Under pressure

Of all the lifeless worlds of our solar system, the gas giant planets Uranus and Neptune are among the most inhospitable.

So strange are these two ringed planets, future astronauts may find black diamonds hailing down on them through the dense poisonous atmospheres. The interiors of both planets contain liquid methane, which according to some researchers may turn into diamonds at the high temperatures and pressures found deep inside.

Once these diamonds form, they fall like raindrops or hailstones toward the center of the planet, according to Laura Robin Benedetti and Raymond Jeanloz of the University of California at Berkeley. The two researchers produced the exact planetary conditions inside what scientists call a diamond anvil cell; they squeezed liquid methane to several hundred thousand times Earth's sea-level atmospheric pressure. When they focused a high-energy infrared laser beam on the liquid - causing the temperature of the methane to reach 5,000 degrees Fahrenheit-diamond dust began to form and "rain" inside the anvil cell.

To produce a rain of diamonds resulting from methane and other hydrocarbon compounds, deep below the cloud covers of Uranus and Neptune, implies planets with complex, high-pressure chemistry. By understanding the interiors of the outer gas giants, scientists also hope to better understand distant brown dwarf stars, small dim stars that are slightly larger than Jupiter. Brown dwarfs are the "missing link" between planets and stars.

"This work is opening the door to study the interesting types of chemical reactions taking place inside planets and brown dwarf suns," 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."

"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," said Benedetti.

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