, given that diamond is thermodynamically unstable, converting to graphite before melting at ambient pressure, and tightly bonded, being the strongest bulk material known. Shock-compression experiments on diamond reported here reveal the melting temperature of carbon at pressures of 0.6–1.1 TPa (6–11 Mbar), and show that crystalline diamond can be stable deep inside giant planets such as Uranus and Neptune. The data indicate that diamond melts to a denser, metallic fluid—with the melting curve showing a negative Clapeyron slope—between 0.60 and 1.05 TPa, in good agreement with predictions of first-principles calculations. Temperature data at still higher pressures suggest diamond melts to a complex fluid state, which dissociates at shock pressures between 1.1 and 2.5 TPa (11–25 Mbar) as the temperatures increase above 50,000 K
Ten pages of supplemental material
Targets consisted of a ~500 μm-thick diamond disk glued to a 50 μm-thick diamond-turned aluminum disk. A plastic ablator was used to minimize hard x-ray generation in the laser-plasma region. Both single-crystal, natural, type-1a diamond, and poly-crystalline, synthetic, type 2a diamond disks with an average grain size of about 130 μm were used. The polycrystalline diamond wafers were made by chemical vapor deposition (CVD) on a silicon substrate with an initial seed-grain size of ~10 nm; there was little preferred orientation.
The OMEGA laser at the University of Rochester was used to produce strong shocks by focusing 1-ns pulses with up to 3 kJ of 351-nm laser light to a flat spot ~600 μm in diameter.