Berkeley - October 1, 1999 - If experiments at the University of California, Berkeley, are any indication, future explorers of our solar system may well find diamonds hailing down through the atmospheres of Neptune and Uranus.

　　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.

　　"Once these diamonds form, they fall like raindrops or hailstones toward the center of the planet," said Laura Robin Benedetti, a graduate student in physics at UC Berkeley.

　　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.

　　"This is opening the door to study of the interesting types of chemical reactions taking place inside planets and brown dwarfs," Jeanloz said. "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."

　　"It is not amazing that chemistry like this happens inside planets, it's just that most people haven't dealt with the chemical reactions that can occur," Benedetti said. "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.

　　"What's exciting to us is the application of this high-pressure chemistry to understanding the outer planets," Jeanloz said.

　　"As more planets are found in unexpected orbits around other stars, the effects of internal chemical processes will need to be further clarified in order to obtain a general understanding of planet formation and evolution," the authors concluded in the Science paper.

　　Our solar system's other gas giant planets -- Jupiter and Saturn -- may also contain diamonds produced under such conditions, though they contain proportionately less methane than Neptune and Uranus. 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. (See graphic for presumed internal structure of Neptune,