These planets contain a high proportion of methane, which UC Berkeley researchers have now shown can turn into diamond at the high temperatures and pressures found inside these planets. The team, led by Benedetti and Raymond Jeanloz, professor of geology and geophysics, produced these conditions inside a diamond anvil cell, squeezing liquid methane to several hundred thousand times atmospheric pressure. When they focused a laser beam on the pressurized liquid, heating it to some 5,000 degrees Fahrenheit, diamond dust appeared. They report their experimental findings in a paper in the Oct. 1 issue of Science. The demonstration that methane can convert to diamond as well as other complex hydrocarbons in the interiors of giant planets like Neptune hint at a complex chemistry inside gaseous planets and even brown dwarf stars.
Brown dwarfs are small, dim stars barely larger than the largest gas giant planets.
Now that technology is able to reproduce the high pressures and temperatures found there, we are getting much better quality information on the chemical reactions taking place under these conditions. The interior of these planets may be much more complicated that our current picture. A simple calculation, for example, shows that the energy released by diamonds settling to the planet's core could account for the excess heat radiated by Neptune, that is, the heat given off by Neptune in excess of what it receives from the sun. Based on theoretical calculations, Neptune and Uranus are estimated to contain about 10 to 15 percent methane under an outer atmosphere of hydrogen and helium. Several groups of researchers have suggested that the methane in these planets could conceivably turn into diamond at fairly shallow depths, about one tenth of the way to the center.