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The black diamonds of Neptune

Of all the strange, 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 Earths sea-level atmospheric pressure. When they focused a high-energy infrared laser beam on the liquidcausing the temperature of the methane to reach 5,000 degrees Fahrenheitdiamond 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.

Benedetti and Jeanloz squeezed liquid methane to make diamond dust. The liquid methane, cooled with liquid nitrogen, was placed in the diamond anvil cell and squeezed to 10 and 50 billion pascals (gigapascals), or about 100,000-500,000 times atmospheric pressure on Earth. Next, the researchers heated the methane with an infrared laser to 3,000 Kelvin (5,400 degrees Fahrenheit). The laser caused the methane to turn black because of the diamonds created. The black diamond specks float in a clear hydrocarbon liquid melted by the laser.

Jupiter and Saturn may also contain diamonds produced under such conditions, though they contain proportionately less methane than Neptune and Uranus. Neptune and Uranus are estimated to contain 10 to 15 percent methane under an outer atmosphere of hydrogen and helium.

Louis Varricchio lives in Vermont. He was a NASA science writer and freelance science reporter for public radio. He is the NASA/JPL solar system ambassador to Vermont. He is available as a speaker about space to local schools and public groups.

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