- Like ice on water, solid diamond floats on liquid diamond.
- The finding explains possible liquid diamond oceans on other planets.
- Diamond oceans may cause off-kilter planetary tilts.
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 J. H. Eggert of Lawrence Livermore National Laboratory in Livermore, California. "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.
The graphite, and not the diamond, then melts into a liquid. The trick for the scientists was to heat the diamond up while simultaneously stopping it from transforming into graphite.
Eggert and his colleagues took 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 like those found on gas giants like Neptune and Uranus.
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 floated 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 Uranus' and Neptune'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.
If Earth's magnetic field were that far off it would place the magnetic north pole in Texas instead of off a Canadian island. A swirling ocean of liquid diamond could be responsible for the discrepancy.
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.
The idea that there are oceans of liquid diamond on Neptune and Uranus is not a new idea, said Tom Duffy, a planetary scientist at Princeton University.
The new Nature Physics article makes diamond oceans "look more and more plausible," said Duffy.
More research on the composition of Neptune and Uranus is needed before a truly definitive conclusion can be made, however, and this kind of research is very difficult to conduct.
Scientists can either send spacecraft to these planets, or they can try to simulate the conditions on Earth. Both options require years of preparation, expensive equipment, and are subject to some of the toughest environments in the universe.