Imagine an iceberg floating in an ocean — except the iceberg is diamond, the ocean is liquid carbon, and the whole scene is unfolding deep inside a planet billions of miles away. A new experiment at Lawrence Livermore National Laboratory (LLNL) is giving scientists a better understanding of how such a bizarre landscape could exist inside ice giants such as Uranus and Neptune.
The study, published in Nature Physics, found that diamond melts under extreme pressure at the toasty temperature of 1,300 degrees Fahrenheit (700 degrees Celsius), but much lower than previous experimental estimates. Using powerful ultraviolet lasers, researchers compressed tiny diamond samples to pressures greater than those found at the centers of Uranus and Neptune and temperatures hotter than the surface of the Sun.
The breakthrough helps resolve a two-decade-old disagreement between laboratory experiments and computer simulations about diamond's melting point. It also confirms something particularly strange about diamond: under these extraordinary conditions, liquid carbon is denser than solid diamond.
That means a chunk of solid diamond could float in a sea of molten carbon — much like an ice cube floats in water. LLNL researchers have dubbed these hypothetical floating chunks “diamond bergs.”
The finding adds an intriguing new dimension to the long-standing “diamond rain” theory. Scientists believe the enormous pressure and heat deep inside Uranus and Neptune can break apart methane and other carbon-containing molecules, allowing carbon to crystallize into diamonds. The crystals would then sink deeper into the planet, creating a spectacular form of precipitation that never reaches a surface.
The new melting temperature estimates suggest that as those diamonds descend into increasingly hotter regions, they could eventually encounter conditions where the surrounding carbon becomes liquid. Because the liquid is denser, some solid diamond could theoretically remain suspended or float within it.
The result could be less like diamonds simply raining toward a planetary core and more like a strange, deep planetary ocean containing floating chunks of diamond. The latest experiment doesn't prove that diamond bergs are actually present inside Uranus or Neptune, but it gives planetary scientists a much stronger physical basis for modeling that possibility.
But how can diamond melt without burning? The answer is oxygen. Burning and melting are two entirely different processes. On Earth, a diamond heated to sufficiently high temperatures in the presence of oxygen can combust, reacting chemically with oxygen rather than turning into a puddle of liquid carbon.
Deep inside an ice giant planet, however, there is no ordinary atmosphere surrounding the diamond. Under tremendous pressure and extraordinary heat, the carbon crystal instead reaches the point where its tightly organized atomic structure can no longer remain solid. It changes phase and becomes liquid carbon.
Credit: Image by The Jeweler Blog using aichatapp.ai.