Eric Bland in Discovery News:
Oceans of liquid diamond, filled with solid diamond icebergs, could be floating on Neptune and Uranus, according to a recent article in the journal Nature Physics.
The research, based on the first detailed measurements of the melting point of diamond, found diamond behaves like water during freezing and melting, with solid forms floating atop liquid forms. The surprising revelation gives scientists a new understanding about diamonds and some of the most distant planets in our solar system.
“Diamond is a relatively common material on Earth, but its melting point has never been measured,” said Eggert. “You can't just raise the temperature and have it melt, you have to also go to high pressures, which makes it very difficult to measure the temperature.”
Other groups, notably scientists from Sandia National Laboratories, successfully melted diamond years ago, but they were unable to measure the pressure and temperature at which the diamond melted.
Diamond is an incredibly hard material. That alone makes it difficult to melt. But diamond has another quality that makes it even harder to measure its melting point. Diamond doesn't like to stay diamond when it gets hot. When diamond is heated to extreme temperatures it physically changes, from diamond to graphite.
More here. [Thanks to Sean Carroll.]